D-rating

[Ben C]

I have a 3/4 PVC conduit with 4 120v circuits and 4 neutrals and a ground. A neutral per circuit. I am using #12 thhn solid wire. Would this b d-rated using the 90 degree column at 30 amps for #12(also knowing that a #12 can be on no larger than a 20 amp which means the Eire would be good for 21 amps???. Also, would the neutrals be considered current carrying conductors in this application??

 

[Dennis Alwon]

The neutrals are CCC and you are correct in your assessment. 8 CCC using 90C is 21 amps so a 20 amp OCPD is good,

 

[Ben C]

The neutrals are CCC and you are correct in your assessment. 8 CCC using 90C is 21 amps so a 20 amp OCPD is good,

When are the neutrals not considered CCC. I saw in another questions where they weren't and was just curious??

 

[kwired]

When are the neutrals not considered CCC. I saw in another questions where they weren't and was just curious??

The neutrals are not current carrying (only for deration purposes) when they only carry unbalanced current from the associated phase conductors of the circuit. An example is a multiwire branch circuit - two opposing "hots" and a "neutral" connected to 120/240 single phase supply. If each "hot" is carrying 10 amps the neutral is not carrying any current. If one hot is carrying 7 amps and the other 12 amps the neutral is only carrying the unbalance which is 5 amps.

If you have two phases of a wye connected system plus a neutral then it is considered current carrying because the neutral current when both phases are carrying the same current is approximately the same amount of current that is in the phase conductors. But a multiwire circuit with all three phases and a neutral is only carrying the imbalance of all three phase conductors, and is not a CCC for deration purposes.

Addition of harmonic currents can complicate things but this is the basics if you disregard those circuits with those associated problems.

Hopefully this is understandable

 

[augie47]

The neutrals are not current carrying (only for deration purposes) when they only carry unbalanced current from the associated phase conductors of the circuit. An example is a multiwire branch circuit - two opposing "hots" and a "neutral" connected to 120/240 single phase supply. If each "hot" is carrying 10 amps the neutral is not carrying any current. If one hot is carrying 7 amps and the other 12 amps the neutral is only carrying the unbalance which is 5 amps.

If you have two phases of a wye connected system plus a neutral then it is considered current carrying because the neutral current when both phases are carrying the same current is approximately the same amount of current that is in the phase conductors. But a multiwire circuit with all three phases and a neutral is only carrying the imbalance of all three phase conductors, and is not a CCC for deration purposes.

Addition of harmonic currents can complicate things but this is the basics if you disregard those circuits with those associated problems.

Hopefully this is understandable

well worded, IMO.

 

[Ben C]

well worded, IMO.

Thanks. That what we were thinking but on another blog a similar question was answered different so just wanted to make sure about the neutrals. Thanks

 

[jumper]

I have a 3/4 PVC conduit with 4 120v circuits and 4 neutrals and a ground. A neutral per circuit. I am using #12 thhn solid wire. Would this b d-rated using the 90 degree column at 30 amps for #12(also knowing that a #12 can be on no larger than a 20 amp which means the Eire would be good for 21 amps???. Also, would the neutrals be considered current carrying conductors in this application??

What type of location is this install in?

You mention PVC conduit, which I generally associate with underground or wet location, and then THHN. Most wire today is dual rated, THHN for dry locations and THWN for wet.

If this is a wet location, then you have to use the 75C column for derating-unless your wires are THWN-2.

 

[kwired]

What type of location is this install in?

You mention PVC conduit, which I generally associate with underground or wet location, and then THHN. Most wire today is dual rated, THHN for dry locations and THWN for wet.

If this is a wet location, then you have to use the 75C column for derating-unless your wires are THWN-2.

It is good to know that information. People that don't realize that get away correctly not knowing they made a mistake because most conductor we all buy is rated THHN, THWN, as well as THWN-2. That is what there is demand for and therefore most suppliers stock that particular type.

If supplier is not stocking that type all it takes is one inspector to notice that conductors are commonly not sized correctly and start writing correction notices. Then it will not take long for requests for the supplier to start supplying conductors rated as THWN-2.

 

[jumper]

It is good to know that information. People that don't realize that get away correctly not knowing they made a mistake because most conductor we all buy is rated THHN, THWN, as well as THWN-2. That is what there is demand for and therefore most suppliers stock that particular type.

If supplier is not stocking that type all it takes is one inspector to notice that conductors are commonly not sized correctly and start writing correction notices. Then it will not take long for requests for the supplier to start supplying conductors rated as THWN-2.

I admit I screwed this up for a long time until Augie/Gus explained it to me.

 

[kwired]

I admit I screwed this up for a long time until Augie/Gus explained it to me.

So did I, (can't remember who or when I was straightened out on it) but then I realized most of the time I wasn't necessarily wrong because the conductors were likely THWN-2 anyway.

 

[infinity]

Thanks. That what we were thinking but on another blog a similar question was answered different so just wanted to make sure about the neutrals. Thanks

Not sure what the other guys said but the correct answers are in this thread. To expand a little on Kwired's excellent post here's a post from an old thread:

Here's some examples of when to count the neutral as a CCC:

208Y/120 volt system-different circuit types:

A)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C)- 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B)- In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C)- In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with one exception, *if the current is more than 50% nonlinear then the neutral would count as a CCC.

120/240 volt system-different circuit types:

D)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E)- In this circuit the neutral will only carry the imbalance between the two ungrounded condcutors so the neutral is not counted as a CCC

 

[Coelec]

Hi folks,
can someone give the section in the code for using 90 degree column when derating?

Thanks in advance

 

[Dennis Alwon]

Hi folks,
can someone give the section in the code for using 90 degree column when derating?

Thanks in advance

Look at 110.14(C) last sentence

 

[Coelec]

Thanks Dennis

 

[Dennis Alwon]

Thanks Dennis

No problem & welcome to the show :thumbsup:

 

[infinity]

Thanks. That what we were thinking but on another blog a similar question was answered different so just wanted to make sure about the neutrals. Thanks

No problem & welcome to the show :thumbsup:

If you really want to learn or just have discussions with like minded people stick around here. :thumbsup: