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Circuit Breaker Question

B

Bill Phillips

Well Known Member
Dear Fellow Builders, I have a 60 amp alternator on my 0-360. I'm trying to figure out how big the CB should be. The battery is 680 amp Odyssey. What size CB should I have? I have looked at other planes and 30 amp CB is the largest I have seen. What am I supposed to look at to get the correct size? Best regards and thanks, Bill RV-8a Wiring

P.S. I know my true RV builder Buds will not so tactfully say in response
"Bill, go look first for some guy to wire it for youl" Ha! That's you Tony
 
For a 60a alternator,

Typical would be a 60a breaker on the alternator output, and a separate breaker for the field circuit-----mine is 5a, but yours may be different, check with the manufacture.
 
CB Question

I assume that you are talking about the alternator output. A 60 amp alternator is quite a honker for an RV-6/7. What is your load analysis showing for max demand?

You have a couple choices. The first thing is to size it. Remembering that CBs and fuses protect wires, not devices, you should first determine that the wires can carry the load that the CB allows. You could put in a 60 amp CB for example as long as your wires can carry it. Assuming no wire restrictions the output CB is typically about 10% or so higher than the max load expected to avoid spurious tripping.

You can also use a fuse instead of a CB. They are reliable and a lot cheaper. Mount them right at the alternator output.

The easy answer is to put a 60 amp fuse on the output. In the unlikely event that it blows you have the battery to get you back home.
 
Hey Bill,

Get away from that wheelbarrow! You don't know nuttin 'bout machinery! :) (quote from a former boss of mine)

I used an ANL-60 from here. You need a base and the ANL-60. I put it in line between the alternator and the battery. I also used one of these coming off the battery on the main line feeding into the cabin area. I just didn't like the idea of having that large cable running into the cabin area with no protection. You may have noticed them mounted with Adel clamps to the engine mount near the battery.

I thought about going the circuit breaker route. But, again, I didn't like the idea of running a large unprotected cable all the way to the panel. Others apparently don't mind doing this, though. So, it is up to you.

I hope you are doing well. I have been working too much lately to get anything done. If this keeps up I'll be able to do my transition training in your airplane! :)
 
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I think I'd phrase that a little differently

w1curtis said:
You want a circuit breaker sized to the maximum output of the alternator with the appropriate AWG sized wire to it.
Click to expand...

I think I'd phrase that a little differently.

You want the appropriate AWG sized wire to match the MAXIUMUM CALCULATED LOAD in your aircraft, along with a breaker sized to match it.

It's the load that determines how much power is drawn from the alternator, not the other way around.
 
jonbakerok said:
I think I'd phrase that a little differently.

You want the appropriate AWG sized wire to match the MAXIUMUM CALCULATED LOAD...
Click to expand...
The alternator breaker/fuse is a bit of a special case. The maximum load will be the load of all the electronic gadgets PLUS the maximum charging load. I don't know how exactly to calculate the charging load, but experience suggests that if the battery is flat that that load will be approximately the maximum output of the alternator.

I would suggest that the appropriate size for a 60 amp alternator would be about 70 amps. Under some conditions an alternator can put out more than its rating. The wire should be sized to handle this load. Yes this is the reverse of the normal advice to size a breaker to protect the wire.
 
I'd also suggest sizing the wire for the max output of the alternator and using an appropriate CB. This is so that later on you can add gadgets to your electrical buss without re-wiring.
 
I think not

jonbakerok said:
I think I'd phrase that a little differently.

You want the appropriate AWG sized wire to match the MAXIUMUM CALCULATED LOAD in your aircraft, along with a breaker sized to match it.

It's the load that determines how much power is drawn from the alternator, not the other way around.
Click to expand...

And how do you determine the "MAXIUMUM CALCULATED LOAD" exactly? You forget about the big variable of the battery condition. You want your alternator to be able to put out its maximum rating. You are just asking for trouble if you sized your alternator on some "CALCULATED LOAD" value. See also Larry's response above.

Whenever I come to a decision point, I first see how it is done in the certified aircraft world. If I don't see any reason to change it, I follow that approach. A circuit breaker sized to the maximum output of the alternator is the way it is done on most certified piston singles. I also looked into an ANL fuse. The fuse and base is over $30, the alternator CB from Spruce is $15--costs less, follows conventional wisdom--easy decision.
 
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