Proposed Changes to Art. 210.19

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jwilson

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A proposal ROP 2-166 was made by Mike Holt, that resulted in the CMP #2 adding a new exception to 210.19(A)(1) as follows:

" Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permtted to be sized at 100 percent of the continuous and noncontinuous load.".

According to my view, that means a circuit with a continuous load of 80amps must use a breaker rated at 100amp and the ungrounded conductor has to be #3 CU rated at 100amp (based on 75C wire and breaker terminal requirements of 75C), both meeting the 125% coninuous load requirement.

The grounded neutral leg, however, only has to be sized at 100%, or 80amp. So, I can use #4 CU at 85amp capacity. What happens if the load malfunctions and the current goes up to 95amps? That's within the continuous range of the breaker and the ungrounded conductor, but 12% over the grounded conductor rating. WHAT KIND OF PROTECTED CIRCUIT IS THAT?

Please help an Engineer with only 37 limited years of experience understand.
 
The 80% rule comes from the fact that circuit breakers aren't listed to carry 100% of their rating continously. If the nuetral isn't connected to a breaker, why should it have to be sized at 125% of the continous load?

If you have problems with this new exception, how do you feel about 240.4(B)? Or 230.90(A) Exception 3?
 
The 100% rated conductor is NOT adequately protected by the 125% rated breaker. Are you comfortable with 95amp continuously on 85amp rated wire?

As for 210.4(B) changes, I can see the logic to that more so that the 210.19 change.
 
Are you comfortable with 95amp continuously on 85amp rated wire?
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No, of course not, it violates 310.10. But we have rules for ungrounded conductors that allow the same thing.
As for 210.4(B) changes, I can see the logic to that more so that the 210.19 change.
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Not 210.4(B), 240.4(B). The rule that says we can protect a 380A condcutor with a 400A breaker. Or a 130A conductor with a 150A breaker. Or a 310A conductor with a 350A breaker. Do these rules bother you?
 
Quote 1: "If the neutral isn't connected to a breaker, why should it have to be sized at 125% of the continuous load?"

The neutral is part of the circuit and is insulated the same as the ungrounded conductors and is subject to the same requirements that it be rated in accorcance with the setting of the OC device -- 210.3 & 210.18(A)(1) for Branch Circuits and 215.2(A)(1) for Feeders

The ONLY reason, for requiring a 125% rated ungrounded conductor be used with a 125% rated OC device, is NOT to provide an adequate heat sink for the device as was implied in the substantiating comments for the subject code change and your quote above. If that was the only reason, then I would be able to use #3 TW wire (60Deg C @ 85ampacity) on a 100amp molded case breaker with 75 Deg C rated terminals. That would be in lieu of #3 THHW (75 Deg C @ 100 ampacity). I'd have a problem with that also.

The primary reason to require 125% rated conductors is for adequate protection by the 125% rated OC device. The same applies to the grounded conductor as it's part of the circuit.

The heat sink issue is covered in 110.14

Quote 2: "If you have a problem with this new exception, how do you feel about 240.4(B)? Or 230.90(A)."

I'm not particularly uncomfortable with either of these, even though they allow the use of an OC device rated higher than the circuit conductors; but, only under certain conditions. I DO feel that it's a compromise, but it's minimalized by the fact the 125% rated conductors have a 25% protection margin factored in.

I WOULD feel uncomfortable it the "next higher standard overcurrent device rating" was being used with a 100% rated grounded conductor.
 
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