I have an old family farm house with a 125 amp Square D breaker box. The service disconnect is a 100 amp Square D cartridge fuse. I have never had the main blow, but I decided that I better have a spare pair of fuses on hand just in case, as in this rural area it would be difficult to locate one in a hurry. I drove to the nearest city and located fuses at Lowes, but I am not sure which one of the two types they had should be used for my application. The heavy-duty time-delay one said it is "for motor and main panel protection. (FRN-R)" It indicated that "for lighting or electric heating circuits use general purpose fuses.(NON)" I use window AC units for cooling, and 1500 watt portable heaters for infrequent use in winter heating. Water heaters are also electric. (This is generally just a week-end house.) Which type of fuse would be the most appropriate for my use?
I would go with the FRN. That will tolerate a short time spike in current. The biggest opportunity for a spike in current is when you have a power glitch and the service would see locked rotor from every motor that was running at that time.
Yeah, I agree with Greg. You really need something with a bit of time delay on it for a main fuse. After all, the fuse/CB that is electrically closest to the fault (ie, a sub-circuit fuse/CB) should operate first, in any electrical system. Anything less is poor design.