ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Any women electricians here?
by gfretwell - 01/17/22 01:44 PM
12 Lead Motor Connection Weird
by Elektrik - 01/12/22 11:09 PM
Well There's your problem.
by gfretwell - 01/08/22 12:17 AM
Happy New Year!!
by HotLine1 - 01/04/22 09:50 AM
Sangamo s309.2
by Robski - 12/31/21 04:54 PM
New in the Gallery:
240/208 to a house
240/208 to a house
by wa2ise, October 9
Now you know.
Now you know.
by Tom_Horne, September 7
Who's Online Now
0 members (), 51 guests, and 13 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Joined: Sep 2005
Posts: 99
T
Tripp Offline OP
Member
I'm trying to figure out whether it's okay to go from a #14 wire NM to #12 NM when extending an existing circuit. (OCPD is 15a) I've tried voltage drop; I've tried Ohms Law; but neither theory is helping. I can't find anything against it, however. And as far as I can tell, as long as the circuit remains protected at 15a everything should be fine.

Counterpoint anyone?

Stay up to Code with the Latest NEC:


2020 NEC & Related Reference & Study Guides
2020 NEC & Related Reference & Study Guides
Pass Your Exam the FIRST TIME with these Exam Prep Combos:
 

>> Master Electrician Exam Prep     >> JourneyMan Electrician Exam Prep
 

Joined: May 2005
Posts: 984
Likes: 1
G
Member
There's nothing that prohibits using a wire of greater ampacity that the OCP rating.
It's quite commonly done to combat voltage drop.


Ghost307
Joined: Jul 2004
Posts: 9,677
Likes: 9
G
Member
The only caveat is that you also need to upsize the EGC in proportion to the circuit conductors. Other than that there is no issue.


Greg Fretwell
Joined: Jan 2005
Posts: 354
P
pdh Offline
Member
I take it you are referring to 250.122(B) for the EGC upsizing. I'm trying to understand the technical reasons for this.

First, 250.122(B) excludes ampacity adjustment factors. But it seems to me that when it is necessary to the EGC be sized for no less than the OCPD rating, a condition that would make that OCPD related sizing crucial (e.g. some fault condition is pulling full load current through the EGC), the thermal aspects in the raceway would still be effectively the same. I don't understand why this exception exists.

However, in the case of a long circuit being upsized only for a voltage drop condition, I don't see an issue like that. But maybe it could be the case that a fault to EGC could fail to open the OCPD where the circuit length has to much resistance?

Of course there is the concern that if upsized conductors are present at the circuit origination, some handyman might think he can upsize the OCPD. That risk is also present if the branch circuit changes to a smaller size at a later point.

What about cases where the neutral is oversized for reasons of harmonic mitigation? Does the EGC need to match the neutral in that case?

Joined: Mar 2005
Posts: 1,213
S
Member
I see no issues with extending a 15A circuit with #12. As noted, it's common for dealing with voltage drop.
Originally Posted by pdh
What about cases where the neutral is oversized for reasons of harmonic mitigation? Does the EGC need to match the neutral in that case?
No, because the neutral is oversized in those cases for overload. The ground is more for fault clearing/shock protection, and is never going to see the extended overload like the neutral serving switched power supplies could.

Joined: Jan 2005
Posts: 354
P
pdh Offline
Member
Originally Posted by SteveFehr
Originally Posted by pdh
What about cases where the neutral is oversized for reasons of harmonic mitigation? Does the EGC need to match the neutral in that case?
No, because the neutral is oversized in those cases for overload. The ground is more for fault clearing/shock protection, and is never going to see the extended overload like the neutral serving switched power supplies could.

250.122(B) doesn't seem to make that exception. Maybe it should? Or is there such an exception elsewhere?

Joined: Jul 2004
Posts: 9,677
Likes: 9
G
Member
The EGC is up-sized in proportion to the ungrounded conductors the way I read it.
I think the power factor of a ground fault is 1 and the harmonic content is zero but I will defer to the engineers.


Greg Fretwell
Joined: Mar 2005
Posts: 1,213
S
Member
Ground fault is typically resistive, but the inductive component comes into play, too. The NEC tables still apply- basically, you just calculate what current would cause 120V of voltage drop, and that's an upper limit for your fault current. The capacity of the source (transformer, generator, etc) comes into play, too, and there are some differential equations and all, but it's not hard to make conservative estimates. The important thing is that the ground wire be able to conduct current enough to trip the OCP before the wires explode, and that's what NEC is mandating.
Originally Posted by pdh
250.122(B) doesn't seem to make that exception. Maybe it should? Or is there such an exception elsewhere?
250.122(B) only refers to the ungrounded conductors; the neutral is implicitely excepted. None of the other items in 250.122 appear to apply to an oversized neutral, either.


Link Copied to Clipboard
Featured:

Tools for Electricians
Tools for Electricians
 

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
Theelectrikid
Theelectrikid
Levittown, PA
Posts: 811
Joined: April 2004
Top Posters(30 Days)
NORCAL 3
grich 1
Popular Topics(Views)
287,879 Are you busy
220,049 Re: Forum
206,042 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5