tome
Well-Known Member
Short answer is that your bog might just be 60 feet cable run from the battery
Do I need to re-wire my toilet...?
- Thread starter Karel
- Start date
misterg
Well-Known Member
[ QUOTE ]
I just reckonned out that a 240v 30amp fuse would take an energy throughput of 7200 watts before blowing, and again came up with misterg's figure that at 12v the current would have to be 600amps to produce the same amount of energy, and surely its this energy that causes the fuse to blow.The figure does some a tad excessive, but what am I missing here?
[/ QUOTE ]
AMPS AMPS AMPS! /forums/images/graemlins/smile.gif Your energy figures are fine, but fuses blow due to amps (whatever the voltage). I'm trying to avoid getting into a "when does an 'x' amp fuse blow discussion, because (as VicS obviously knows) the answer isn't straightforward if you get into the detail.
Voltage ratings *are* important (as is the AC/DC rating), but these are to do with the fuse/breaker being able to break the circuit being supplied at that voltage, at the appointed current without it exploding, melting, etc. which is where your energy consideration comes in.
Recently went through replacing some 100 amp circuit breakers at work - each one was the size of a suitcase, and in case they exploded, they had to be closed by a clockwork spring released by a lanyard operated from outside the building - Oh, they were working a 11KV /forums/images/graemlins/grin.gif
Bottom line is just use a fuse or circuit breaker that's designed for the type of application you're considering, at the recommended rating - automotive stuff (intended for DC + damp) is fine.
(600 amps not my figure, btw /forums/images/graemlins/frown.gif)
Bet someone's already posted this reply...
Andy
I just reckonned out that a 240v 30amp fuse would take an energy throughput of 7200 watts before blowing, and again came up with misterg's figure that at 12v the current would have to be 600amps to produce the same amount of energy, and surely its this energy that causes the fuse to blow.The figure does some a tad excessive, but what am I missing here?
[/ QUOTE ]
AMPS AMPS AMPS! /forums/images/graemlins/smile.gif Your energy figures are fine, but fuses blow due to amps (whatever the voltage). I'm trying to avoid getting into a "when does an 'x' amp fuse blow discussion, because (as VicS obviously knows) the answer isn't straightforward if you get into the detail.
Voltage ratings *are* important (as is the AC/DC rating), but these are to do with the fuse/breaker being able to break the circuit being supplied at that voltage, at the appointed current without it exploding, melting, etc. which is where your energy consideration comes in.
Recently went through replacing some 100 amp circuit breakers at work - each one was the size of a suitcase, and in case they exploded, they had to be closed by a clockwork spring released by a lanyard operated from outside the building - Oh, they were working a 11KV /forums/images/graemlins/grin.gif
Bottom line is just use a fuse or circuit breaker that's designed for the type of application you're considering, at the recommended rating - automotive stuff (intended for DC + damp) is fine.
(600 amps not my figure, btw /forums/images/graemlins/frown.gif)
Bet someone's already posted this reply...
Andy
Karel
Well-Known Member
So you think the wiring might just be ok? And the tech knew what he was doing? (except for the fuse and the connectors...) I ought to check voltage drop first with a meter to know for sure. And if I lose no more than 0.5V I'm ok. That would be a huge relief.
Do other folk here concur?
Do other folk here concur?
tome
Well-Known Member
What can I say?
misterg
Well-Known Member
[ QUOTE ]
Good idea tome - measure it!.
[/ QUOTE ]
No problem with the pragmatic approach, and reluctant to cut across one so senior in the pack /forums/images/graemlins/laugh.gif ...but...(!) Chances are the motor will not be doing any work, so wouldn't draw much current. This will give an unrealistically optimistic result. Highest current will be drawn if the motor is stalled - It won't do it any good to test it like this though. You don't want your wiring melting if you have a "pound of Dundee cake" to dispose of
[ QUOTE ]
Now here's a question about circuit breakers; some specify "combined breaker/switch" What does that mean?
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Means that the same gizmo can be used to routinely switch the circuit on and off as well as acting as the "fuse". (It's not good practice to use a pure circuit breaker this way, as they aren't designed for it and wear out quickly.)
Good idea tome - measure it!.
[/ QUOTE ]
No problem with the pragmatic approach, and reluctant to cut across one so senior in the pack /forums/images/graemlins/laugh.gif ...but...(!) Chances are the motor will not be doing any work, so wouldn't draw much current. This will give an unrealistically optimistic result. Highest current will be drawn if the motor is stalled - It won't do it any good to test it like this though. You don't want your wiring melting if you have a "pound of Dundee cake" to dispose of
[ QUOTE ]
Now here's a question about circuit breakers; some specify "combined breaker/switch" What does that mean?
[/ QUOTE ]
Means that the same gizmo can be used to routinely switch the circuit on and off as well as acting as the "fuse". (It's not good practice to use a pure circuit breaker this way, as they aren't designed for it and wear out quickly.)
ningcompoop
Well-Known Member
Oops, sorry misterg, not your 600amps! Still can't get my head round that amps thing, but I guess thats where a forum comes in useful! /forums/images/graemlins/grin.gif
tome
Well-Known Member
Quite right, have your assistant eat well and make sure the bog is macerating at max amps before taking measurements. If your assistant draws 60 Amps think about replacing him with lighter crew
I wouldn't worry too much about upsetting 'seniors in the pack' as they're quite good at doing this for themselves...
I wouldn't worry too much about upsetting 'seniors in the pack' as they're quite good at doing this for themselves...
HeadMistress
Well-Known Member
I've only skimmed through all the posts, so I may be repeating what's already been said....but the worst I can do here is sum it all up:
Any electric toilet should be on its own separate dedicated circuit, shared by nothing else that can be on at the same time to pull power away from it...low voltage to the toilet can not only result in sluggish discharge, but also will severely shorten the lifespan of the motor.
Wire size isn't a constant....there are mininums based on the amperage draw, but the actual wire size is determined by the distance--which is always measured ROUND TRIP, not one way--from the power supply (battery) to the toilet. And any fuse or breaker should be the same as the rated amperage to the toilet. Wire size that's too small will not only result in low voltage to the toilet, but is also a fire hazard. It's hard to have too large a wire size, but too small a fuse will blow prematurely, preventing use of the toilet.
I suggest that you contact Vetus and have a chat with them about the correct wire size and proper for YOUR installation. You might also have a qualified marine electrician inspect your installation and advise you as to what--if anything--needs to be corrected before doing anything.
Any electric toilet should be on its own separate dedicated circuit, shared by nothing else that can be on at the same time to pull power away from it...low voltage to the toilet can not only result in sluggish discharge, but also will severely shorten the lifespan of the motor.
Wire size isn't a constant....there are mininums based on the amperage draw, but the actual wire size is determined by the distance--which is always measured ROUND TRIP, not one way--from the power supply (battery) to the toilet. And any fuse or breaker should be the same as the rated amperage to the toilet. Wire size that's too small will not only result in low voltage to the toilet, but is also a fire hazard. It's hard to have too large a wire size, but too small a fuse will blow prematurely, preventing use of the toilet.
I suggest that you contact Vetus and have a chat with them about the correct wire size and proper for YOUR installation. You might also have a qualified marine electrician inspect your installation and advise you as to what--if anything--needs to be corrected before doing anything.
tome
Well-Known Member
Hi Peggie, enjoying a high fibre breakfast I hope
It's the volts what jolts and the amps what bangs!
It's the volts what jolts and the amps what bangs!
William_H
Well-Known Member
Right listen up. A voltage rating on a fuse is a maximum voltage circuit that it can be used in. Nothing else. When a fuse operates (blows) there is a small voltage drop across the wire in the fuse. This voltage drop at(just above) the rated current is enough to heat the wire and melt it. So if you are replacing the old style house fuses with fuse wire. Heavier wire has less resistance and being bigger can take more heat before it melts. The voltage drop is a small fraction of a volt regardless of the voltage of the circuit. It is dictated by the resistance of the wire and the current. So lets not hear this 240v x 35 amp bit. In a short circuit situation in a 240v circuit the majority of the voltage (power) is dissipated in the wiring.
However when a fuse does blow the ends of the wire as they part can end up with the circuit voltage across them. At 240 volts this can easily start an arc. Usually this only melts further fuse wire until the gap gets so big the arc fails. Consider though that at high voltages the arc may continue across the gap even when the fuse wire has melted. So a high voltage circuit fuse needs to be longer in the fuse wire while a low voltage circuit can be much shorter. Hence the voltage rating of the fuse and indeed the holder.
Usually however the fuse and holder on the smallest (cheapest) is rated at 240volt (max).
Circuit breakers however are more ciritical on voltage rating because they have contacts which open a small distance so any arcing as they open may destroy the contacts and make the circuit breaker unusable again.
In DC the rating may be lower than AC because if you break a circuit of high current that includes an inductance (typically a motor), the act of breaking the circuit can create a high back EMF votlage much higher than 12 volts which can induce an arc. So often devices will be rated at one current for 240vAC and a lower current for 12VDC. (to cover inductive loads situation).
Lastly fuses are often described by their time delay characteristics. So you get slow blo, standard or surge proof and various other names. Slow blow is good where a motr draws a lot of current for a short period but normal current is low. So that the rating can be less than the surge current.
Auto fuses are generally do not have the time characteristics described and are a standard thin bit of wire.
olewill
However when a fuse does blow the ends of the wire as they part can end up with the circuit voltage across them. At 240 volts this can easily start an arc. Usually this only melts further fuse wire until the gap gets so big the arc fails. Consider though that at high voltages the arc may continue across the gap even when the fuse wire has melted. So a high voltage circuit fuse needs to be longer in the fuse wire while a low voltage circuit can be much shorter. Hence the voltage rating of the fuse and indeed the holder.
Usually however the fuse and holder on the smallest (cheapest) is rated at 240volt (max).
Circuit breakers however are more ciritical on voltage rating because they have contacts which open a small distance so any arcing as they open may destroy the contacts and make the circuit breaker unusable again.
In DC the rating may be lower than AC because if you break a circuit of high current that includes an inductance (typically a motor), the act of breaking the circuit can create a high back EMF votlage much higher than 12 volts which can induce an arc. So often devices will be rated at one current for 240vAC and a lower current for 12VDC. (to cover inductive loads situation).
Lastly fuses are often described by their time delay characteristics. So you get slow blo, standard or surge proof and various other names. Slow blow is good where a motr draws a lot of current for a short period but normal current is low. So that the rating can be less than the surge current.
Auto fuses are generally do not have the time characteristics described and are a standard thin bit of wire.
olewill
HeadMistress
Well-Known Member
[ QUOTE ]
Hi Peggie, enjoying a high fibre breakfast I hope
[/ QUOTE ]
Hardly...It's 6 hours earlier here than in the UK...I'm enjoying after dinner coffee now. But you're either up very late or very early!
Hi Peggie, enjoying a high fibre breakfast I hope
[/ QUOTE ]
Hardly...It's 6 hours earlier here than in the UK...I'm enjoying after dinner coffee now. But you're either up very late or very early!
ex-Gladys
Well-Known Member
"Professional" wiring can be amazing. Our Wallas Cooker (wiring harness withit has 30A cable) had both supplies linked to a single 5A cable... Now wonder it complained at the voltage drop over about 20'...
VicS
Well-Known Member
[ QUOTE ]
if a 30A domestic fuse is meant to carry 30A, when does it blow? At 31A?
[/ QUOTE ]
I have seen figures but I can't remember where and I certainly can't quote them. Almost certainly more than 31amps but time also comes into the reckoning as well. So it will carry various %age overloads for different lengths of time and will need quite a significant overload to blow instantaneously.
One thing for certain is that if your toilet has been fitted with a fuse small than the recommended one and it blows then it was too small and the recommended size should be fitted. You can only resolve the cable size issue by determining the size that has been used. If it is too small then voltage drop may be an issue but length of cable run will be a factor here but that has already been mentioned and will only be overcome by fitting a heavier cable than suggested. The most serious implications of using cable that is too small will be firstly; heating of the cable but if it is only slightly too small that will not be a great problem because, presumably, the toilet is not running for prolonged periods secondly the cable MUSt be rated higher than the fuse that protects it otherwise there is a risk of the cable overheating under fault conditions
You will have to measure the diameter of the wire with a vernier or micrometer and compare that with published figures or easier maybe with a bit of cable known to be of the correct rating.
I must confess that i have only skimmed through the other replies so i may have repeated what others have said. I will comment elsewhere on the power dissipated in the fuse issue and the falacy surrounding the calculations used.
Must dash.
if a 30A domestic fuse is meant to carry 30A, when does it blow? At 31A?
[/ QUOTE ]
I have seen figures but I can't remember where and I certainly can't quote them. Almost certainly more than 31amps but time also comes into the reckoning as well. So it will carry various %age overloads for different lengths of time and will need quite a significant overload to blow instantaneously.
One thing for certain is that if your toilet has been fitted with a fuse small than the recommended one and it blows then it was too small and the recommended size should be fitted. You can only resolve the cable size issue by determining the size that has been used. If it is too small then voltage drop may be an issue but length of cable run will be a factor here but that has already been mentioned and will only be overcome by fitting a heavier cable than suggested. The most serious implications of using cable that is too small will be firstly; heating of the cable but if it is only slightly too small that will not be a great problem because, presumably, the toilet is not running for prolonged periods secondly the cable MUSt be rated higher than the fuse that protects it otherwise there is a risk of the cable overheating under fault conditions
You will have to measure the diameter of the wire with a vernier or micrometer and compare that with published figures or easier maybe with a bit of cable known to be of the correct rating.
I must confess that i have only skimmed through the other replies so i may have repeated what others have said. I will comment elsewhere on the power dissipated in the fuse issue and the falacy surrounding the calculations used.
Must dash.
Karel
Well-Known Member
Thanks Vic.
William, thank you for a very informative post. Despite my being very technical in other respects, I've resigned myself to never fully understanding electricity - the 'water pressure / tap / size of pipe etc.' analogy (which is what i was taught at school) falls apart pretty quickly. What's more this thing of the diameter of a wire seems all wrong to me - the elctrons flow on the surface of a wire, so it should be wire surface area shouldn't it? Which should give an advantage to multi-stranded wiring. Ho-hum...
Peggie, just curious here, but do you find there's a tad more humor here than on BoaterEd? I used to post there before I discovered the much more local ybw - dunno how I managed to miss it.
Ah yes, the 'Dundee Cake Scenario'...
You know, when I first got into boating I thought it was all about chicks in bikinis, instead I find myself having serious online conversations about the technicalities of crew bowel movements! /forums/images/graemlins/crazy.gif
I'm beginning to wonder if this flash new toilet, pride of the marina, could possibly drain my batteries. Consider the scenario that your moored up for a couple of days, maybe you leave the boat in the care of your passengers and so the engine doesn't get a chance to re-charge the batteries, and when you come to start the engine...
Of course there's always a second battery, but relying on a backup makes me uncomfortable. A backup should only ever be a backup, and if used indicates your main system may need a re-think.
Anyway, I'm wondering if the loo needs a re-think of its own, and should maybe have it's own battery located in the cupboard right next to it! The off-shore charger has a third (currently unused) output that could be put to use. What type of battery should I use?
William, thank you for a very informative post. Despite my being very technical in other respects, I've resigned myself to never fully understanding electricity - the 'water pressure / tap / size of pipe etc.' analogy (which is what i was taught at school) falls apart pretty quickly. What's more this thing of the diameter of a wire seems all wrong to me - the elctrons flow on the surface of a wire, so it should be wire surface area shouldn't it? Which should give an advantage to multi-stranded wiring. Ho-hum...
Peggie, just curious here, but do you find there's a tad more humor here than on BoaterEd? I used to post there before I discovered the much more local ybw - dunno how I managed to miss it.
Ah yes, the 'Dundee Cake Scenario'...
You know, when I first got into boating I thought it was all about chicks in bikinis, instead I find myself having serious online conversations about the technicalities of crew bowel movements! /forums/images/graemlins/crazy.gif
I'm beginning to wonder if this flash new toilet, pride of the marina, could possibly drain my batteries. Consider the scenario that your moored up for a couple of days, maybe you leave the boat in the care of your passengers and so the engine doesn't get a chance to re-charge the batteries, and when you come to start the engine...
Of course there's always a second battery, but relying on a backup makes me uncomfortable. A backup should only ever be a backup, and if used indicates your main system may need a re-think.
Anyway, I'm wondering if the loo needs a re-think of its own, and should maybe have it's own battery located in the cupboard right next to it! The off-shore charger has a third (currently unused) output that could be put to use. What type of battery should I use?
G
Guest
Guest
First wire sizes:
Using my tables for wires in boats and not in free air they would recommend 8AWG or 8mm2 conductors. However, you could get away with 12AWG or 3mm2. As it is a motor with high starting and stall currents and the distance is low for a boat I would fit 6mm2 cable as the fittings/connector blocks are more universal. Since it is in the area of obvious water the wire should also be tinned. I also would suggest using a marine type fuse holder with plated contacts capable of taking 6mm2 screw down crimp terminals. You definitely want a fuse that you can remove and put in your pocket when you come to work on the toilet. Just for safety!
Boat wires in general are bunched through holes and have bad environments and turning stresses. Undersized wires will generate heat and can cause the insulation to melt leading to full short faults. I always tell the story from another yachty we met who was sailing along quite happily when 8 feet of unfused battery cable shorted under a locker full of stuff. They only had time to grab the essentials and jump before the yacht burnt to the water line.
Now the Fuse:
As for the speed of blowing the fuse a normal type would be OK. A fuse will blow nearly instantly with a short fault, but with an overload or long stall situation it will take a time to heat up and melt. A fuse is just a short sacrificial wire that will melt BEFORE any other damage occurs to the main wiring in particular the insulation. I always think of fuses protecting the wires and short circuit faults. The actual motor should have its own protection via a resetable overheat device.
One final point:
Vetus sells a 230V version! With my experience of electrical earthing on boats I have seen, I would think this could get a little tingling!
Using my tables for wires in boats and not in free air they would recommend 8AWG or 8mm2 conductors. However, you could get away with 12AWG or 3mm2. As it is a motor with high starting and stall currents and the distance is low for a boat I would fit 6mm2 cable as the fittings/connector blocks are more universal. Since it is in the area of obvious water the wire should also be tinned. I also would suggest using a marine type fuse holder with plated contacts capable of taking 6mm2 screw down crimp terminals. You definitely want a fuse that you can remove and put in your pocket when you come to work on the toilet. Just for safety!
Boat wires in general are bunched through holes and have bad environments and turning stresses. Undersized wires will generate heat and can cause the insulation to melt leading to full short faults. I always tell the story from another yachty we met who was sailing along quite happily when 8 feet of unfused battery cable shorted under a locker full of stuff. They only had time to grab the essentials and jump before the yacht burnt to the water line.
Now the Fuse:
As for the speed of blowing the fuse a normal type would be OK. A fuse will blow nearly instantly with a short fault, but with an overload or long stall situation it will take a time to heat up and melt. A fuse is just a short sacrificial wire that will melt BEFORE any other damage occurs to the main wiring in particular the insulation. I always think of fuses protecting the wires and short circuit faults. The actual motor should have its own protection via a resetable overheat device.
One final point:
Vetus sells a 230V version! With my experience of electrical earthing on boats I have seen, I would think this could get a little tingling!
VicS
Well-Known Member
The falacy is using Watts + volts X amps where volts is the supply.. You must use the PD across the fuse, but we don't know that! A better formula would be watts = Amps squared X resistance of the fuse in ohms. Even that although valid can't really beused because the resistance of the fuse wll increase as its temperature increases.
Sorry written very quickly. Have A level physics as well .
really must dash now!
Sorry written very quickly. Have A level physics as well .
really must dash now!
Karel
Well-Known Member
"You definitely want a fuse that you can remove and put in your pocket when you come to work on the toilet."
Hadn't thought of that. Good point.
Actually the cabling runs through the engine compartment, and it's a petrol engine....
I'm not going to chance it. I'm putting another battery in. Any suggestions? This will be in a cupboard in the shower/loo, so a damp environment, but not actually 'wet'.
In fact, when moored up there's a 230V supply very close by, so another option is to also install a trickle charger in the loo cupboard which would save me a lot of effort running cables aft to the existing one. I looked into chargers in some depth a couple of years ago before buying the current unit, but it's worth asking again here because there's always something new.
<span style="color:red"> - I'm starting a fresh thread on this - </span>
Hadn't thought of that. Good point.
Actually the cabling runs through the engine compartment, and it's a petrol engine....
I'm not going to chance it. I'm putting another battery in. Any suggestions? This will be in a cupboard in the shower/loo, so a damp environment, but not actually 'wet'.
In fact, when moored up there's a 230V supply very close by, so another option is to also install a trickle charger in the loo cupboard which would save me a lot of effort running cables aft to the existing one. I looked into chargers in some depth a couple of years ago before buying the current unit, but it's worth asking again here because there's always something new.
<span style="color:red"> - I'm starting a fresh thread on this - </span>
Karel
Well-Known Member
"it should be wire surface area shouldn't it?" I said
Ah - just realised! wiring is rated by the cross-sectional surface area isn't it? Not the diameter (as I first thought). And that is proportional to the actual surface area of the outside of a wire (the bit the electrons travel in). But multi-stranded should still have the advantage by having more area...?
Have I gone off-topic? /forums/images/graemlins/grin.gif
No dammit! I'm wrong!! (am I driving everyone nuts yet /forums/images/graemlins/grin.gif ) - the outside surface area is proportional to the cross-sectional circumference of the wire. So the cross-sectional area is misleading as a measure. This makes more sense for multi-stranded wires: you add all the seperate diameters together, multiply by pi and you get a measure of the actual area the electrons travel in. The cross-sectional surface area (for the purpose of comparing cables) would give a completely different result.
/forums/images/graemlins/crazy.gif
Ah - just realised! wiring is rated by the cross-sectional surface area isn't it? Not the diameter (as I first thought). And that is proportional to the actual surface area of the outside of a wire (the bit the electrons travel in). But multi-stranded should still have the advantage by having more area...?
Have I gone off-topic? /forums/images/graemlins/grin.gif
No dammit! I'm wrong!! (am I driving everyone nuts yet /forums/images/graemlins/grin.gif ) - the outside surface area is proportional to the cross-sectional circumference of the wire. So the cross-sectional area is misleading as a measure. This makes more sense for multi-stranded wires: you add all the seperate diameters together, multiply by pi and you get a measure of the actual area the electrons travel in. The cross-sectional surface area (for the purpose of comparing cables) would give a completely different result.
/forums/images/graemlins/crazy.gif
dratsea
Well-Known Member
The Ancor Marine web site has most of the information you need about cross section and wire run. I keep a copy of their catalogue on the boat and it comes second only to Nigel Calder's book as my most used source of information about things electrical.
davidpbo
Well-Known Member
Forgive me if I am repeating someone else's post. It is the cross sectional area of the wire that is important.
= Pi multiplied by R(adius) Squared.
Measure the diameter of the conductor, half it, multiply the result by itself then by 3.142 (I think).
David
= Pi multiplied by R(adius) Squared.
Measure the diameter of the conductor, half it, multiply the result by itself then by 3.142 (I think).
David
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