Conductor derate.

[JdoubleU]

Is it code compliant to run 10 #12 THHN current carring conductors in one raceway to feed 120volt general purpose recepticals being fed from a 20 amp OCPD. This is asumeing that the race ways so more that 10 feet long. I would say no because the conductors are derated below the 20amp overcurrent device being used. In the 90 degree column #12 is rated 30 amps but when derated its only good to carry 15 amps. Am I right in saying this is against the NEC?

 

[Sparky Marky]

I would agree, based on an ambient temperature of 26 degrees Celsius or higher. If the ambient temperature is 25 degrees Celsius or below, then I believe it would actually be permissible for copper THHN based on the correction factors of 310.15(B)(2)(a), and the allowance of 240.4(B). I'm sure that's a very rare "real life" situation, however it may be used in an exam question.

 

[infinity]

I would agree, based on an ambient temperature of 26 degrees Celsius or higher. If the ambient temperature is 25 degrees Celsius or below, then I believe it would actually be permissible for copper THHN based on the correction factors of 310.15(B)(2)(a), and the allowance of 240.4(B). I'm sure that's a very rare "real life" situation, however it may be used in an exam question.

240.4(B) cannot be applied to multi-receptacle circuits.

 

[Sparky Marky]

240.4(B) cannot be applied to multi-receptacle circuits.

That is correct. I forgot about that rule. Thanks!

 

[infinity]

That is correct. I forgot about that rule. Thanks!

You brought up an interesting point about temperature factor correction, if you can nudge the derated value above 15 amps (30*50%) you can go up to the next standard size of 20 amps, providing there is not multiple receptacles on the circuit.

 

[GoldDigger]

You brought up an interesting point about temperature factor correction, if you can nudge the derated value above 15 amps (30*50%) you can go up to the next standard size of 20 amps, providing there is not multiple receptacles on the circuit.

A subtle point, as I tend to say: The receptacle count only comes into play if they are for cord-and-plug-connected portable loads [2011].
So, for example, dedicated receptacles for dishwasher, disposal, refrigerator, built-in microwave, etc. would NOT be counted in this test.

 

[infinity]

A subtle point, as I tend to say: The receptacle count only comes into play if they are for cord-and-plug-connected portable loads [2011].
So, for example, dedicated receptacles for dishwasher, disposal, refrigerator, built-in microwave, etc. would NOT be counted in this test.

Does the NEC actually define a portable load inclusive of all of the things that you've listed or are we to decide on our own?

 

[don_resqcapt19]

Does the NEC actually define a portable load inclusive of all of the things that you've listed or are we to decide on our own?

Rob, If the NEC defined everything there would be no need for this forum

 

[kwired]

A subtle point, as I tend to say: The receptacle count only comes into play if they are for cord-and-plug-connected portable loads [2011].
So, for example, dedicated receptacles for dishwasher, disposal, refrigerator, built-in microwave, etc. would NOT be counted in this test.

Feed those with a single receptacle on a dedicated circuit and you don't have multiple outlets in the circuit. If you want to be a little lenient you maybe consider a single duplex dedicated for that one appliance to still be one outlet, but otherwise it is generally considered two outlets.

 

[kwired]

Is it code compliant to run 10 #12 THHN current carring conductors in one raceway to feed 120volt general purpose recepticals being fed from a 20 amp OCPD. This is asumeing that the race ways so more that 10 feet long. I would say no because the conductors are derated below the 20amp overcurrent device being used. In the 90 degree column #12 is rated 30 amps but when derated its only good to carry 15 amps. Am I right in saying this is against the NEC?

For general purpose lighting/receptacles circuits you are correct. If supplying single outlet circuits then the 12 AWG would be acceptable if the load is 15 amps or less - even if on a 20 amp overcurrent device. I have done this many times with motor circuits, and the 12 AWG may even be on more than a 20 amp breaker with motor circuits. Bottom line is finding the minimum ampacity necessary. With a motor that comes from the tables in art 430. With a multiple outlet general purpose receptacle circuit the minimum ampacity necessary is based on the rating of the overcurrent protection, so 20 amp device needs a 20 amp conductor.

 

[david luchini]

In the 90 degree column #12 is rated 30 amps but when derated its only good to carry 15 amps. Am I right in saying this is against the NEC?

If supplying single outlet circuits then the 12 AWG would be acceptable if the load is 15 amps or less - even if on a 20 amp overcurrent device.

Wouldn't protecting a conductor with an ampacity of 15 with an OCPD rated for 20A be a violation of 240.4?

 

[GoldDigger]

No

Wouldn't protecting a conductor with an ampacity of 15 with an OCPD rated for 20A be a violation of 240.4?

Not if the conductor is protected against long term low level overload by, for example, motor overloads at the far end.
You can generally consider that the only fault probable along the run of the wire would be a short circuit which would still trip the higher current OCPD just fine.

 

[david luchini]

Not if the conductor is protected against long term low level overload by, for example, motor overloads at the far end.
You can generally consider that the only fault probable along the run of the wire would be a short circuit which would still trip the higher current OCPD just fine.

Not referring to motor circuits, but to "single outlet circuits." As kwired noted in his post, for motor circuits, #12 may be protected at higher than 20A.

If supplying single outlet circuits then the 12 AWG would be acceptable if the load is 15 amps or less - even if on a 20 amp overcurrent device.

 

[GoldDigger]

Not referring to motor circuits, but to "single outlet circuits." As kwired noted in his post, for motor circuits, #12 may be protected at higher than 20A.

IMHO it still comes down to the degree of assurance you have that the device will only draw 15A
If the device has the potential to pull more current and does not contain integrated OCPD, then it may still be OK by code but not a good idea.

 

[david luchini]

then it may still be OK by code but not a good idea.

My point was that is is NOT OK by Code to protect a conductor with an ampacity of 15 with an OCPD with a rating of 20A (with the exception of the specific conductor applications in 240.4).

 

[GoldDigger]

My point was that is is NOT OK by Code to protect a conductor with an ampacity of 15 with an OCPD with a rating of 20A (with the exception of the specific conductor applications in 240.4).

The context of this part of the thread is exactly one of those exceptions, and the question I am concerned with is how this exception is justified, not whether it applies.

 

[kwired]

Not referring to motor circuits, but to "single outlet circuits." As kwired noted in his post, for motor circuits, #12 may be protected at higher than 20A.

Isn't a circuit supplying a single motor also a "single outlet circuit"?

A single receptacle dedicated to a particular piece of equipment is very similar. Though I can't say I have done this for 5-15 or 5-20 receptacles, I have done it many times for other receptacle configurations when supplying an individual motor. Motors seems to be where you may apply this the most often.

I would probably look harder at rules if confronted again with such an install as I think there may have been changes and the receptacle may need rated for the HP these days, or at least meet a certain other rating, where I don't believe that was required in the past.

I still commonly supply L16-20 receptacles with less than 20 amp circuits (like 1 or 2 amp fuses are often the short circuit protection), but that is kind of the other extreme of the topic here.

 

[david luchini]

Isn't a circuit supplying a single motor also a "single outlet circuit"?

A single receptacle dedicated to a particular piece of equipment is very similar. Though I can't say I have done this for 5-15 or 5-20 receptacles, I have done it many times for other receptacle configurations when supplying an individual motor. Motors seems to be where you may apply this the most often.

I would probably look harder at rules if confronted again with such an install as I think there may have been changes and the receptacle may need rated for the HP these days, or at least meet a certain other rating, where I don't believe that was required in the past.

I still commonly supply L16-20 receptacles with less than 20 amp circuits (like 1 or 2 amp fuses are often the short circuit protection), but that is kind of the other extreme of the topic here.

Yes, but isn't an individual branch circuit supplying an 1800w, 120v heater also a "single outlet circuit?" Wouldn't the branch circuit required to be protected by at least a 20A OCPD? And wouldn't the required ampacity of the conductor to supply the load be 15? BUT, wouldn't a conductor with an ampacity of 15 being protected by a 20A OCPD be a violation of 240.4? There is no general rule that permits a conductor with an ampacity of 15 to be protected by a 20A OCPD for a "single outlet circuit." You'd have to look at the specific conductor applications in 240.4.

 

[kwired]

Yes, but isn't an individual branch circuit supplying an 1800w, 120v heater also a "single outlet circuit?" Wouldn't the branch circuit required to be protected by at least a 20A OCPD? And wouldn't the required ampacity of the conductor to supply the load be 15? BUT, wouldn't a conductor with an ampacity of 15 being protected by a 20A OCPD be a violation of 240.4? There is no general rule that permits a conductor with an ampacity of 15 to be protected by a 20A OCPD for a "single outlet circuit." You'd have to look at the specific conductor applications in 240.4.

True, but you have the "small conductor rule" requiring the minimum size here and not the general ampacity rules. Now go back to the OP situation and supply that 1800w heater with a 12AWG derated to 50% because of number of conductors in the raceway. The conductor ampacity is adjusted to 15 amps, the load is 15 amps 125% for continuous load only applies to termination and 10AWG is sufficient for the termination temp. Overcurrent device still needs to be 125% of load so it can not be a 15 amp device.

210.19 Conductors ? Minimum Ampacity and Size.

(A) Branch Circuits Not More Than 600 Volts.

(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.


Exception: If the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

Informational Note No. 1: See 310.15 for ampacity ratings of conductors.

Informational Note No. 2: See Part II of Article 430 for minimum rating of motor branch-circuit conductors.

Informational Note No. 3: See 310.15(A)(3) for temperature limitation of conductors.

Informational Note No. 4: Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation. See Informational Note No. 2 of 215.2(A)(3) for voltage drop on feeder conductors.

(2) Branch Circuits with More than One Receptacle. Conductors of branch circuits supplying more than one receptacle for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit.

I need to correct myself, 210.19(A)(2) only says circuits with more than one receptacle, I think I earlier said "with multiple outlets".

(A)(1) says conductor ampacity needs to be at least the amount of load served, and if continuous load minimum size is determined before any other adjustments.

 

[david luchini]

True, but you have the "small conductor rule" requiring the minimum size here and not the general ampacity rules. Now go back to the OP situation and supply that 1800w heater with a 12AWG derated to 50% because of number of conductors in the raceway. The conductor ampacity is adjusted to 15 amps, the load is 15 amps 125% for continuous load only applies to termination and 10AWG is sufficient for the termination temp. Overcurrent device still needs to be 125% of load so it can not be a 15 amp device.

The "small conductor rule" specifies a maximum OCPD size for small conductors, not a minimum.

If the #12 conductor has an adjusted ampacity of 15, it cannot be protected by a 20A OCPD (unless it falls under one of the specific conductor applications.)