310.15(B)(3)(c) & Copper.Org

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iwire said:
It might be interesting to compare NEC ampacities with other standards. I have no idea how they would compare. Is the NEC more conservative or less than other major wiring standards. :?
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The NEC is certainly much more conservative than UK standards.
Here we use metric cable sizes. 2.5mm is similar to #14 but is rated at 27 amps in most circumstances, and is regularly used on 20 amp circuits, and fairly regularly misused on 30/32 amp circuits.
4.0mm is similar to #12, but is rated at well over 30 amps and normally used on 32 amp circuits, and often misused on 40 amp circuits.

I have seen electric cookers (ranges in USA terminology) that have been incorrectly wired in 2.5mm "twin and earth" cable which is similar to romex, on a 40 amp fuse.
After decades of use the cable appeared as good as new.

Australian standards are somwhat similar to UK ones, but I believe that in Australia one can purchase 27 amp circuit breakers in order to fully utilise the capacity of 2.5mm cable.
 
I am glad I started this, everyone's input has helped me. I am still pondering the issue, haven't quite formulated my next response yet. A commute or two and I'll bring a new turn to the conversation. :)
 
This is not exactly on point with George's original post, but I have question along the same lines. If you have a circuit that needs #10 due to roof top heat, but a #12 would otherwise be required, can you run the non roof portion in #12 and run the roof portion with #10 and be compliant?
 
Certainly. In fact, 310.15(A)(2) exception may permit the use of 12 all the way - which is part of the turn I intend to take this. Not completely thought out yet though.
 
George Stolz said:
Certainly. In fact, 310.15(A)(2) exception may permit the use of 12 all the way - which is part of the turn I intend to take this. Not completely thought out yet though.
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I'm familiar with 310.15(A)(2) Exception. Assume 50' non roof and 100' roof. I say #12 non roof to J box then #10 on roof.
 
texie said:
This is not exactly on point with George's original post, but I have question along the same lines. If you have a circuit that needs #10 due to roof top heat, but a #12 would otherwise be required, can you run the non roof portion in #12 and run the roof portion with #10 and be compliant?
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Yes you can.
Although permitted by code it is IMHO poor practice and should only be considered if there is a substantial saving.
I feel that there is a risk that in the future that somone will observe the #10 and upsize the breaker to suit #10, not realising that part of the circuit is in #12.
Should be OK if the load center end is #12, but a bit dubious if the load center end is #10 and smaller wire is used out of sight of someone working on it in future.
 
broadgage said:
Yes you can.
Although permitted by code it is IMHO poor practice and should only be considered if there is a substantial saving.
I feel that there is a risk that in the future that somone will observe the #10 and upsize the breaker to suit #10, not realising that part of the circuit is in #12.
Should be OK if the load center end is #12, but a bit dubious if the load center end is #10 and smaller wire is used out of sight of someone working on it in future.
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I'm confused - you seem to say it's poor practice and then recommend it in the same breath. :?

Anyone who would slap in a 30 without checking out the circuit had no business in the panel in the first place, IMO. Quite common for voltage drop.
 
broadgage said:
Yes you can.
Although permitted by code it is IMHO poor practice and should only be considered if there is a substantial saving.
I feel that there is a risk that in the future that somone will observe the #10 and upsize the breaker to suit #10, not realising that part of the circuit is in #12.
Should be OK if the load center end is #12, but a bit dubious if the load center end is #10 and smaller wire is used out of sight of someone working on it in future.
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If someone increases that breaker size they did wrong even if it was #10 all the way. What they failed to do is calculate the derated ampacity necessary for that circuit.

If it is supplying a rooftop HVAC unit it very well may require a 30 amp breaker and a 12 AWG conductor and be compliant anyway - again an evaluation of the installation is needed before condemning the fact we have conductors and overcurrent devices that apparently do not match each other.
 
I have often wondered if similar adjustments need to be made to any conductor exposed to direct sunlight and not just roof tops. I understand roof tops may be a little different in nature, but some places still get pretty hot that are not on roof tops, but only for a few hours at a time during summer months.
 
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