Proper fusing of various charging sources

[R_C]

I'm improving my charging sources on my trawler and I'm confused on the proper fusing required. I have a 450Ah house bank with the positive cable connected to a DC power bus bar through a 100A fuse on the battery. I have an Iota 90A charger and backup Xantrex 40A charger connected to the bus bar. I will be eliminating a battery isolator and connecting the main engine's 70A alternator output to the bus bar and adding a new Blue Sky solar charge controller. I'll use a Xantrex echo-charger to charge the start battery. Should each charging source have its own appropriately sized fuse between it and the bus bar?

[djmarchand]

RC:

Generally speaking, fuses and circuit breakers are sized to protect the wire that they are hooked to. The battery is a BIG source of amperage and any wire connected to the buss bar that can't take 100 amps (the size of the main fuse) needs a separate fuse on its wire or alternately upgrade its wire size.

#6 is the smallest wire size that can carry 100 amps, so any circuit with a wire size less than that needs a separate fuse. That fuse should be placed near the buss bar so it can blow and protect all of the wire downstream of it.

But the charging source needs to be considered as well. If your 90 amp charger is wired to the buss bar with #6 or greater then there is no need for a fuse at the charger (or any fuse for that matter).

Your spare 40 amp charger can be wired with #12 wire which will safely carry the 40 amps of current but it would need to be protected with a 40 amp fuse near the buss bar (or preferrably #10 wire with a 50 amp fuse).

The general rule of thumb is to size the wire to keep the voltage drop reasonable (3% or so) and then protect the wire from any large current sources (like the battery) with a fuse.

David

[R_C]

David,

Thanks for the detailed explanation. I am using 6 AWG wire from the alternator and both 90A and 40A chargers to the bus bar so they are all protected by the 100A fuse at the battery. I do have a 50A fuse on the wire from the 40A charger to the bus bar, which was in place when it was the only charge source. Wouldn't that fuse also protect the charger and not just the wire? I'll add another 50A fuse on the 10AWG wire to the solar charge controller.

[FlyingCloud1937]

Originally Posted by R_C

David,

Thanks for the detailed explanation. I am using 6 AWG wire from the alternator and both 90A and 40A chargers to the bus bar so they are all protected by the 100A fuse at the battery. I do have a 50A fuse on the wire from the 40A charger to the bus bar, which was in place when it was the only charge source. Wouldn't that fuse also protect the charger and not just the wire? I'll add another 50A fuse on the 10AWG wire to the solar charge controller.

Consider the Pos Bus the battery, as it's just an extension of the pos. post.

Now each wire attached to the bus should be sized to the load it supports, both in amperage and in voltage drop. Then each wire should be fused to the over-current protection of the wire in case of ground fault.

Allowing the 100 amp main fuse to accommodate the other wires, eve though they could carry the short term fault, would be a huge mistake.

One fault can take out the whole system instead of just that circuit.

Now just imagine that the alternator is connected to the bus and the engine is running...then a fault develops on one of the other wires and blows the 100 amp main fuse, that event has just caused a loss of connection from the alternator to the battery, and that will then blow all the diodes in the alternator.

So now after you find and cure the fault, replace the main fuse, you will also have to replace the alternator.

So saving the costs of the proper wiring and fusing is a bad decision.

djmarchand, if you're going to give advise, you better be willing to stand behind the warranty.

Lloyd

[djmarchand]

RC:

Fuses generally protect wire, not the source. The 40 amp charger can only put out 40 amps and the #6 wire can easily carry that much current.

Flying Cloud does have a good point. It is better to protect branch circuits with individual fuses sized for the expected current flow, rather than simply installing big wire, for the reason he notes.

David

[twistedtree]

I too would protect each circuit for maintenance and redundancy, if not just safety. As stated, you don't want a bad charger to take out everything which is what will happen if you blow the 100 battery fuse.

You might also want to consider increasing the main battery fuse from 100A to 200A or maybe even more. Your collective charge sources look to me like they can exceed 100A (Iota + Xantrex = 130A) which obviously can blow that 100A fuse. If you go to 200A, just be sure the cabling from battery to fuse to bus bar is sized for 200A.

And last, I agree that the purpose of fusing is to protect the wiring, but the attached device needs to be taken into consideration too. Each charger should say what fusing requirements it has, and these should be followed. In the same way a short can cause a wire to catch fire if not properly fused, the wiring inside the device (charger for example) is similarly susceptible to fire if the short is internal and current is not limited by a fuse or breaker.

This fall a pleasure trawler on the ICW developed some sort of short and burned to the water line is frighteningly short order. Everyone got off, but it all happened VERY fast from the accounts I read.

Fuses are your friend, fire is not.

[chala]

Originally Posted by FlyingCloud1937

Now just imagine that the alternator is connected to the bus and the engine is running...then a fault develops on one of the other wires and blows the 100 amp main fuse, that event has just caused a loss of connection from the alternator to the battery, and that will then blow all the diodes in the alternator.
Lloyd

How that?

[twistedtree]

Originally Posted by chala

How that?

Without a battery to sink the current from the alternator, they risk going over-voltage and blowing the diodes. This is why you frequently see warnings about not running an engine with the battery disconnected, or with the alternator disconnected.

[R_C]

Thanks everyone. Protecting each branch circuit makes sense. Lloyd, thanks for highlighting the alternator scenario. It's not an issue right now since the alternator output is connected to both an isolator to charge the house bank and directly to the start battery. But once I eliminate the isolator, the alternator's output will only be connected to the busbar and, as you noted, if I lose the path to the battery it's goodbye alternator. Thanks.

[David M]

You gauge the wire to the load. You size the fuse to the wire. The larger loads need their own circuits and fuses. Loads that are critical should not be combined in the same circuit with the same fuse with other loads that are also critical.

[CarlF]

The right solution to the alternator issue is run the alternator cable directly to the battery with it fused as close to the alternator as practical. Better a blown alternator diode than a fire.

I also take a look every year for signs of chafe at any red cable that gets anywhere close to engine and put a piece of split hose as extra chafe guard at worrisome points. Engine vibrations can make short work of insulation.

And if the alternator cable is quite long there's an argument that you should put a fuse at both ends of the cable in case the cable chafes. That's probably a bit over the top but electrical fires are really scary.


Carl

[R_C]

I really appreciate this discussion. The attached crude sketch shows what I am now planning. Anyone see any problems with this configuration? The wire to the DC breaker panel is probably 1/0 AWG because of its length and the 100A fuse at the battery has been in place for some time. I'll add the additional 100A fuse at the busbar. Note that the Iota and Xantrex chargers are never run at the same time so the 100A fuse at the battery is sufficient. Others may disagree, but I've been using the rule that FLA batteries should be charged at no more than 25% of their Ah capacity.

[Maine Sail]

Originally Posted by CarlF

The right solution to the alternator issue is run the alternator cable directly to the battery with it fused as close to the alternator as practical. Better a blown alternator diode than a fire.



Carl

Carl,

If only installing one fuse in that feed you want the fuse as close to the battery bank as possible. ABYC want's to see it within 7" but that is often unrealistic unless using MRBF fuses. So 9" -12" is often where Class T or ANL fuses are placed.

Alternators are considered "self limiting" thus you want to protect the wire at the "source" end, which in the case of an alternator, is still the battery bank. If your alternator can melt the wire connected to it then it has been sized it incorrectly.

If there were a short to ground between the battery and the alternator fuse there would be a big problem. You can also fuse the alt if you feel it necessary, but should not ignore the battery end of the alt feed wire, if you're fusing for safety..

[CarlF]

MainSail. Hmm. Learn something new everyday. Now I know why there are two fuses in the alternator lead. Always thought the one at the alternator end was the more important but your explanation makes perfect sense.

Carl

[twistedtree]

Originally Posted by David M

You gauge the wire to the load. You size the fuse to the wire. The larger loads need their own circuits and fuses. Loads that are critical should not be combined in the same circuit with the same fuse with other loads that are also critical.

True, especially when building a circuit from scratch.

The only thing I'd caution is that often times a small load is connected to a big wire which is protected by a big fuse/breaker. In this case, the small load is the weak link. I think this is quite common with an "electronics" breaker that feeds a bunch of small devices.

In this case, each of the small loads needs to be fused according to it's capacity/specs.

The situation posed in this thread is essentially the same where there are many charge sources converging on a single battery bus bar.