Originally Posted by RichH
Bill (or others) - could you elaborate a bit more on the need for AIC rating, with respect to battery banks. What Im really asking is - then how EXACTLY should such a battery fuse be sized/selected with respect to AIS ratings, etc. etc. etc. Thanx
Sure. The needed AIC is a function of the size of the battery bank, specifically, it's total cold cranking amps (CCA) rating. For banks larger than 1100CCA -- which means most house battery banks on cruising boats -- a circuit protection device (CPD) with an AIC rating of at least 5,000 amps is required. The purpose of the high rating is to avoid disaster in the event of a catastrophic short-circuit. The potential amperage flow is a function of CCA. CPDs with less that the indicated rating could themselves arc over, short out, and fail to interrupt the circuit, leading to overheating and fires.
Blue Sea Systems has a nice little primer on DC circuit protection here:
DC Circuit Protection - Resources - Blue Sea Systems
Normally, CPDs are sized to protect the wire, not the connected devices. Branch circuit breakers and fuses are designed to protect the (smaller wires) and, often, the attached devices.
I'm a conservative kinda guy with regard to CPDs. If I KNOW that there will never be a demand greater than, e.g., 80 amps on a wire connected to the battery -- even a very large wire capable of carrying much more than that load -- then I'll use a CPD of, say, 100 amps on that wire, providing that it has an AIC of at least 5,000 amps.
Example: I use AWG6 or AWG4 battery cable direct to the house batteries to provide power to HF/SSB radios. I fuse them near the house batteries on both positive and negative cables with 30-40 amp fuses (normally, terminal-type or ANL fuses), since I know the load will never exceed that amount, even though the AWG6 or AWG4 cables are capable of carrying a lot more amperage than that.