DC Combiner Box Ampacity size?

[ElectricPersonality]

If you get a calculated wire size of 413 amps from your Combiner box output circuit (panel short circuit current x 156% x the number of strings connected to combiner) to the inverter, would it stand to reason that the combiner box bus size would need to be greater than or equal to 404 amps? Is there any exception in the code that would allow you to use a 400 amp combiner box? Article 690 doesn't specifically talk about combiner boxes as it relates to ampacity size.

 

[ggunn]

If you get a calculated wire size of 413 amps from your Combiner box output circuit (panel short circuit current x 156% x the number of strings connected to combiner) to the inverter, would it stand to reason that the combiner box bus size would need to be greater than or equal to 404 amps? Is there any exception in the code that would allow you to use a 400 amp combiner box? Article 690 doesn't specifically talk about combiner boxes as it relates to ampacity size.

It depends. Some pieces of gear are rated for 100% use, others are not. Example: Square D discos without fuses are, those with fuses are not. For an unfused disco the current calculation is (Isc)(# of strings)(1.25), but for a fused disco it's (Isc)(# of strings)(1.25)(1.25).

 

[NorthwestPV]

It depends. Some pieces of gear are rated for 100% use, others are not. Example: Square D discos without fuses are, those with fuses are not. For an unfused disco the current calculation is (Isc)(# of strings)(1.25), but for a fused disco it's (Isc)(# of strings)(1.25)(1.25).

Is there a code reference to support this comment. Not having to add the addition 25% could save us huge deniro.

 

[ggunn]

Is there a code reference to support this comment. Not having to add the addition 25% could save us huge deniro.

Search for "continuous use".

 

[jaggedben]

Is there a code reference to support this comment. Not having to add the addition 25% could save us huge deniro.

See 690.9(B)(1)Exception. (Also 705.60(B)Exception, for the AC side.)

To my reading the language in both would apply only to assemblies with overcurrent devices, not unfused discos or unfused combiners (which is seemingly opposite of what ggunn said?) Most combiners are fused so I'd guess it applies.

I've never debated it with an AHJ or heard any war stories about someone doing so.

 

[ggunn]

See 690.9(B)(1)Exception. (Also 705.60(B)Exception, for the AC side.)

To my reading the language in both would apply only to assemblies with overcurrent devices, not unfused discos or unfused combiners (which is seemingly opposite of what ggunn said?) Most combiners are fused so I'd guess it applies.

I've never debated it with an AHJ or heard any war stories about someone doing so.

In the 2008 and 2011 NEC, 690.9(B) is a section on power transformers and there is no 690.9(B)(1). 690.8(B)(1) appears to be relevant but says what I have been saying in the exception, i.e., that "an assembly together with its overcurrent devices, that is listed for continuous use at 100% of its rating shall be permitted to be utilized at 100% of its rating." Square D discos without fuses are so listed, but those with fuses are not because the fuse holders are not listed at 100% continuous use. I found that information on the Schneider website and I have talked to their tech support to confirm it. My interpretation is that on the DC side, that means that you must use 1.56 X Isc to determine max current for fused Square D discos but only 1.25 X Isc for unfused discos.

 

[ggunn]

Clarification example: Say I am designing an array that contains combiner boxes which have a combined Isc of 67.4A.

(67.4)(1.25) = 84.2A
(67.4)(1.25)(1.25) = (67.4)(1.56) = 105.3A

If I were to use an unfused Square D DC disco on the output circuit of a combiner, I could use one rated at 100A, but if it were a fused disco, I'd have to go to the next larger size. Unfused Square D discos are listed for 100% continuous use but fused ones are not.

 

[Garrison]

All string fuses/fuse holders and wire need to be rated for String Isc x 1.25 x 1.25 (or 1.56), but you do not need to apply the additional 125% for your output circuit. See 690.8(2). The fuses in the combiner are at the string level, there is no fuse on the combiner output circuit in the box, so 690.8(B)(1) or (B)(2) should not apply to this calculation. Check with your combiner manufacturer to confirm their box is rated for continuous use at 100% of its rating. Since a disconnect is required at/near the combiner (690.16) you also need to make sure this is appropriately rated, and of course you need to add in the additional 125% for wire sizing.

 

[jaggedben]

690.8(B)(1) appears to be relevant

Yes, that's the section I meant. Oops.

but says what I have been saying in the exception, i.e., that "an assembly together with its overcurrent devices, that is listed for continuous use at 100% of its rating shall be permitted to be utilized at 100% of its rating." ... My interpretation is that on the DC side, that means that you must use 1.56 X Isc to determine max current for fused Square D discos but only 1.25 X Isc for unfused discos.

I guess what confuses me is the wording 'together with its overcurrent devices'. To me this implies either that the overcurrent device must be part of the assembly, or at least that the overcurrent device must also be rated for continuous use at 100% of its rating. That's just my take on the wording, I have never had this discussion with anyone before, AHJ or otherwise.

 

[ggunn]

Yes, that's the section I meant. Oops.



I guess what confuses me is the wording 'together with its overcurrent devices'. To me this implies either that the overcurrent device must be part of the assembly, or at least that the overcurrent device must also be rated for continuous use at 100% of its rating. That's just my take on the wording, I have never had this discussion with anyone before, AHJ or otherwise.

It says to me that if the assembly has overcurrent devices, they must also be listed for continuous use. If it doesn't have them, then it's a non-issue. Of course, there I go applying common sense to the NEC...

 

[ggunn]

All string fuses/fuse holders and wire need to be rated for String Isc x 1.25 x 1.25 (or 1.56), but you do not need to apply the additional 125% for your output circuit. See 690.8(2). The fuses in the combiner are at the string level, there is no fuse on the combiner output circuit in the box, so 690.8(B)(1) or (B)(2) should not apply to this calculation. Check with your combiner manufacturer to confirm their box is rated for continuous use at 100% of its rating. Since a disconnect is required at/near the combiner (690.16) you also need to make sure this is appropriately rated, and of course you need to add in the additional 125% for wire sizing.

I believe that is consistent with what I have been saying; everything in the DC path before and after the combiner is subject to the 1.25 multiplier due to excess insolation. The other 1.25 multiplier (it would be less confusing to some if it were a different number ) is for continuous use. As to the wiring size, you should use the excess insolation multiplier, and then either the continuous use multiplier or the conditions of use derating(s), whichever results in the larger wire size (per John Wiles).

 

[jaggedben]

All string fuses/fuse holders and wire need to be rated for String Isc x 1.25 x 1.25 (or 1.56), but you do not need to apply the additional 125% for your output circuit. See 690.8(2). The fuses in the combiner are at the string level, there is no fuse on the combiner output circuit in the box, so 690.8(B)(1) or (B)(2) should not apply to this calculation.

You seem to be saying that 690.8(B) applies to source circuits only and not output circuits. I do not agree with this. The section refers to 'system currents' and 'circuit conductors' and does not make any finer distinction.

 

[ggunn]

You seem to be saying that 690.8(B) applies to source circuits only and not output circuits. I do not agree with this. The section refers to 'system currents' and 'circuit conductors' and does not make any finer distinction.

I thought the same thing when I read it. It seemed to me like he was saying that the 1.25X excess insolation factor need only be applied to source circuits, which is incorrect. It applies to all components on the DC side.