Battery Combiner and Circuit Protection


Hi Jeff, I have been watching your seminars for a while now and can't thank you enough for the excellent content you put out for all of us. Thank you so much! You're probably the most knowledgeable I've come across on the internet. I'm hoping you could help me determine what size circuit breakers to use on the ACR install I'm doing. My previous install was a 65A "add a battery" from Blue Sea, it provided a table with this information.  Now I am installing two of their much larger ACR's on a larger system. I have one house battery, and two 105A alternators. The instructions for these ACR's does not include a table specifying what size fuses/breakers to use. It recommends to use their "circuit wizard" to determine size, but I'm unsure if I should be inputting the alternator rating as the load.

I used the "circuit wizard" on their website. I input 105A as the load (alternators) and it recommends 130A breakers/fuses. What's throwing me off is that when I did the add a battery on a different boat the fuse rating recommendations seemed to be about 150% that of the alternator output. Please help me. I'm a bit lost, and would really appreciate some guidance. -Kevin


Hi Kevin, when protecting a wire and an appliance with a circuit breaker, the multiplier to avoid nuisance tripping is 125% to 150%. The larger the multiplier the less likely of nuisance tripping. Also, if your battery combiner is rated at a maximum of 120 amps, your circuit breaker should not be larger then 120 amps. Your wire connected to that battery combiner should be able to handle 120 amps continuous without any trouble.  

When fusing an alternator, we commonly fuse the alternator circuit connection at 125% of rated max capacity. We also make sure the alternator positive and negative wire are sized to handle 125% of max alternator capacity. -Jeff