310.15(B)(16) wire size requirement vs. breaker lug size....

[wwhitney]

No, it is not NEC compliant to supply a 48A continuous load with #6NM cable, per 210.19(A)(1).

OK, that section seems clear enough. This is a case of the NEC text not corresponding to my understanding of the principles involved, so I'm trying to figure out which one should be adjusted.

As I see it, the only reason that the notion of 125% of continuous loads occurs in the NEC is that standard circuit breakers are not 100% rated. That is, the circuit breaker is the weak link; everything else in the circuit is rated for continuous loads.

Therefore the checks as I would see it should be:

1) Circuit breaker rating >= 125% continuous plus noncontinuous.
2) Circuit breaker rating <= Ceiling(Wiring Ampacity) "next size up rule"
3) Wiring ampacity >= continuous plus noncontinuous.
4) Termination limitations.

The #6 NM, 48A continuous, 60A breaker example satisfies all of those. Is there a principle I'm missing, or does the language in the NEC miss this corner case?

Thanks, Wayne

 

[david luchini]

OK, that section seems clear enough. This is a case of the NEC text not corresponding to my understanding of the principles involved, so I'm trying to figure out which one should be adjusted.

As I see it, the only reason that the notion of 125% of continuous loads occurs in the NEC is that standard circuit breakers are not 100% rated. That is, the circuit breaker is the weak link; everything else in the circuit is rated for continuous loads.

Therefore the checks as I would see it should be:

1) Circuit breaker rating >= 125% continuous plus noncontinuous.
2) Circuit breaker rating <= Ceiling(Wiring Ampacity) "next size up rule"
3) Wiring ampacity >= continuous plus noncontinuous.
4) Termination limitations.

The #6 NM, 48A continuous, 60A breaker example satisfies all of those. Is there a principle I'm missing, or does the language in the NEC miss this corner case?

Thanks, Wayne

You are missing the principle listed in 210.19(A)(1). The minimum branch circuit conductor size, BEFORE the application of any adjustment or correction factors, shall have an allowable ampacity of not less than the noncontinuous load plus 125% of the continuous load.

48A * 125% = 60A. #6NM has an allowable ampacity of 55A, which is less than 60A.

 

[infinity]

The #6 NM, 48A continuous, 60A breaker example satisfies all of those. Is there a principle I'm missing, or does the language in the NEC miss this corner case?

Thanks, Wayne

You're using the next size up rule to put a 60 amp OCPD on a 55 amp conductor however you can only use that rule when the load (in this case 48 * 125% or 60 amps) is less than or equal to the conductor ampacity. If you want to use the next size up rule the load on a 55 amp conductor cannot exceed 55 amps.

 

[wwhitney]

You are missing the principle listed in 210.19(A)(1).

From my point of view, 210.19(A)(1) is not a principle. It's a rule written to implement an underlying principle. The question I have is whether that section is properly written to promulgate the purpose of the NEC, e.g. 90.1(A).

So is there an operational issue (e.g. the breaker will nuisance trip, the reason for the 125% continuous load rule) or a safety issue (e.g. under operation the conductor insulation or terminals may exceed their temperature ratings) that the #6 NM / 48A continuous load / 60A breaker installation presents? I.e. is that configuration prohibited on purpose, or just by accident as a side effect of how the principles have been turned into rules?

Thanks,
Wayne

 

[wwhitney]

In other words, say I propose rewriting 210.19(A)(1) along the following lines (using somewhat informal language, and adjusting 215.2(A)(1) accordingly):

"Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the OCPD protecting the branch circuit shall be not less than the noncontinuous load plus 125 percent of the continuous load. See also 240.4(B) and 240.4(C)."

Would this result in any safety or operational issue? I don't see any.

Thanks, Wayne

 

[ramsy]

You've lost me here...could you show your calculation?

Perhaps another thread. Lots of other confusion to settle here.

 

[wwhitney]

So is there an operational issue (e.g. the breaker will nuisance trip, the reason for the 125% continuous load rule) or a safety issue (e.g. under operation the conductor insulation or terminals may exceed their temperature ratings) that the #6 NM / 48A continuous load / 60A breaker installation presents? I.e. is that configuration prohibited on purpose, or just by accident as a side effect of how the principles have been turned into rules?

Should I conclude from the lack of any response that no one can think of an operational or safety issue with this configuration (compared to any NEC-compliant installation), and that the prohibition is an unintended consequence of writing 210.19(A)(1) in terms of conductor ampacity instead of OCPD rating?

Thanks, Wayne

 

[kwired]

Should I conclude from the lack of any response that no one can think of an operational or safety issue with this configuration (compared to any NEC-compliant installation), and that the prohibition is an unintended consequence of writing 210.19(A)(1) in terms of conductor ampacity instead of OCPD rating?

Thanks, Wayne

It is just as coincidental as 4/0 aluminum being allowed to be protected by next size up device of 200 amps. but the actual load on the conductor can't be more then 180 amps. If you have non continuous load of 180 amps then it can be on 200 amp breaker. If you have continuous load you must add 25% to it, if continuous x 1.25 plus non continuous results in say 185 amps then you need a larger conductor but still can have the 200 amp overcurrent protection.

With the 6 AWG what gets people in trouble is the fact NM must be selected from 60 C ampacity table, where most every other method they encounter it can come from 75C table.

 

[ramsy]

..As I see it, the only reason that the notion of 125% of continuous loads occurs in the NEC is that standard circuit breakers are not 100% rated. That is, the circuit breaker is the weak link; everything else in the circuit is rated for continuous loads..

Don't know if OCP rules in NEC 240.4(B) & (C), relate to circuit breaker trip curves, or if 100% rated have different trip curves?

I thought Article 100 defines "Continuous Load" @ 3-hours for full-heat build up of conductor temperature, since NEC Ampacity Tables exclude this condition and others by default, without special adjustments.

My effort to select Ampacity by relying on Temperature formulas from Note 2 in Table-8, does simplify some adjustments, but defaults to full ampacity, and full-heat build up, which excludes non-continuous conditions. Need to find more flexible NEC Temperature formula.

 

[david luchini]

Should I conclude from the lack of any response that no one can think of an operational or safety issue with this configuration (compared to any NEC-compliant installation), and that the prohibition is an unintended consequence of writing 210.19(A)(1) in terms of conductor ampacity instead of OCPD rating?

The intended consequence of writing 210.19(A)(1) in terms of conductor ampacity is to get the conductor to operate at a lower temperature.

 

[david luchini]

Don't know if OCP rules in NEC 240.4(B) & (C), relate to circuit breaker trip curves, or if 100% rated have different trip curves?

I don't believe the rules in 240.4(B) & (C) relate to trip curves.

I thought Article 100 defines "Continuous Load" @ 3-hours for full-heat build up of conductor temperature, since NEC Ampacity Tables exclude this condition and others by default, without special adjustments.

The NEC defines ampacity as the maximum current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

My effort to select Ampacity by relying on Temperature formulas from Note 2 in Table-8, does simplify some adjustments, but defaults to full ampacity, and full-heat build up, which excludes non-continuous conditions. Need to find more flexible NEC Temperature formula.

I'm still unclear on how you are using the formula in Chapter 9, Table 8, Note 2, which tells you how the resistance of a conductor will change with different temperatures, to select ampacity. Perhaps you can demonstrate.

 

[wwhitney]

The intended consequence of writing 210.19(A)(1) in terms of conductor ampacity is to get the conductor to operate at a lower temperature.

OK, in which case 210.19(A)(1) could be more accurately written in terms of the ampacity of the conductors at the temperature rating of the OCPD terminals, independent of the conductor's insulation temperature. The OCPD's performance doesn't depend on the conductor insulation.

This would only make a difference for 60C insulation conductors (or those limited to 60C, like NM) landed on 75C rated OCPD terminals. In the currently prohibited example I've been referencing (#6 Cu, 60C insulation, 48A continuous load, 60A OCPD now specified to have 75C rated terminals), I don't see how any of the equipment would be operating outside of its allowed temperature range.

Chers, Wayne

 

[david luchini]

OK, in which case 210.19(A)(1) could be more accurately written in terms of the ampacity of the conductors at the temperature rating of the OCPD terminals, independent of the conductor's insulation temperature. The OCPD's performance doesn't depend on the conductor insulation.

This would only make a difference for 60C insulation conductors (or those limited to 60C, like NM) landed on 75C rated OCPD terminals. In the currently prohibited example I've been referencing (#6 Cu, 60C insulation, 48A continuous load, 60A OCPD now specified to have 75C rated terminals), I don't see how any of the equipment would be operating outside of its allowed temperature range.

Chers, Wayne

Some people don't feel that you are allowed to connect a 60C insulation conductor to a 75C rated OCPD terminal. I'm not sure that I agree with them.

However, it seems simpler to me the way the Code is currently written than to have a bunch of IF you have this insulation with this terminal rating, THEN do this, and IF you have this insulation with this terminal rating, THEN do this...

 

[wwhitney]

Some people don't feel that you are allowed to connect a 60C insulation conductor to a 75C rated OCPD terminal. I'm not sure that I agree with them.

OK, I probably meant to say 60C/75C rated terminals, I'm not well conversant on the distinctions.

However, it seems simpler to me the way the Code is currently written than to have a bunch of IF you have this insulation with this terminal rating, THEN do this, and IF you have this insulation with this terminal rating, THEN do this...

Well, it wouldn't have to do that. It could add the phrase "at the the overcurrent device's termination temperature rating" to the second sentence of 210.19(A)(1).

Anyway, I'm mostly wondering if there is a principle I'm missing, or if the code language is just overly broad (for simplicity or otherwise). So far, it sounds like the latter.

Cheers, Wayne

 

[GoldDigger]

One thing you may be missing is that some heating or lighting devices *require* a particular minimum temperature rating for the wires in their built-in terminal enclosure.
This is totally separate from the maximum temperature rating of the terminals themselves (except of course that they must be rated for at least that temperature also.)

Sent from my XT1585 using Tapatalk

 

[wwhitney]

One thing you may be missing is that some heating or lighting devices *require* a particular minimum temperature rating for the wires in their built-in terminal enclosure.
This is totally separate from the maximum temperature rating of the terminals themselves (except of course that they must be rated for at least that temperature also.)

How does that relate to 210.19(A)(1)?

Cheers, Wayne