Wiring size calculator

[Tim22]

I downloaded both the ABYC and Blue Sea wire size calculator and noticed that there is a discrepancy in the suggested wire size. The Blue Sea asks if the wire is terminated at a fuse and recommends a larger wire size if it does. The ABYC calculator doesn‘t care about the fuse. For a 300A 6 foot run 105 degree single wire with 3% voltage drop ABYC recommends 2/0 while Blue Sea recommends 4/0 when terminated on a fuse. Is the ABYC standard acceptable or is it necessary to upsize. It seems to me that most circuits on a boat terminate at a fuse so shouldn’t ABYC take this into consideration?

Thanks
Tim

 

[dlochner]

I just tried the BlueSea calculator.

12 v, 300 amp, 6 feet, fixed load, fused, 105 temp, and 3% drop.

Left the duration blank. The answer was 2/0.

When I added the time of the load at 60 minutes and 120 minutes the recommended gauge was 3/0.

Did you list a time for the load? It would be unusual for a 300 amp load to run for a long time on a 31 ft boat. Is this for the primary battery wire?

 

[Tim22]

Hi Dave
This is for the primary battery wire which has a 300A terminal fuse. I used 60 minute duration, single wire, not in engine room. When terminated on a fuse I get 4/0. With no fuse I get 2/0. I agree that 300A for 60 minutes is unrealistic. if I drop it down to 15 minutes I still get 4/0, but at 10 minutes it drops to 2/0.
I was just surprised to find this discrepancy as I had always looked on ABYC as the safety standard but this got me to questioning it.

 

[dlochner]

Hi Dave
This is for the primary battery wire which has a 300A terminal fuse. I used 60 minute duration, single wire, not in engine room. When terminated on a fuse I get 4/0. With no fuse I get 2/0. I agree that 300A for 60 minutes is unrealistic. if I drop it down to 15 minutes I still get 4/0, but at 10 minutes it drops to 2/0.
I was just surprised to find this discrepancy as I had always looked on ABYC as the safety standard but this got me to questioning it.

Blue Sea is giving a more specific rating, while ABYC and others are more generic. By specific, I mean the formula is including a time element and assuming the current draw is for that time.

As current is drawn through a wire, it heats up. The more current, the more heat. As wire temperature increases, resistance increases which causes the wire to heat up faster. To get around this, the wire has to be fatter.

The BlueSea calculator is telling you that for loads of 300 amps or less for a duration of 10 minutes or less there will not be a significant increase in resistance and the 2/0 cable is fine. However, if the load is longer than 10 minutes, resistance will increase and a larger cable is needed if you are to maintain the 3% voltage drop.

Try your calculations again, only this time use a 10% drop. At 60 minutes duration you'll need 3/0 cable, but at 15 minutes a 2/0 cable will be fine.

When I'm wiring devices on my boat, I almost always use larger wires than ABYC or BlueSea call for. The voltage drop is simply wasted energy that I worked hard to get. The more voltage I get to the device the less is wasted on heating wire.