# 10 Romex overcurrent protection

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noon9elec

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Bourne, MA
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Self employed Electrical Contractor
I am far from an engineer but am curious about what is the real amperage capabilities of # 10 copper. 310.16 says 35 amps if it is rated at 70 C. and is THHN , which I believe Romex is. I am on a job where the installation is over 50 yrs old and they protected the 10/3 circuit with a 2-P 40-amp breaker. I want to change breaker, which includes new panel, but engineer says I am over thinking it and leave it alone. thoughts? thank you
 
NM cable must be sized using the 60C column except when applying adjustment factors. As Larry asked what type of load is being supplied? The 40 may be ok.
 
noon9elec said:
I am far from an engineer but am curious about what is the real amperage capabilities of # 10 copper. 310.16 says 35 amps if it is rated at 70 C. and is THHN , which I believe Romex is. I am on a job where the installation is over 50 yrs old and they protected the 10/3 circuit with a 2-P 40-amp breaker. I want to change breaker, which includes new panel, but engineer says I am over thinking it and leave it alone. thoughts? thank you
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I figured I'd blow away the cobwebs upstairs and do some research on this because normally I simply wouldn't exceed a 30 A breaker but your 310.16 mentioned as you stated, 35 Amp. I found that NM (older romex) is rated for 60C making it 30A. NM-B claims its internal conductor is either a THHN being 90C or a THWN rated at 75C .. noted on article 310.104 . Now this is were it gets interesting because notice the 10** that ( **) now directs one to 240.4(D) which then sends us to... shall not exceed D1 - 7 ... look at 240.4 (D) (7) 10AWG 30A. I wont say what I think of that big circle of research other than I'd stick with a 30A. breaker. or if they really want a 40 amp throw in an 8/3 better safe than sorry.
 
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LarryFine said:
I still say it depends on the load - HVAC for example.
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That is true, a number 10 can probably handle 40 amps as mentioned for short periods before the copper starts turning blue and baking the insulation a tad. My thought on that short spurt duration clause is .. is it really worth pushing a conductor to its molecular ability.
 
noon9elec said:
I am far from an engineer but am curious about what is the real amperage capabilities of # 10 copper. 310.16 says 35 amps if it is rated at 70 C. and is THHN , which I believe Romex is. I am on a job where the installation is over 50 yrs old and they protected the 10/3 circuit with a 2-P 40-amp breaker. I want to change breaker, which includes new panel, but engineer says I am over thinking it and leave it alone. thoughts? thank you
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Last comment regarding what you want to do and what the engineer wants you to do, If its your license and your reputation, stick to your plan or let him put someone else's name on it, I don't do what others tell me to do especially if I know or think its wrong .. your call.
 
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Another C10 said:
Last comment regarding what you want to do and what the engineer wants you to do, If its your license and your reputation, stick to your plan or let him put someone else's name on it, I don't do what others tell me to do especially if I know or think its wrong .. your call.
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Lawyers claim we know more than engineers, and will try to hang us if we do what the engineer says, especially if we have an inkling that it might not be right. Been there, done that.
 
Another C10 said:
That is true, a number 10 can probably handle 40 amps as mentioned for short periods before the copper starts turning blue and baking the insulation a tad. My thought on that short spurt duration clause is .. is it really worth pushing a conductor to its molecular ability.
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For the HVAC situation would need to be one with a motor/compressor load before you will be allowed to go over 30 amp breaker. The reason is to allow starting that motor or compressor, yet running current will remain below the conductor ampacity, and the motor overload will shut it down if it were to exceed motor rating anyway. This true for any size conductor for motors.
 
If it is a simple change out of a 2 pole breaker, code exception or not I would consider changing to a 30 amp breaker or 35 amp breaker.
If the breaker pops due to the change, I would consider using the 40 amp breaker with #8 wire.
 
powerpete69 said:
If it is a simple change out of a 2 pole breaker, code exception or not I would consider changing to a 30 amp breaker or 35 amp breaker.
If the breaker pops due to the change, I would consider using the 40 amp breaker with #8 wire.
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Still need to know what it is supplying, if a HVAC compressor, nameplate might even allow more than 40 amp breaker but still need at least 10 AWG conductor. MCA 29 MOCP 45 would be somewhat normal and would be allowed on 10 AWG NM cable.
 
Adding info in case anyone out there is wondering why it might be ok... Yes, 240.4 (D) states that 30A OCPD is needed for 10AWG, but it also says "unless specifically permitted in 240.4(E) or (G)" and 240.4 (G) would point you to Art. 440 if it's for HVAC for example.
 
bwat said:
Adding info in case anyone out there is wondering why it might be ok... Yes, 240.4 (D) states that 30A OCPD is needed for 10AWG, but it also says "unless specifically permitted in 240.4(E) or (G)" and 240.4 (G) would point you to Art. 440 if it's for HVAC for example.
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334.80 Will restrict #10 romex to 60C in all situations, the insulation is not THHN, unless its marked.
 
tortuga said:
334.80 Will restrict #10 romex to 60C in all situations, the insulation is not THHN, unless its marked.
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And for art 430/440 applications you still could need a conductor ampacity of up to 30 amps but also have OCPD of more than 30.
 
kwired said:
And for art 430/440 applications you still could need a conductor ampacity of up to 30 amps but also have OCPD of more than 30.
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Hmmm good point
s.jpg
Say noon9elec gives us more details, he has put in a standard loadcenter that takes residential plug in breakers (inverse time breaker) the motor served is a 5HP 208V Air Compressor motor single phase
1740 Nameplate RPM
Continuous Duty Rated
Nameplate FLA 23A @ 208
(non thermally protected Capacitor-Start/Run)
Its on a single branch circuit #10 NM-B cable. Breaker and motor terminations are marked 75C.

What is the ampacity of the #10 NM-B cable ? (A) 30A (B) 35 A (C) 50A
What is the minimum AWG NM-B cable allowed to serve this motor?
(A) 12 (B) 10 (C) 8
What is the maximum size inverse time breaker allowed to protect the NM-B cable?
(A) 30 (B) 35 (C) 40
 
tortuga said:
Hmmm good point
View attachment 2551910
Say noon9elec gives us more details, he has put in a standard loadcenter that takes residential plug in breakers (inverse time breaker) the motor served is a 5HP 208V Air Compressor motor single phase
1740 Nameplate RPM
Continuous Duty Rated
Nameplate FLA 23A @ 208
(non thermally protected Capacitor-Start/Run)
Its on a single branch circuit #10 NM-B cable. Breaker and motor terminations are marked 75C.

What is the ampacity of the #10 NM-B cable ? (A) 30A (B) 35 A (C) 50A
What is the minimum AWG NM-B cable allowed to serve this motor?
(A) 12 (B) 10 (C) 8
What is the maximum size inverse time breaker allowed to protect the NM-B cable?
(A) 30 (B) 35 (C) 40
Click to expand...
Regardless of what your nameplate indicates this motor must have conductor ampacity of 38.5.
The max inverse time breaker would be 80 amp.
The max ampacity of #10 NM is 30.
 
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