Question on ampacity of conductors, adjustment factors and #12 THHN on 20A circuits.

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What is the maximum number of single pole 20 amp circuits on #12 awg THHN conductors (one hot, one neutral) that can be installed in a 3/4" EMT conduit and still meet the requirements of the NEC? At what number of circuits, or conductors, will it be either required to; reduce the overcurrent protection device from 20 amp single pole circuit breakers to less than 20 amps, increase the number of conduits to be installed, or increase the awg wire size from #12 THHN to #10 THHN?

Background: The conduits are installed indoors in a temperature controlled enviroment. The circuits are general purpose receptacle circuits, general lighting circuits. Ambient temperature factors or correctional factors for temperature are not applicable in this question. The calculated load or load on the circuit breakers are not factors in this equation, it is anticipated that the actual load demand will be small, the circuits are for general flourescent strip lighting and general office use such as desktop computers and an occassional printer.

Is a grounded conductor "neutral" in a single-phase two wire circuit consisting of one ungrounded conductor and one grounded conductor always considered a current carrying conductor for the purposes of ampacity adjustment factors?

Is there an NEC code reference that states that in a single-phase two wire circuit consisting of one ungrounded conductor and one grounded conductior that the grounded conductor is considered a current carrying conductor?

In the questions submitted, is the type voltage relevant such as 120/208V or 120/240V?
 

iwire

Moderator
Staff member
Location
Massachusetts
What is the maximum number of single pole 20 amp circuits on #12 awg THHN conductors (one hot, one neutral) that can be installed in a 3/4" EMT conduit and still meet the requirements of the NEC? At what number of circuits, or conductors, will it be either required to; reduce the overcurrent protection device from 20 amp single pole circuit breakers to less than 20 amps, increase the number of conduits to be installed, or increase the awg wire size from #12 THHN to #10 THHN?

In that situation in most cases the max number of current carrying conductors is 9 before you have to increase. This usally is true with 15, 20, and 30 amp circuits.
 

Smart $

Esteemed Member
Location
Ohio
...

Is a grounded conductor "neutral" in a single-phase two wire circuit consisting of one ungrounded conductor and one grounded conductor always considered a current carrying conductor for the purposes of ampacity adjustment factors?

Is there an NEC code reference that states that in a single-phase two wire circuit consisting of one ungrounded conductor and one grounded conductior that the grounded conductor is considered a current carrying conductor?

In the questions submitted, is the type voltage relevant such as 120/208V or 120/240V?
The simple answer is one wire cannot be the only current carrying conductor. Deductive reasoning concludes that 2 wires are necessary for a complete circuit and both will carry current. It does not matter what the voltage is, or whether one conductor is a neutral or grounded or not. No NEC code reference... just Electric 101.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
OK, so the right answer has been given above. But the reason that that is the right answer is equally important.

Start with the fact that THHN wire is rated for a 90C rise above the ambient of 30C, and with the fact that at 90C #12 THHN has an ampacity of 30 amps. Now apply the derating factors of Table 310.15(B)(2)(a). If there are up to 6 CCCs, the derating factor is 80%, and 80% of 30 amps is 24 amps. Finally, apply the limitation of 240.4(D), and we conclude that we can use a 20 amp breaker for this installation.

Next try up to 9 CCCs. The derating factor is 70%, and 70% of 30 amps is 21 amps. Finally, apply the limitation of 240.4(D), and we conclude that we can use a 20 amp breaker for this installation.

Next, try 10 CCCs. The derating factor is 50%, and 50% of 30 amps is 15 amps. That is too low to allow us to use a 20 amp breaker for this installation.

Therefore, using a 20 amp breaker and #12 THHN wire, the maximum number of CCCs we can install in a single conduit is 9.

By similar reasoning, you can see that for a 30 amp breaker and #10 THHN wire, the maximum number of CCCs we can install in a single conduit is 6.
 

iwire

Moderator
Staff member
Location
Massachusetts
By similar reasoning, you can see that for a 30 amp breaker and #10 THHN wire, the maximum number of CCCs we can install in a single conduit is 6.

I am going to stick with 'it depends' as it really is dependent on 240.4(B) and if it can be applied or not. :)
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Even if 240.4(B) does apply, you can't use it to justify 9 #10 THHN wires in a conduit and a 30 amp breaker unless you also meet the requirement in 210.19(A)(1) that the load is no higher than 28 amps. I would say that is rare that we calculate the load on a 30 amp circuit, and discover that it is 28 or lower. So I might take issue with a statement that 9 CCCs can "usually" work for a 30 amp circuit.
 

Smart $

Esteemed Member
Location
Ohio
... I would say that is rare that we calculate the load on a 30 amp circuit, and discover that it is 28 or lower. ...
I would say it is common that the calculated load of a 30A ocpd #10 THHN circuit is 28A or less.

The main question I have is whether the ocpd would have to be changed if we found the calculated load to be 25A or less. The "next size up" would then be a 25A ocpd. But yet the circuit is still capable of going to 28A.
 

jumper

Senior Member
I would say it is common that the calculated load of a 30A ocpd #10 THHN circuit is 28A or less.

The main question I have is whether the ocpd would have to be changed if we found the calculated load to be 25A or less. The "next size up" would then be a 25A ocpd. But yet the circuit is still capable of going to 28A.


WHs come to mind.
 

kwired

Electron manager
Location
NE Nebraska
I would say it is common that the calculated load of a 30A ocpd #10 THHN circuit is 28A or less.

The main question I have is whether the ocpd would have to be changed if we found the calculated load to be 25A or less. The "next size up" would then be a 25A ocpd. But yet the circuit is still capable of going to 28A.

I think as long as calculated load is not over 28 amps then a 30 amp overcurrent device is acceptable. There is no reason a 1 amp load on a 10 AWG conductor can not have 30 amp overcurrent protection as long as the conductor was not derated to less then 25 amps. This is general rules - motors and other similar loads can usually have higher overcurrent protection levels than other loads in general
 

kwired

Electron manager
Location
NE Nebraska
What is the maximum number of single pole 20 amp circuits on #12 awg THHN conductors (one hot, one neutral) that can be installed in a 3/4" EMT conduit and still meet the requirements of the NEC? At what number of circuits, or conductors, will it be either required to; reduce the overcurrent protection device from 20 amp single pole circuit breakers to less than 20 amps, increase the number of conduits to be installed, or increase the awg wire size from #12 THHN to #10 THHN?

Background: The conduits are installed indoors in a temperature controlled enviroment. The circuits are general purpose receptacle circuits, general lighting circuits. Ambient temperature factors or correctional factors for temperature are not applicable in this question. The calculated load or load on the circuit breakers are not factors in this equation, it is anticipated that the actual load demand will be small, the circuits are for general flourescent strip lighting and general office use such as desktop computers and an occassional printer.

Is a grounded conductor "neutral" in a single-phase two wire circuit consisting of one ungrounded conductor and one grounded conductor always considered a current carrying conductor for the purposes of ampacity adjustment factors?

Is there an NEC code reference that states that in a single-phase two wire circuit consisting of one ungrounded conductor and one grounded conductior that the grounded conductor is considered a current carrying conductor?

In the questions submitted, is the type voltage relevant such as 120/208V or 120/240V?
There are other factors that come to play. Lets just stick with the mentioned 12 AWG on 20 amp circuit and up to 9 current carrying conductors.

If you have 120 volt circuits with neutral for each circuit - you will have 8 current carrying conductors with 4 circuits. If you try to go to 5 circuits you have 10 CCC and you are now in the 50% derate category and your 12AWG are not large enough.

If you have 120/240 volt multiwire branch circuits you can have 8 circuits with 4 neutrals - the neutrals are carrying only imbalanced current in this case and do not count as current carrying conductors.

If you have 120/208 volt multiwire branch circuits and have one neutral per two phase conductors - the current on the neutrals is not imbalanced current - it does not get any higher than the higher level phase current but it is still a current carrying conductor - so you can have 3 such circuits (6 poles) without going over the 9 current carrying conductor limit for deration purposes on 12 AWG.

If you have 120/208 volt multiwire branch circuits and have one neutral per each set of all three phase conductors you now have a neutral that only carries the unbalance of the three and can have a total of 3 such circuits (9 poles) without going over the 9 current carrying conductor limit for deration purposes on 12 AWG.
 

Smart $

Esteemed Member
Location
Ohio
I think as long as calculated load is not over 28 amps then a 30 amp overcurrent device is acceptable. There is no reason a 1 amp load on a 10 AWG conductor can not have 30 amp overcurrent protection as long as the conductor was not derated to less then 25 amps. ...
I agree with that premise :D
 
There are other factors that come to play. Lets just stick with the mentioned 12 AWG

There are other factors that come to play. Lets just stick with the mentioned 12 AWG

Thank you kwired. I agree with your comments and your response helped to affirm my understanding of the rules.

The situation that prompted the post is where the technician had shoved (15) #12 conductors (7 circuits & 1 egc) into a 3/4" conduit and was arguing that he did not have to reduce the OCPD from 20A to 15A because the neutrals were carrying the unbalanced load. I tried to explain to him that in a single phase circuit with one ungrounded conductor and one grounded conductor that the ungrounded conductor carries all of the current back to the source, it isn't the unbalanced portion, it is all of it (all of the circuits were on single pole 20A circuit breakers).

I also appreciated the comment referring to basic E101. However, I think there is still quite a bit of misunderstanding out there. I think because the NEC does not explicitly state that in a single phase circuit where there is only one ungrounded conductor and one grounded conductor (one hot and one neutral) that the grounded conductor is considered a current carrying conductor for the purposed of ampacity adjustment, that when people read the the section to see if the neutral is counted or not, they are left either confused or they interpret it incorrectly and believe that it does not count, again because the code does not explicitly state that it does.

I appreciate all of the helpful responses.
 

Smart $

Esteemed Member
Location
Ohio
...

I also appreciated the comment referring to basic E101. However, I think there is still quite a bit of misunderstanding out there. I think because the NEC does not explicitly state that in a single phase circuit where there is only one ungrounded conductor and one grounded conductor (one hot and one neutral) that the grounded conductor is considered a current carrying conductor for the purposed of ampacity adjustment, that when people read the the section to see if the neutral is counted or not, they are left either confused or they interpret it incorrectly and believe that it does not count, again because the code does not explicitly state that it does.

...
I agree.

I think this is a result of the Code changing definitions of "grounded" and "neutral" conductors, i.e. in a two-wire circuit there is no "neutral" conductor. The result of the change is we can now call the grounded conductor of a two-wire circuit a "neutral".

PS: Better to call it a two-wire circuit than a single-phase circuit. A 120/240 mwbc circuit is still a single-phase circuit which has a neutral.
 
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