Cooktop issue

[blueheels2]

Had an emergency service call on Sunday for a cooktop that had quit working. Determined that a faulty connection (possible overload) was the problem. Wirenut on the black was melted and there were obvious smells of burning. Remade the connections and it worked. However nameplate of the cooktop was 7.5kw at 240 and the branch circuit was #10 NM-B. That is a 31.25 amp load and was verified by my meter. All eyes on and it was 31.5 amps. So I’m writing up the invoice citing code 210.19(A)3 and suggesting upsizing the wire to meet code and prevent fires etc.

However the #10 showed no real sign of overheating. So question is was the overload really the culprit or was it just a bad connection?

 

[gadfly56]

Had an emergency service call on Sunday for a cooktop that had quit working. Determined that a faulty connection (possible overload) was the problem. Wirenut on the black was melted and there were obvious smells of burning. Remade the connections and it worked. However nameplate of the cooktop was 7.5kw at 240 and the branch circuit was #10 NM-B. That is a 31.25 amp load and was verified by my meter. All eyes on and it was 31.5 amps. So I’m writing up the invoice citing code 210.19(A)3 and suggesting upsizing the wire to meet code and prevent fires etc.

However the #10 showed no real sign of overheating. So question is was the overload really the culprit or was it just a bad connection?

My opinion, and worth every penny you're paying for it, is that it was just a poorly made connection. That is an entirely separate issue from the undersized conductor. If the subtext is, "Well, with a proper connection it'll be fine", I wouldn't go there if it was my license.

 

[blueheels2]

This is what I was thinking. Technically wrong but is it really a fire hazard? I don’t think so but I can’t put that in writing nor do I want to say it’s just fine. I think a statement of fact that sites a code violation is probably sufficient as well as siting a faulty connection is the real issue this time. Another blurb about to be code compliant wire would need to be upsized to #8 or a smaller cooktop installed.

 

[mbrooke]

In any part other part of the world this cooktop would get thinner wire.

The THHN in NM-B is rated 90*C, so 31.5 amps would not hit that temp limit or the 75*C temp limit of the OCPD.

I'd literally worry more about the smaller ground in #8 NM than #10 carrying 31.5 amps.

TLDR: I won't lose sleep over this.

 

[GoldDigger]

In any part other part of the world this cooktop would get thinner wire.

The THHN in NM-B is rated 90*C, so 31.5 amps would not hit that temp limit or the 75*C temp limit of the OCPD.

I'd literally worry more about the smaller ground in #8 NM than #10 carrying 31.5 amps.

Yet the conductors inside the NM sheath will reach a higher temperature for a given load than in either free air or conduit.
And, of course, what will the 90*C temperature do to the NM sheath?

 

[mbrooke]

Yet the conductors inside the NM sheath will reach a higher temperature for a given load than in either free air or conduit.
And, of course, what will the 90*C temperature do to the NM sheath?

Don't you mean lower?

In Canada that same sheath is rated 90*C.

 

[GoldDigger]

Don't you mean lower?

In Canada that same sheath is rated 90*C.

Why would you think that the same guage conductor carrying the same current would be cooler inside NM that in free air? Are you figuring that the sheath is good thermal conductor?

 

[mbrooke]

Why would you think that the same guage conductor carrying the same current would be cooler inside NM that in free air? Are you figuring that the sheath is good thermal conductor?

I thought you meant the cable assembly in free air vs fiberglass insulation.


Even though Table 310.16 says 60, 75 and 90*C the conductor doesn't actually get that hot even in a wall in contact with thermal insulation.

 

[petersonra]

I think it is kind of silly to determine solely by code that putting a plastic sheath around three conductors makes them hotter than if those same three conductors were in say a piece of plastic conduit.

 

[Dennis Alwon]

In most case there should not be an issue but code does require larger than #10 in this case. Note 4 to T220.55