Anyone care to check my boat electrics calculation?

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FairweatherDave

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Been studying Calder and Tony Brooks website trying to get to grips with basic boat electrics and first principles. I want to see if I have got this right. And apologies if I have got the terminology wrong!
I have already got the VHF radio installed. However rather than going back to the bus bar I would like to "common up", ie T off from the VHF circuit the AIS receiver, the Glomex splitter and the Garmin 72 handheld unit, (ie 3 things in parallel circuits). I want to check I have the right size cable and fuse that I can T off. So I need to total up my total current flow.

VHF is an XM DSC 25/1 watt radio. The manual spec does not give me any other obvious figure so at 12v that makes 2 amps?
The Garmin manual for the GPS says use a 2 amp fuse with a 12volt power source. So 2 amps. (probably OTT?)

The Glomex splitter uses 25mA at 5v so 0.125Watts
The Quark AIS again uses a micro USB from 12v to 5v step down thing, so 270mA (x5) = 1.35Watts
Combine those two Watt figures and round up to 1.5Watts and divide by 12 = 0.125 amps

So a total current of 4.125 amps on that original VHF circuit which would have a fuse sized for that (so 5 Amp fuse)

However the wire for each of the circuits I am adding on in parallel only has to be sized to the whatever I am putting in it, ie the Glomex splitter and the AIS can be pretty thin. The Glomex comes with an inline fuse too.

I think my confusion arises from the fact I am planning to add three circuits to an existing circuit and I'm not sure you simply add up the amps of each unit.
So anyone care to point out how many mistakes I have made :) I'm sure I am taking this all a bit far as the bits and pieces I'm adding are all small but it seemed a good opportunity to learn some theory. Thanks for any pointers.
 
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Spotted first mistake with the vhf,the output watts is not what the unit draws from the battery more like around 5amps give or take.
 
Hi Dave, I’m sure someone very knowledgeable will be along shortly but I can tell you that parallel circuits are a bit more complicated. The total reisistance of the combined circuits is less than the resistance of the lowest resistance of the parallel circuits if that makes sense. I would fuse all three circuits separately.
 
Thanks both. My head has been spinning with this stuff and so after doing the book route I feel it is time to learn via the forum for a while.............I'm hoping I'll understand it quicker (wading through Calder yesterday I could have only done on a proper "rainy day" = snow day......)
 
FairweatherDave said:
VHF is an XM DSC 25/1 watt radio. The manual spec does not give me any other obvious figure so at 12v that makes 2 amps?
The Garmin manual for the GPS says use a 2 amp fuse with a 12volt power source. So 2 amps. (probably OTT?)
Click to expand...

The VHF radio will take about 0.5A on standby, but will take 5A to 6A when transmitting at 25W, so you need to fuse the supply cable appropriately.

A typcal Garmin GPS will use about 0.5A max.
 
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So no one has shot me down (so far) for not going back to the bus bar. Good.
No one has said put the smaller units on one circuit in series.
The answer seems to be rate the VHF supply to 6+0.5+0.125 = 6.625 amps
And then fuse each of the smaller circuits appropriately
 
KenMcCulloch said:
Hi Dave, I’m sure someone very knowledgeable will be along shortly but I can tell you that parallel circuits are a bit more complicated. The total reisistance of the combined circuits is less than the resistance of the lowest resistance of the parallel circuits if that makes sense. I would fuse all three circuits separately.
Click to expand...

I offer the works of Gustav Kirchhoff as worthy of a little study.:)
 
My vhf is fused 7.5 amp as per manual,fuse rating needs to be above current draw of instrument otherwise fuses will forever be blowing say 50 percent more.
 
I've done something similar with my instruments. VHF & Instruments are on separate switches as that's the way the boat came, but there's no real logic for that especially as the VHF is integrated via NMEA0183, so they're generally turned on together.

I put the instrument wiring to an 8-way blade fuse holder and then fused each instrument or group of instruments separately, mainly with 2A fuses.
 
Even if you wanted separate circuits you would not go back to the busbar. The busbar should supply a fuse holder, which would then supply switches for items that don't have their own switch or direct to items which do have their own switches.

There is nothing wrong with grouping equipment. Ideally you want to group stuff that you would always have on together, with nothing included that you would never have on, on its own. If some of those items do not have their own switches, the supply cable should go through a switch. Your VHF and GPS have their own switches, but i don't know if the other two items do, if either of them does not, you need a switch.

So, from busbar to a fuse/fuse holder. from there to a switch, from the switch to a four way blade fuse holder, accessible but close to the equipment, from the fuse holder to each item. Bin any inline fuses as they end up all over the boat, in hard to reach places and will mostly be different sizes, so you need to carry a million spare fuses.

Cable sizing from the switch to the 4 way fuse holder would be ok at 1.5mm if it's a short run, fused with a 15a fuse, over rated, but you're protecting the cable, not the equipment. The fuses in the 4 way fuse holder protect the equipment/equipment cables and should be rated accordingly.

I would question the wisdom of putting the VHF in with the other three items, you may want to turn everything off except the VHF :confused:
 
Having the VHF on its own circuit and dedicated fuse is also a safety consideration. The VHF should be available in the worst of circumstances, when interest in the GPS and AIS may become of secondary importance, or indeed may not be available for other reasons. I would suggest also at least 7 amps on the VHF supply.
 
Thanks for all the further posts. Really glad I posted.

superheat6k said:
Having the VHF on its own circuit and dedicated fuse is also a safety consideration. The VHF should be available in the worst of circumstances, when interest in the GPS and AIS may become of secondary importance, or indeed may not be available for other reasons. I would suggest also at least 7 amps on the VHF supply.[/QUOTE

I understand the importance of VHF. But I am curious how if I am tee_ing off that circuit it compromises it? Is that about if your battery is getting low? Genuine question.
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FairweatherDave said:
Thanks for all the further posts. Really glad I posted.

superheat6k said:
Having the VHF on its own circuit and dedicated fuse is also a safety consideration. The VHF should be available in the worst of circumstances, when interest in the GPS and AIS may become of secondary importance, or indeed may not be available for other reasons. I would suggest also at least 7 amps on the VHF supply.[/QUOTE

I understand the importance of VHF. But I am curious how if I am tee_ing off that circuit it compromises it? Is that about if your battery is getting low? Genuine question.
Click to expand...

No it is not about battery getting low. Fusing is all about protecting wiring (and perhaps to a lesser degree the actual components in the box) which can cause a fire if it gets a short circuit and gets very hot. So a fuse rating relates to the wire size (fuses at less current than wire can safely carry) but of course must be larger than total current. Wire size is dictated by current but in a boat IMHO should be robust enough to survive years of corrosion and possibly physical damage. The wire plus the connection device of course. So I would use fairly robust wire even for low current drawing devices. Which means you can lump whatever you like together on one fuse 7.5 or 10 amp should do it. The only reason to have separate supplies and fuses is in the remote chance you have a short and take out a fuse (or more likely the fuse fails from vibration or corrosion) you only lose that service on that fuse. On my little boat I have one fuse for VHF cabin light and nav lights all lumped together. No problems. Just no impressive switch box.
olewill
Click to expand...
 
William_H said:
No it is not about battery getting low. Fusing is all about protecting wiring (and perhaps to a lesser degree the actual components in the box) which can cause a fire if it gets a short circuit and gets very hot. So a fuse rating relates to the wire size (fuses at less current than wire can safely carry) but of course must be larger than total current. Wire size is dictated by current but in a boat IMHO should be robust enough to survive years of corrosion and possibly physical damage. The wire plus the connection device of course. So I would use fairly robust wire even for low current drawing devices. Which means you can lump whatever you like together on one fuse 7.5 or 10 amp should do it.
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Not for the sort of instrument electronics the OP is discussing. For all sorts of practical reasons the wiring generally used can take far more current than the device should ever draw in normal operation. It just doesn't make sense to fuse for the wiring rather than the device.
 
lpdsn said:
Not for the sort of instrument electronics the OP is discussing. For all sorts of practical reasons the wiring generally used can take far more current than the device should ever draw in normal operation. It just doesn't make sense to fuse for the wiring rather than the device.
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Certainly does not make sense to fit a 10a fuse to protect the main wiring, then step down the wiring gauge to the sort of rating for a Glomex.

I'm absolutely with you on the additional fuse holder method.
 
I may have used the wrong terminology Paul (maybe not!)
I am attaching a sketch of what I think I am doing based on this thread........
when I can resize it :o
 
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DSC_0071%20again.jpg
 
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