Nec 705.12

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erickench

erickench

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NEC 705.12(D)(2) states the following:

The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

But the beginning phrase of NEC 705.12(D)(7) states:

Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent ratings supplying it,.....


I was under the impression that a panelboard and it's busbar would have the same rating. Am I wrong?
 
A panelboard that has busbars rated for 200A, could be protected by a 150A c/b, for example.

705.12(D)(2) would allow a utility-interactive inverter to be connected to this panel with a 90A c/b. 200*1.2=240 => 150+90 =240.

NEC 705.12(D)(7) would allow an inverter connected to this panel with a 45A c/b to be positioned at other than the opposite end from the main circuit breaker.
 
erickench said:
I'm sorry but I'm just not clear on NEC 705.12(D)(7). The language is too difficult to understand.
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I agree its poorly written, but its not not too hard to follow once you understand the intent. The intent is that if the OCPDs from the two interconnected power supplies exceed the rating of the panel bus, then the two OCPDs must be installed at opposite ends of the bus.

For example, if I had a 200A panel with a 200A MCB (mounted at the top of the panel) and I wanted to connect a PV array to the panel, I could add a c/b for the PV array that doesn't exceed 40A (per 705.12(D)(2)). The sum of the OCPDs for my to supplies to the panel (the 200A mcb and the 40A pv c/b) is 240Amps. Since this sum exceeds the rating of the panel bus, the pv c/b MUST be installed at the opposite end of the panel, ie, the bottom of the panel.

But if I had a 200A panel with a 150A MCB (at the top) and I wanted to add a 40A c/b for a PV array, the sum of the supply OCPDs would be 190A. Since the rating of the panel (200A) is not less than the sum of the OCPDs (190A), the pv c/b could be installed anywhere within the panel. It could be the first breaker location below the main for instance.
 
I understand the theory. It's just that when they use words like "unless" and "not" in the same sentence things become a little more difficult to understand even if you use a dictionary. I wish they would write this damn code in more straight forward language.
 
Okay, I would like to continue this thread with another question. NEC 705.12(B) and (C) permits outputs to be interconnected at a point or points elsewhere on the premises. These rules follow NEC 705.12(A) which permits outputs to be connected to the supply side of the service disconnect. Do (A) and (B) actually permit connection on the load side as well as the supply side?
 
The way I read it, 705.12(A) permits an Interconnected Source on the supply side of the service disconnecting means. (B) (C) & (D) permit connection to the load side of the service disconnecting means as long as you comply with the provisions in those sections.
 
Now regarding NEC 705.12(D) it states that utility-interactive inverters are permitted to be connected to the load side of the service disconnect. But NEC 705.12(A) permits the connection to the supply side and actually refers to NEC 230.82(6). NEC 230.82(6) specifies a rule for PV and fuel cell sources permitting connection to the supply side and these sources are DC supplies requiring inverters. Are utility-interactive inverters permitted on both supply and load side of service disconnect?
 
erickench said:
Are utility-interactive inverters permitted on both supply and load side of service disconnect?
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Yes, they could be connected to either side. On the supply side per 230.82(6) or on the load side complying with the restrictions in 705.12(D).

690.64 reiterates this.
 
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