May 5, 2008 05:50 am
Re: Panel to converter in-line fuse size?
Buckle up!
Wire is sized according to the expected number of amps that will be running through them.
Manufactures of UL (Underwriters Laboratories) listed electricity consuming devices must post on that device the amount of electricity it consumes or in the case of PV modules what they produce.
Manufactures of UL listed wire do not have to post on the wire itself how many amps each wire can safely conduct but it does have to be posted somewhere. Typically we find these in the NEC (National Electrical Code) manual or handbook. There are so many types of wire and situations, such as ambient air temperature or conduit fill, to name a few, where the ampacity of a wire can change. All of this is in the NEC book.
That 156% that Mr. Cormican mentioned, is actually two different requirements. One is the addition of 125% to satisfy the NEC. The other is 125% to satisfy the UL.
125% x 125% = 156%
You see both the NEC and UL, individually believe that under certain circumstances a PV module can exceed its manufactures rated short circuit amperage output.
An example: We know from the NEC that #12 THHN copper wire is predominately good for 20 amps. Lets say, for example, that you have 5 - PV modules wired in parallel and their short circuit rating is 3 amps each for a total of 15 amps. #12 THHN can handle this, but we still have to add 156% for a total of 23.4 and so now we know #12 will not do. There are other factors to consider as well, but I will have to get back you on those.
