fuse vs circuit breaker

hello all,

first time post here so please pardon any ignorant questions. a friend of mine is installing a magnetek aurora and is running into a problem with the inspector. apparently the manual calls for a 20a, 600v fuse on the pv side though the NEC does not require it. for the sake of safety, he installed a like-spec'd circuit breaker and the inspector has told him to switch it out per the manual.

is there a difference? isn't the nec supposed to supercede all other requirements?

thanks for any input or experiences shared.


  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: fuse vs circuit breaker

    Most circuit breakers are AC rated, not DC. AC tends to extinguish the disconnect arc as the voltage swings thru 0V. DC does not, and may weld the breaker closed

    your mileage may vary
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    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: fuse vs circuit breaker
    apparently the manual calls for a 20a, 600v fuse on the pv side though the NEC does not require it.

    The NEC does require overcurrent protection and a DC disconnect between the PV array and the inverter (Para 690.9 and 690.14 respectively, I think). As Mike indicated, it's pretty tough, if not impossible, to find a circuit breaker rated for 600 VDC. A breaker's VAC rating doesn't count. I'd suspect that the fuse voltage would have to accommodate the PV array's nameplate Voc specs multiplied by your local temperature correction factor (NEC 690-7).

    Fused disconnects like those used for air conditioner compressors and hot tubs seem to be popular solutions.

    Jim / crewzer
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: fuse vs circuit breaker

    This might be an alternate solution to discuss with your friend's inspector:


    Jim / crewzer
  • RoderickRoderick Solar Expert Posts: 253 ✭✭
    Re: fuse vs circuit breaker

    The cheapest way to go might be to replace the DC disconnect switch with a fused type disconnect switch. Watch eBay for a while, and a bargain will probably show up.

    For example, if your friend has a Square D HU361 disconnect, change it for a H361. I'm assuming that the inverter has no protection of its own. In the inverter I have, there is already an internal fuse and GFI on the DC input, so no extra fuse is required outside.
  • FrankFrank Solar Expert Posts: 54 ✭✭✭
    Re: fuse vs circuit breaker

    Manufacturer's instructions have to be followed to satisfy Listing requirements. Put them both in!
  • DLwindsunDLwindsun Administrators Posts: 34 admin
    Re: fuse vs circuit breaker

    You need to use a combiner box and put this fuse holder inside, you can order the correct size fuse needed.

    Fuse holder - http://store.solar-electric.com/obfh.html
    Combiner box - http://store.solar-electric.com/pspv.html
  • System2System2 Posts: 6,290 admin
    Re: fuse vs circuit breaker

    he has the combiner and the fuses in them but the inspector was more concerned with an acutal disconnect on the pv side. we explained that that was the purpose of the fuses in the combiner but that didn't seem to matter...the whole thing was very bizarre....needless to say there's some serious circuit protection redundancy on this system.

    the inverter has its own protection as well...but again, that didn't matter to this inspector. so we folded to the demands of the inspector so we can get this customer's system up and running already. i suppose this was one of those "pick your battles" sorta deals.
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