Fuses, Shunts, Circuit Breakers For Charging Circuit?

[demonboy]

Thanks to SJF I'm working on rewiring my charging circuit and in his instructions he mentions the use of fuses but doesn't show them for the sake of simplicity. Unfortunately I'm a bit of a simpleton myself and don't know what fuses I should be using where. Also I don't understand why circuit breakers aren't used.

I have the West Marine catalogue here and there are a whole host of fuses. Should I be looking at someting like the Blue Sea Systems Fuse blocks shown here?

For my charging circuits (from the Sterling to the battery, for example) what rating should I be using if the Sterling is rated at 80amp?
What about fuses from the alternator?
And fuses from the battery to the distribution panel?
Finally, what is this? (Excuse poor photo - camera shoved in a tight spot)

Is it a shunt rated at a certain ampage? It used to connect the battery charger to the isolator. Should I be using more of these?

 

[whipper_snapper]

I use circuit breakers for most purposes, but big fuses to protect big feeds like the main battery feed. In theory, a breaker has a MAX interuptable current unlike a fuse. In theory a big enough current can melt it and prevent it tripping, hence really big cables are usually protected by big fuses, almost like a fusible link in a car.

The value of the breaker or fuse is determined by the current carrying capacity of the wire, which in turn is matched to the current it is expected to supply.


That is a shunt which is intended to allow current measurement. The blue and brown wires measure the voltage dropped across it as a current flows, this voltage is proportional to the current. The shunt needs to be matched to the ammeter used, there are several different likely values of voltage per Amp.

 

[mitiempo]

demonboy

The ANL fuses you linked to are excellent for battery cable wiring, as well as windlasses or thrusters for that matter. I use ANL's for all battery circuits, house banks as well as starting batteries on any engine under about 60 hp or so, as is common on a sailboat. The fuses should be rated between the expected load but not more than the wire's ampacity

Agree the pic is of a shunt to measure amperage.

 

[demonboy]

Thank you, both.

Determining the ampage capacity of my wire is difficult as I bought them from a car shop in India. There is no marking on the cable but they are what I would call 'battery' cables - would that make them 25mm2? If so, what would be the guestimated rating?

Additionally I'm looking for some advice on what size fuses to fit and where. Using SJF's diagram, where would you place the fuses?

If my (new) alternator is 70amp, my engine 60hp, my Sterling 80amp max, my Mastervolt mains 45amp, my house batteries between 350-400aH (as yet undecided), what sizes are we talking about?

 

[vyv_cox]

Determining the ampage capacity of my wire is difficult as I bought them from a car shop in India. There is no marking on the cable but they are what I would call 'battery' cables - would that make them 25mm2? If so, what would be the guestimated rating[/img]

just use pi r2. 6 mm diameter wire is 25 mm2, 7 mm is 35 mm2 and 8 mm is 48 (50) mm 2.

 

[demonboy]

Thanks, Vyv. That makes them 35mm2.

 

[pvb]

Fuses protect wire from the danger of fire in the case of short circuits. You should have a fuse as close as possible to the positive terminals of your start battery and domestic bank. These can be ANL type, or I prefer MegaFuses. As you have an emergency parallel switch, both these fuses (and all the associated wiring) need to be able to handle starting current.

Then, if you're connected a smaller cable to feed a lower current circuit, that cable ought to be protected by a suitable fuse. So, for example, the feed to the "Always On" should be fused appropriately.

 

[demonboy]

I'm surprised Oyster never installed fuses in the first place, unless a previous owner had removed them when rewiring. Either way, there are no fuses between battery and switches, which is why this is a new subject to me.

PVB - what is a typical starter current?

 

[BabaYaga]

Fuses protect wire from the danger of fire in the case of short circuits. You should have a fuse as close as possible to the positive terminals of your start battery and domestic bank. These can be ANL type, or I prefer MegaFuses. As you have an emergency parallel switch, both these fuses (and all the associated wiring) need to be able to handle starting current.

Then, if you're connected a smaller cable to feed a lower current circuit, that cable ought to be protected by a suitable fuse. So, for example, the feed to the "Always On" should be fused appropriately.

To size both the main fuses to handle the starting current could result in a rating that is unnecessarily high on the domestic side (depending of course on what domestic loads you have).
To avoid this you could place the emergency paralleling switch on the battery side of the two main fuses. At least that is my thinking and what I have done in my own boat.

However, the down side of a paralleling switch like this is that when it is closed there could be a rush of current into the then flat starter battery. So perhaps it is better to wire the emergency switch between the domestic battery (or battery side of fuse) and the starter side of the main switch in the starter circuit – bypassing any fuse on that rare occasion?

FWIW, my starting circuit is 50mm2 for a 20 hp engine and the fuse is 250 A. Domestic feed to busbar and distribution panel is 25 (or is it 35?), fused by 80 A.

I agree you should have a separate fuse block for the always on leads, this could also be used for incoming leads from mains charger, solar etc

 

[pvb]

To size both the main fuses to handle the starting current could result in a rating that is unnecessarily high on the domestic side (depending of course on what domestic loads you have).
To avoid this you could place the emergency paralleling switch on the battery side of the two main fuses. At least that is my thinking and what I have done in my own boat.

I certainly wouldn't recommend wiring anything to the battery side of a main fuse - it rather defeats the purpose of the fuse!

However, the down side of a paralleling switch like this is that when it is closed there could be a rush of current into the then flat starter battery. So perhaps it is better to wire the emergency switch between the domestic battery (or battery side of fuse) and the starter side of the main switch in the starter circuit – bypassing any fuse on that rare occasion?

I've already suggested to demonboy in another thread that the parallel should go to the starter side of the engine isolator. Again though, I certainly wouldn't recommend wiring anything to the battery side of a main fuse.

 

[pvb]

PVB - what is a typical starter current?

It obviously depends on the engine. My 43hp Volvo has a 250A MegaFuse on the starter circuit. MegaFuses have a "slow blow" characteristic and will tolerate overcurrent for short periods.

 

[BabaYaga]

I certainly wouldn't recommend wiring anything to the battery side of a main fuse - it rather defeats the purpose of the fuse!




I've already suggested to demonboy in another thread that the parallel should go to the starter side of the engine isolator. Again though, I certainly wouldn't recommend wiring anything to the battery side of a main fuse.

Yes I agree the cable from the domestic battery to the emergency switch will be unprotected (as are the short cables to the fuses). That is an increase of risk of course.
But perhaps an oversized domestic main fuse is also?

 

[pvb]

But perhaps an oversized domestic main fuse is also?

As I've said before, fuses protect the wiring. There's no reason why the wiring to the domestic distribution panel couldn't be suitably sized.

 

[BabaYaga]

As I've said before, fuses protect the wiring. There's no reason why the wiring to the domestic distribution panel couldn't be suitably sized.

Do you advice then that the cable to the distribution panel should be the same size as the one in the starter circuit, e g 50mm2?
That is, matching the cable to the fuse rather than to the expected load?

 

[pvb]

Do you advice then that the cable to the distribution panel should be the same size as the one in the starter circuit, e g 50mm2?
That is, matching the cable to the fuse rather than to the expected load?

If an emergency parallel switch is required, the domestic battery bank fuse will need to cope with starter loads. In that case, there's a choice - install another lower-rated fuse to protect a smaller cable to the domestic distribution board, or run a starter-size cable to the domestic distribution board. In most boats, the cost would be about the same. Not many people will use 50mm2 cable for the starter circuit; most will probably have 25mm2.

 

[BabaYaga]

If an emergency parallel switch is required, the domestic battery bank fuse will need to cope with starter loads. In that case, there's a choice - install another lower-rated fuse to protect a smaller cable to the domestic distribution board, or run a starter-size cable to the domestic distribution board. In most boats, the cost would be about the same. Not many people will use 50mm2 cable for the starter circuit; most will probably have 25mm2.

Thanks for clarifying.

A separate, lower-rated fuse for the cable to the distribution panel is probably the correct approach.
For my own part though, I think I would be prepared to take the of risk of leaving the emergency circuit unfused, considering how rarely it (hopefully) will be used. After all, many boats, like the OP's, have no main fuses at all.

 

[mitiempo]

You fuse near the batteries for the wire size. ABYC recommends within 7" but that isn't always practical so if within 12" should be the goal. 35mm2 wire is quite small for a starting circuit on a 60 hp diesel. I usually use 1/0 (53mm2) or 2/0 (67mm2) and the result is better starting. That is for any diesel, even my 1 cylinder Yanmar. After rewiring the start times were reduced to a few seconds. That is not a wire you want to get warm. If the engine starts immediately with smaller wire there is not a problem but the time you have to crank a bit to start a smaller wire can get warm or hot. I would recommend upsizing the wire to at least 50mm2.

I prefer ANL fuses which are unique in how they blow. 35mm2 wire (close to 2 awg) will handle 178 amps in an engine compartment and an ANL fuse rated at 175 amps will handle 290 amps for a period of 500 seconds and a lot more amperage for shorter periods. Using these fuses for starting circuits I never get nuisance blows.

 

[pvb]

You fuse near the batteries for the wire size. ABYC recommends within 7" but that isn't always practical so if within 12" should be the goal. 35mm2 wire is quite small for a starting circuit on a 60 hp diesel. I usually use 1/0 (53mm2) or 2/0 (67mm2) and the result is better starting. That is for any diesel, even my 1 cylinder Yanmar. After rewiring the start times were reduced to a few seconds. That is not a wire you want to get warm. If the engine starts immediately with smaller wire there is not a problem but the time you have to crank a bit to start a smaller wire can get warm or hot. I would recommend upsizing the wire to at least 50mm2.

For most small boat installations, upgrading a 35 sq mm cable to a 50 sq mm cable will make a negligible difference to starting efficiency. With, say, 200A cranking current and 4 metres of cable, the bigger cable will only give a 0.15v increase in voltage at the starter motor, not enough to make a significant difference.

I prefer ANL fuses which are unique in how they blow. 35mm2 wire (close to 2 awg) will handle 178 amps in an engine compartment and an ANL fuse rated at 175 amps will handle 290 amps for a period of 500 seconds and a lot more amperage for shorter periods. Using these fuses for starting circuits I never get nuisance blows.

I'm not sure where you got those figures from. 290A on a 175A fuse is 165% of rating. Most ANL fuse specifications say that anything much above 140% of rating will blow within 10 seconds or less. Here's the BlueSea curve...

 

[mitiempo]

Directly from Blue Seas - see below. More detail than the curve shows.

 

[pvb]

Directly from Blue Seas - see below. More detail than the curve shows.

Strangely, that doesn't seem to agree with the curve they publish.