Error in 2005 NEC Section 690?

[trosepe]

A local solar contractor (really nice guy)has installed a 15KW PV system on the property of a multi-millon dollar home. There has been some type of dispute and the owner is requesting an engineer to inspect the system for compliance to 2005 NEC.

The inverter output circuit connects to an 80 amp, 2 pole circuit breaker in the 400 amp service with 400 amp main.

Section 690.64 (B)(2) states that the sum of the overcurrent devices supplying power to a bus or cable shall not exceed the rating of the bus or cable.

The exception for dwellings states that the sum of the overcurrent devices shall not exceed the 120% percent the rating of the bus or cable.

Note the absence of the word 'supplying power' in the exception. If you go by the words in the exception - this guy is in trouble.

It seem obvious to me that the authors of this artical intended the exception to allow 120 percent of the overcurrent devices 'supplying power' but that is not what it says.
He can't derate the system and use a smaller circuit breaker as his contract sets an output level that requires the 80 amp circuit breaker.

Since this will be likely litigated, and we all know how lawyers are about what the words say and not what they really intended; what do you all think? Did the the code authors leave out the 'supplying power'in the exception as a mistake or intentionally?

 

[alehman]

That would have to be the case or you couldn't have any branch breakers in a 400A panel with a 400A main breaker.

Have you looked in the 2005 NEC handbook? I don't have a copy at home, but sometimes it can provide clarification as to intent. You're probably aware that the 2008 code was revised to use the 120% supplying power rule for all applications, not just residential. I suppose that might help your argument.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[trosepe]

alehman,
I did check the 2005 hnadbook and the 2002 NEC; same thing. You would have think the issue would have come up in the 2002 to 2005 cycle and they would have corrected it in the 2005. As you said they did correct it in the 2008 but there are so many other problems with the 2008 that I don't know when if ever CA will adopt it.

 

[rbulsara]

I don't understand the issue. If the overcurrent devices are not "supplying" the bus/cable then they are not protecting the bus/cable anyway, why should it be clarified? Branch breakers are never in the protection scheme at all.

Plus 120% of 400A is 480A. Sum of OCPDs is also 400+80=480A.

 

[jfpe]

There must be some existing bad feelings for this to lead to a law suit.

The exception needs to be taken in the context of the rule it is modifying. The words "supplying power to" are implied in the exception to 690.64(B)(2).

One document that could be useful is "Photovoltaic Power Systems and the 2005 National Electric Code: Suggested Practices". This document was written by the same author who drafted the text for article 690 in the 2005 and 2008 NEC Handbooks. It is availble free on the internet; just google it.

 

[alehman]

I agree. This seems like a non-issue. Whoever is raising it is clueless.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[resqcapt19]

While the wording of the exception is poor, I don't think that there is any question of the intent of the exception. The NEC Style Manual specifies that the exception applies to the rule immediately before it. The rule immediately before only applys to the OCPDs that supply the busbar.

NEC Style Manual 2.6 Exceptions.
2.6.1 Placement and Order. Exceptions shall immediately follow the main rule to which they apply. Where exceptions are made to items within a numbered list, the exception shall clearly indicate the items within the list to which it applies. Exceptions containing the mandatory terms shall or shall not are to be listed first in the sequence. Permissive exceptions containing shall be permitted are to follow any mandatory exceptions and be listed in their order of importance as determined by the Code-Making Panel.

 

[trosepe]

Even the base rule has issues.

Hypothetical example: I have a 120/208V, 4,000 amp, 100% rated draw-out, air frame circuit breaker supplying a 4,000 amp switchboard with no feeder circuit breakers installed; no load at all. I want to connect a 15 amp circuit breaker in the switchboard for connection of a 250 watt inverter. Section 690.64(B)(2)will not allow that connection.

How does that make sense?

 

[rbulsara]

I normally do not open code books to answer here, but this is bugging me do I did.

2008 (not 2005) NEC 690.64(B)(2) sets the limit to 120% and there is no exception! I guess that takes care of the issue. Even if not adopted by state laws, almost all code officials will accept compliance with a code of later edition.

Also in your hypothetical 4000A case in your latest post, you can install a 400A subpanel with a 400A main and then connect the 250W inverter to that. Code language may not be very straight forward, but they are never impractical.

trospe: What is your connection/role with the project you referred to in your original post? Just curious.

 

[trosepe]

rbulsara,
In California you can only go by the California Electrical Code which is the 2005 NEC. If you design a project using the 2008 NEC and there is a conflict with the 2005; no plan checker, city, county or state electrical inspector would buy your 'later code' argument. You would have to pay for the correction.

As for connecting a 400 amp sub panel and then connecting the PV circuit breaker to that; you still have the same problem. The 400 amp and the 15 amp circuit breaker are the 'supply' breakers and the sum is grater than 400 amps (the rating of the sub panel). You would have to install a 400 amp breaker in the switchboard, connect it to a 600 amp panel and then install the 15 amp breaker in the 600 amp subpanel. Then 400 + 15 is less the 600. My whole issue here is that there are no provisions to apply good engineering practices to the connection of the PV system. It seem the code authors tried to come up with a simple rule.

My connection to the project is to help the local solar contractor present a case that his installation is compliant with the CEC. It is obvious what the intent of the exception (for dwellings) is but the language is not accurate. Someone has told the owner that this contractor did not do the project per the CEC and he has over $50K in the project.

 

[rbulsara]

Yes you are right, that is what I meant (came here to clarify this morning) to say a sub panel with a main of rating less than its bus.

I still do not see a problem.

 

[trosepe]

rbulsara,

The problem is, as with my hypothetical, there is no provisions for engineering judgment. The 15 amp PV circuit in the 4,000 amp switchboard with no load; it is obvious there would be no problem with that installation but the code section will not allow it. You would have to do something like you suggested with the sub-panel. The rule just seem too restrictive to me.

 

[rbulsara]

What you and I suggest is exactly what is spelled out in the last sentence of the paragraph in question in 2008 version of the code.

 

[trosepe]

I hope CA adopts the 2008 this Jan. Thanks for your responses.