10 AWG on 40 amp breaker

Originally Posted By: jonofrey
This post was automatically imported from our archived forum.



Some time ago, I called out a 40 amp breaker connected to 10 AWG copper. The electrician that installed the panel called me to say that depending on what the circuit is serving, he could bump up the breaker size.


That's news to me? When I check conductor size I've always referenced NEC 310-16.

What's up with that?

BTW - please look me in the eyes and quit staring at my breasts. ![icon_lol.gif](upload://zEgbBCXRskkCTwEux7Bi20ZySza.gif)


--
Inspection Nirvana!

We're NACHI. Get over it.

Originally Posted By: dhadler
This post was automatically imported from our archived forum.



John, I’m trying not to look but I just can’t help it icon_exclaim.gif


It seems to be OK on certain items such as Electric motors and the like. Such as AC condensers for example. They only have a significant load at start up and after that it drops dramatically once the fan/load is spinning.
They are allowed to use the larger breaker to avoid nuisance tripping under the start-up phase. It does have some type of formula to find out what and how large that is, but I don't know it, I'm just a HI.

Some of the other guy's with the electrical background will probably be able to fill you in on the details.


--
Darrell Hadler
Five Star Home Inspections
Medicine Hat, Alberta CANADA

Originally Posted By: jpope
This post was automatically imported from our archived forum.



okay (as a rule for the HI).



Jeff Pope


JPI Home Inspection Service


“At JPI, we’ll help you look better”


(661) 212-0738

Originally Posted By: jmyers
This post was automatically imported from our archived forum.



John,


It would depend on what the circuit was serving. Units such as HVAC have to be installed using the nameplate ratings on the unit. Other than HVAC equipment, I have never heard of bumping up a rating, although a nameplate rating would supercede the code requirements.


--
Joe Myers
A & N Inspections, Inc.
http://anii.biz

Originally Posted By: bbadger
This post was automatically imported from our archived forum.



I agree with the others that certain loads may be wired differently than you expect.


As far as the nameplate it can be shown on the nameplate but not always.

Most HVAC equipment is labeled directly, a motor will not be.

In an extreme case you might have a 70 amp breaker with 14 AWG feeding a motor.

The following are types of equipment with 'different' requirements.

Quote:
Air-conditioning and refrigeration equipment circuit conductors

Capacitor circuit conductors

Control and instrumentation circuit conductors (Type ITC)

Electric welder circuit conductors

Fire alarm system circuit conductors

Motor-operated appliance circuit conductors

Motor and motor-control circuit conductors


Phase converter supply conductors

Remote-control, signaling, and power- limited circuit conductors

Secondary tie conductors


I made bold the ones that you may run into in a house.


--
Bob Badger
Electrical Construction & Maintenance
Moderator at ECN

Originally Posted By: Joey D’Adamo
This post was automatically imported from our archived forum.



10 Gauge on a 40A breaker is very common here for A/C installs.


Originally Posted By: jmyers
This post was automatically imported from our archived forum.



Quote:
The concept that you need to grasp is that for motors and HVAC equipment, the overload devices for the circuit are in the unit itself, not in the overcurrent device that is protecting the circuit. The device that is protecting the circuit is providing short circuit and ground fault protection, not overload protection.

It is possible to have a #12 Cu. NM protected by a 40 ampere short circuit and ground fault protective device.


This is the link if you care to read it for yourself.

http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=11;t=005738


--
Joe Myers
A & N Inspections, Inc.
http://anii.biz

Originally Posted By: dhartke
This post was automatically imported from our archived forum.



We’ve had this discussion before and if my memory serves me someone added even more information about the condensing unit having its own overcurrent protection. I still have a problem with the “safety first” NEC or UL allowing for inadequate protection of the wire between the panel and the condensing unit disconnect. What happens if an accident along the way causes the #12 hot to contact the neutral or grounding wire and it is protected by a 40 amp breaker without a GFCI? I’m not doubting the interpretation of the NEC codes. I’m doubting the NEC safety continuity. Pun intended.


Originally Posted By: dhartke
This post was automatically imported from our archived forum.



OK Joe beat me to the punch but that 40 amp breaker still does not protect the #12 wire between the panel and the condenser disconnect. Does it?


Originally Posted By: Joey D’Adamo
This post was automatically imported from our archived forum.



dhartke wrote:
What happens if an accident along the way causes the #12 hot to contact the neutral or grounding wire and it is protected by a 40 amp breaker without a GFCI?


The breaker should still trip. My understanding is that the short circuit/ground fault tolerance barely, if at all, changes between breaker sizes.


Originally Posted By: dhartke
This post was automatically imported from our archived forum.



How hot will a #12 wire get carrying 40 amps?


Originally Posted By: Joey D’Adamo
This post was automatically imported from our archived forum.



dhartke wrote:
How hot will a #12 wire get carrying 40 amps?


For continuous loads, it would get pretty warm and likely damage the insulation. Of course that's not what we're talking about...

But I remember one time I had a bad lamp socket that would short out if you put a regular light in it (tri-light socket) and flipped the switch a certain way. When this happened, the breaker immediately tripped and the 18 gauge lamp cord was very very warm. But it wasn't damaged because the breaker tripped in time, despite the fact that the wire was pretty damn warm.

If the same thing happens with a short of 12 ga on a 40 A feeding a motor, I don't personally see a problem with it.

I'm only defending this issue for motor loads, though. Otherwise I never think it would be okay.


Originally Posted By: dhartke
This post was automatically imported from our archived forum.



Joey


I think I'm getting closer to understanding. The key terminology is perhaps "continuous load", and a dead short rapidly spikes an amperage that is much, much greater than 40 amps, tripping the breaker in a flash.

That sparks my memory (another silly pun intended) of a stunt I pulled when I was a kid trying to hot wire a car like they do in the movies. I was under the hood, near the battery and starter relay with a coat hanger wire. Wow, burned a groove in all 10 fingers and the palms of both hands before I could let go of the wire.

Thanks for your patience.


Originally Posted By: Joey D’Adamo
This post was automatically imported from our archived forum.



dhartke wrote:
I think I'm getting closer to understanding. The key terminology is perhaps "continuous load", and a dead short rapidly spikes an amperage that is much, much greater than 40 amps, tripping the breaker in a flash.


I'm pretty sure that's the idea behind it, yes. I know there are cases where breakers don't trip properly, such as the whole Federal Pacific/Federal Pioneer and Zinsco mess. However, I think in those cases even if the wire was properly sized it may be damaged during a short.

With that aside, assuming a working breaker for shorts and ground faults, the only issue left is overload protection. Assuming, once again, that some type of overload protection exists, the wires will never be put in a position where they draw excess current for very long. Motors usually do have their own form of protection, and would usually only draw extra current for a few seconds during startup.

Quote:
Thanks for your patience.


I wouldn't worry about that... we're all friends here and just trying to learn. I don't even know that i'm entirely correct either. I just go after what makes sense to me and see what other people say in response.