705.14(c) and redundant breakers

[310 BLAZE IT]

We have an abandoned PoCo 5000A switchgear with its full capacity available and are interconnecting both a 2000A building maximum connected load and a 1600A solar array @480v

The problem is that the 2000A connected load is fed through (2)2000A breakers with a kirk key interlock, putting us above 5000A with the sum of the OCPDs.

Can anyone explain why the code does not have any exception for this? My thought is that if there are redundant breakers they should not be counted in this equation because the load is remaining the same. We're having issues with a supply side tap because there's no room between the breaker and transformer to easily make a connection.

I know they changed this section significantly in 2014, but I think it deserves some more changes... thoughts?

 

[Carultch]

We have an abandoned PoCo 5000A switchgear with its full capacity available and are interconnecting both a 2000A building maximum connected load and a 1600A solar array @480v

The problem is that the 2000A connected load is fed through (2)2000A breakers with a kirk key interlock, putting us above 5000A with the sum of the OCPDs.

Can anyone explain why the code does not have any exception for this? My thought is that if there are redundant breakers they should not be counted in this equation because the load is remaining the same. We're having issues with a supply side tap because there's no room between the breaker and transformer to easily make a connection.

I know they changed this section significantly in 2014, but I think it deserves some more changes... thoughts?

There is no "exception" written for redundant breakers, or unconnected spare breakers, or to breakers connected to insignificantly low loads, in the rule pertaining to the "sum of the breakers excluding the main supply". That rule pretends that all connected circuits are somehow operating either as all sources or all loads, at full utilization of the trip setting. It is a conservative rule, which may not represent reality.

It sounds like if you have control over the main breaker, you can configure this so that the typical "120% rule" can work, and then you don't need to think about the sum of the breakers.

 

[310 BLAZE IT]

In 705.14(a) does 'rating' imply 'rating and setting'? I would think a similar labeling like in the other two section would need to be applied so that it is not adjusted. If the breaker is under the PoCo's jurisdiction that may change the story here as well.

 

[pv_n00b]

What version of the NEC are you looking at? Mine does not have a 705.14(c) and 705.14 in general has nothing to do with interconnection.

Typically the term OCPD "rating" in the NEC applied to CBs refers to either the non-adjustable instantaneous rating or the plug rating if it has a plug. The adjustments you make with a tweaker to set the overload and ground fault rating do not count as the "rating".

705.12 does not take diversity into account when evaluating the bus ratings for a load side interconnection. Who knows why but it's probably because CMP 4 was looking for a simple fail safe rule to be applied to a panel or SWBD, and counting the CB ratings was pretty simple. It took many code cycles to get them to accept using the inverter current and not the inverter OCPD rating when evaluating the bus, and that was a obvious change.

Since you have a lockout system the AHJ should be amenable only counting only one of the CBs being on at a time when sizing the SWBD even if the code does not say anything about it.

 

[310 BLAZE IT]

What version of the NEC are you looking at?

wow sorry i meant 705.12(2)(3)(c) I totally messed up my code references here

 

[jaggedben]

Is there a main breaker in the switchgear that you neglected to mention? If not then it's a supply side connection and I don't see the problem , although make sure your AHJ agrees.

If there's a 5000A main breaker then, as Carultch suggested, look into replacing it with something between 4000A and 2000A and use the 120% rule.

 

[310 BLAZE IT]

Is there a main breaker in the switchgear that you neglected to mention? If not then it's a supply side connection and I don't see the problem , although make sure your AHJ agrees.

If there's a 5000A main breaker then, as Carultch suggested, look into replacing it with something between 4000A and 2000A and use the 120% rule.

Looks like we'll have to do that, although it will likely be replacing the entire switchgear as it's old and the main tie main is probably 70% of the switchgear cost since there's only 5 breakers in the distribution section that we'll need. or just making a line side connection somehow...

 

[pv_n00b]

Looks like we'll have to do that, although it will likely be replacing the entire switchgear as it's old and the main tie main is probably 70% of the switchgear cost since there's only 5 breakers in the distribution section that we'll need. or just making a line side connection somehow...

I think it's still worth talking to the AHJ about allowing for diversity in this case. AHJs can allow an alternate means that still complies with the spirit of the NEC (NEC 90.4) and I think this applies.

 

[Smart $]

wow sorry i meant 705.12(2)(3)(c) I totally messed up my code references here

Still messed up.

It's 705.12(D)(2)(3)(c). With that said, note this subsection is with respect to panelboards. Switchgear is different equipment. See Article 408. In short, 705.12(D)(2)(3)(c) does not, cannot apply.

For your case, refer to 705.12(C) and see if that can apply.

 

[310 BLAZE IT]

Still messed up.

It's 705.12(D)(2)(3)(c). With that said, note this subsection is with respect to panelboards. Switchgear is different equipment. See Article 408. In short, 705.12(D)(2)(3)(c) does not, cannot apply.

For your case, refer to 705.12(C) and see if that can apply.

Thanks, this helped a whole lot. I was way over thinking this, 408 doesn't really help much as far as the load but im very happy that I have more flexibility.