House & Starter Battery Fuse Sizes

[BenCT]

Ok, I'm pretty much done with the BoatHowTo Electric 101 class. Which is great. I recommend it for any novices. And I've read so many different people talking in general about fuses on house batteries and starter batteries. Almost all for other motors. I understand that you fuse for the wire. There is a back and forth on whether to fuse the starter battery...all that good stuff.

Here with the Atomic 4, everything I read according to Moyer and other sources we use a 4 gauge wire for our batteries. Which I have. Many people say their starting cranking amps is around 150 maybe up to 250.

Can people chime in on what you are doing with fusing for your house and cranking batteries? Specifically what amp fuse are you using, if any, and what gauge cable are you using?

I currently have two 27 group West Marine (800 MCA) house batteries and one 27 group "starter battery" (1050MCA). #4 Gauge wires. Nothing is fused and I'm looking at adding MRBF Terminal Fuses to all once I figure out what size fuse.

[hanleyclifford]

150amps. Breaker is better than fuse.

[hanleyclifford]

This is what I use. https://www.bluesea.com/products/718...ace_Mount_150A The biggest problem is ganging up all the (-) cables on the one terminal. You need an extra post and some creative wiring.

[joe_db]

Ok, I'm pretty much done with the BoatHowTo Electric 101 class. Which is great. I recommend it for any novices. And I've read so many different people talking in general about fuses on house batteries and starter batteries. Almost all for other motors. I understand that you fuse for the wire. There is a back and forth on whether to fuse the starter battery...all that good stuff.

Here with the Atomic 4, everything I read according to Moyer and other sources we use a 4 gauge wire for our batteries. Which I have. Many people say their starting cranking amps is around 150 maybe up to 250.

Can people chime in on what you are doing with fusing for your house and cranking batteries? Specifically what amp fuse are you using, if any, and what gauge cable are you using?

I currently have two 27 group West Marine (800 MCA) house batteries and one 27 group "starter battery" (1050MCA). #4 Gauge wires. Nothing is fused and I'm looking at adding MRBF Terminal Fuses to all once I figure out what size fuse.

I have measured the cranking amps of my engine at about 130. I do not currently have the starter fused, but if I did I would probably use a 200 amp fuse.
4 gauge IMHO is light for the starter wire, I am using 2 gauge.

[ndutton]

4 gauge cable should be considered a minimum and length is a huge factor that may dictate a larger wire.

[joe_db]

I ran the wire calc for 6 feet from battery to engine at 150 amps and 3% voltage drop and got 1 gauge wire.
I use 4 gauge for a lot, but the starter on my boat is on 2 gauge cable.

[BenCT]

That’s what I was getting at. If you run the ABYC amperage calcs the gauge doesn’t jive with the diagrams that I’ve been referring to for the starter battery that seem to be used without an issue by Atomic 4 owners. Having said that, a lot of the documentation I’m reading also says that the cranking load is so quick it’s not the same as a sustained load. The conduits don’t have time to heat up.

[joe_db]

It isn't so much heating the wire up as making the engine hard to start.

[hanleyclifford]

It isn't so much heating the wire up as making the engine hard to start.

Agreed.The starting function is an electrically explosive event. The starter should be fed as much amperage and voltage as it wants so I fully concur with the #2 cable. Just make sure that the path back to the batteries is equal or better.

[Dave Neptune]

As Hanley points out more often than not problems on marine DC systems are often found on the negative side as it "completes" the path. The negative side also is often overlooked by many backyard mechanics and PO's.

Dave Neptune

[hanleyclifford]

That’s what I was getting at. If you run the ABYC amperage calcs the gauge doesn’t jive with the diagrams that I’ve been referring to for the starter battery that seem to be used without an issue by Atomic 4 owners. Having said that, a lot of the documentation I’m reading also says that the cranking load is so quick it’s not the same as a sustained load. The conduits don’t have time to heat up.

ABYC standards are all well and good for planning purposes and nice new installations, but in the salt water environment formerly sound calculations go south real fast. Battery lugs corrode, crimped cable loses strands at the terminals, corrosion creeps up into the cables themselves, and resistance begins to develop. When calculating your negative run to the batteries remember the engine itself is part of the circuit. Does it have resistance? If a cable in the starting circuit heats up you already have a problem. If you have a shunt in the negative side and a breaker (I do) you have created a potential bottleneck. I use 2/0 from the post to the breaker, and from the breaker to the shunt. Every installation is different; the point is, do not create resistance bottlenecks that could ever impede your biggest electrical load - the starter.

[joe_db]

https://www.omnicalculator.com/physics/wire-size

For fun I ran it for 2 feet from engine to battery and still got 1 gauge. At 10 feet from engine to battery it goes to 1/0 gauge.

[ndutton]

You make my point Joe, length is an important factor and I'll repeat, whatever the calculator returns for a wire gauge should be considered a minimum.