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

Status
Not open for further replies.

broadgage

Senior Member
Location
London, England
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. :?

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.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
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. :)
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
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?
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
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.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
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.

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.
 

broadgage

Senior Member
Location
London, England
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?

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.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
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.

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.
 

kwired

Electron manager
Location
NE Nebraska
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.

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.
 

kwired

Electron manager
Location
NE Nebraska
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.
 
Status
Not open for further replies.
Top