# Thread: Source circuit fuse sizing issue

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## Source circuit fuse sizing issue

Under 2011 code I was taught to use fuse sizes (source circuits/strings) in 1 amp increments up to 10A, then 5 A increments from 15A up to 50A and so on.
Under 2014 we were told only use standard values only, and now I am taking some training on 2017 where they are stating standard values only as well.

My issue –

Isc of module = 9.68A x 1.25 x 1.25 = 15.125A fuse

Next standard size is 20A but module says max fuse size is 16A, so using a 20A would break listing of product and violate code.

The way I read the code (last line of 240.6), non-standard values shall be allowed. Am I wrong in thinking I can use a 16A DC/PV rated and marked fuse in this situation?

Any clarification appreciated.

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Originally Posted by jasonbone
Under 2011 code I was taught to use fuse sizes (source circuits/strings) in 1 amp increments up to 10A, then 5 A increments from 15A up to 50A and so on.
Under 2014 we were told only use standard values only, and now I am taking some training on 2017 where they are stating standard values only as well.

My issue –

Isc of module = 9.68A x 1.25 x 1.25 = 15.125A fuse

Next standard size is 20A but module says max fuse size is 16A, so using a 20A would break listing of product and violate code.

The way I read the code (last line of 240.6), non-standard values shall be allowed. Am I wrong in thinking I can use a 16A DC/PV rated and marked fuse in this situation?

Any clarification appreciated.
220.5(B) allows you to round OCPD calculations to the nearest ampere, if after you apply your safety factors, your number ends up being no greater than 0.5A from the OCPD size you use. That would allow you to use standard 15A fuses in this application. This would be my recommendation in the interest of product availability.

If you can find it, and if it is a product your AHJ will accept, you are allowed to use non-standard OCPD ratings, but any articles pertaining to standard OCPD ratings would not apply. As an example, 240.4(B), the "next size up rule". One example I've had, was with inverters that had exotic 330A fuse ratings in the DC recombiner. Use a 350A fuse, and wire only needs a 301A rating to comply with 240.4(B). Use a 330A fuse, and you need the full 330A worth of wire.

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Originally Posted by Carultch
220.5(B) allows you to round OCPD calculations to the nearest ampere, if after you apply your safety factors, your number ends up being no greater than 0.5A from the OCPD size you use. That would allow you to use standard 15A fuses in this application. This would be my recommendation in the interest of product availability.

If you can find it, and if it is a product your AHJ will accept, you are allowed to use non-standard OCPD ratings, but any articles pertaining to standard OCPD ratings would not apply. As an example, 240.4(B), the "next size up rule". One example I've had, was with inverters that had exotic 330A fuse ratings in the DC recombiner. Use a 350A fuse, and wire only needs a 301A rating to comply with 240.4(B). Use a 330A fuse, and you need the full 330A worth of wire.
Outstanding info - thank you sir!

4. Zee
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Shame 20A fuse not allowed. Even the appropriate fuse may burn out if CB exposed to full sun on roof.

In other words, definitely do not use a 20A fuse...but rather ensure fuse location (combiner box) is well shaded. There are some serious heat factors at play on roofs.

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jasonbone: NEC 240.6(A) permits the use of fuses and CBs that have non-standard ampere ratings. So if you can find a 16A fuse feel free to use it but rounding down 15.125 to 15A is allowed by the NEC. The standard rating list just provides a reference and a way to bound values.

Carultch: I don't see where using a non-standard OCPD rating prevents the application of 240.4(B) for using the next size higher standard rating. If you need a 301A OCPD and the next size standard rating is 350A but you use a 330A OCPD then you can still size the conductors for 301A. The 330A OCPD will provide better protection than the 350A and I don't see anything in 240 that says the conductor needs to be sized for 330A in that case. In 240.4(B) it's providing an upper limit for the next size up, not restricting it to only using the next size up. Enlighten me please if I am missing something.

Why are 600A and 601A standard OCPD values?

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