derating help

[killer76]

I have been trying to understand all the answers in my search for de rating , I must be confusing myself, can not remember what I was taught years ago.

I have 4 circuits (1) 20 amp receptacle on each, my wire is #12 THHN the ground has been daisy chained so I have only 9 wires that will run to the intended j-box using the 90deg table

I know that can get 16 # 12 wires in a 3/4" conduit (RMC)

I have been told that I may need to increase the conduit size to 1"


a simple equations would be great, if at all possible, or were in the code can I find the information, and hope to understand how to make adjust , for some reason I have been just increasing the conduit size and see that is not the true issue.

thanks
Robert

 

[roger]

Re: derating help

The size of the conduit doesn't matter, if you have 4,6,8,10, etc... current carrying #12 conductors in a 1/2" or a 4" you have to derate them the same. See Table 310.15(B)(2)(a)

Roger

[ February 09, 2006, 05:37 PM: Message edited by: roger ]

 

[iwire]

Re: derating help

I agree with Roger and add that sixteen 12 AWGs in the same raceway will likely have an ampacity of no more than 15 amps.

By the way, the maximum number of conductors allowed and what you can really install are often very different.

Unless this a very short and straight run I would use a larger raceway or more likely run a second 3/4" EMT parallel to the first and install conductors in each.

Easer to pull and the 12 AWG will still be rated high enough for 20 amp breakers.

[ February 09, 2006, 05:49 PM: Message edited by: iwire ]

 

[celtic]

Re: derating help

What you need to look at is:
Table 310.15(B)(2)(a): Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable

Then, based on the load for any given circuit - not just your example, determine if the neutrals are current carrying conductors (CCC).

Based on what you have given:
4 circuits, 4 N, 1 G

...and what is not given - are the N's CCC?
...but assuming they are CCC, 8 CCC in a raceway requires a derate of 70%.

Looking at Table 310.16: Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60?C Through 90?C (140?F Through 194?F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30?C (86?F)
- a #12 THHN has an ampacity of 30A (@90?C) - for derating purposes.
30Ax.70 = 21A

If need be, you must also derate for ambient temperature. That derate factor is located at the bottom of T310.16

Looking ahead to ???Table C8: Maximum Number of Conductors or Fixture Wires in Rigid Metal Conduit (RMC) [in back of NEC]
- we see that 3/4" RMC can hold 22 #12 THHNs. So your 9 #12 THHNs fit.
If you wanted to, you could put the conductors in a 1/2" RMC (capacity of 13 312 THHN's).
I would NOT reccommend downsizing the RMC - it could prove to be a nightmare under field conditions.


EDIT: Looks messy, reorganized.

[ February 09, 2006, 06:00 PM: Message edited by: celtic ]

 

[iwire]

Re: derating help

Originally posted by celtic:
Looking ahead to ???Table C8: Maximum Number of Conductors or Fixture Wires in Rigid Metal Conduit (RMC) [in back of NEC]
- we see that 3/4" RMC can hold 22 #12 THHNs. So your 9 #12 THHNs fit.
If you wanted to, you could put the conductors in a 1/2" RMC (capacity of 13 312 THHN's).
I would NOT reccommend downsizing the RMC - it could prove to be a nightmare under field conditions.

Maybe....if you pull real hard and use lots of lube.

You where reading the wrong columns.

1/2" RMC can hold 9 - 12 AWGs and 3/4" can hold 16 - 12 AWGs.

In general it does not pay to run more than 9 CCCs in a raceway for 15, 20 or 30 amp branch circuits.

[ February 09, 2006, 06:29 PM: Message edited by: iwire ]

 

[celtic]

Re: derating help

Originally posted by iwire:
]Maybe....if you pull real hard and use lots of lube.

You where reading the wrong columns.

1/2" RMC can hold 9 - 12 AWGs and 3/4" can hold 16 - 12 AWGs.

I had the right column, wrong row ~ I had #14's highlighted in my book....if you lock nuts would stop hiding my glasses, we wouldn't have these problems.

But other than that dyslexic manuver, how was the answer?

 

[celtic]

Re: derating help

Originally posted by iwire:

In general it does not pay to run more than 9 CCCs in a raceway for 15, 20 or 30 amp branch circuits.

Convenient how the derating works out quite nicely w/9 CCCs, isn't it

 

[roger]

Re: derating help

Originally posted by celtic:
But other than that dyslexic manuver, how was the answer?

Long winded.

Roger

 

[celtic]

Re: derating help

Originally posted by roger:

Originally posted by celtic:
But other than that dyslexic manuver, how was the answer?

Long winded.

 

[charlie b]

Re: derating help

Originally posted by killer76:I have 4 circuits (1) 20 amp receptacle on each, my wire is #12 THHN the ground has been daisy chained so I have only 9 wires that will run to the intended j-box. . . .

This may be obvious, but I'll point it out anyway. You have to count all 9 wires for the purposes of conduit fill. But you never count the equipment grounding conductor as being a "current carrying conductor" (CCC), for the purposes of derating. You will note, for example, that Celtic's explanation talked about your situation having up to 8 CCCs.

 

[killer76]

Re: derating help

In my circuits I have one run with 4 H 4 N 1 G

in another conduit I have 3 H 3 N 3 G

I understand now that the conduit size is not the issue, however I would like to be able to explain,

when questioned about why I would not need to use the 1" conduit, I have been doing this for so long I just need a method of calculation

maybe some could show me were we get the 70 degree, is that just the ambient conditions of the location

if so perhaps a simple equation 101


thanks for all your help

 

[1793]

Re: derating help

You will note, for example, that Celtic's explanation talked about your situation having up to 8 CCCs.

If there are 4 separate circuits 4 phase conductors and 4 grounded conductors, neutrals, then would there not be but only 4 ccc per 310.15 (B)(4)(a)

(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying provisions of 310.15(B)(2)(a).

So we could use an adjustment factor of 80%.
Is this not so?

 

[mario]

Re: derating help

for conduit size see annex c table 8 ...

 

[charlie b]

Re: derating help

Originally posted by 1793: If there are 4 separate circuits 4 phase conductors and 4 grounded conductors, neutrals, then would there not be but only 4 ccc per 310.15 (B)(4)(a). . . Is this not so?

It is not.

If you have one circuit that comprises one hot and one neutral, then both of them are current carrying conductors. That particular neutral is carrying every bit of the current that the hot is carrying. It is not just carrying any kind of "unbalanced load." Similarly, if you have four sets of such circuits, comprising four hots and four neutrals, then each and every one of the neutrals will carry 100% of the current that is carried by its respective hot conductor. None of the four is carrying unbalanced current. Therefore, you are looking at 8 current carrying conductors, not four.

You only get to talk about "unbalanced current" when you have one neutral that is associated with two or more hot conductors. The neutral in a 3-wire single phase system and the neutral in a 4-wire three phase system are the most common examples. These will only carry unbalanced current from the 2 (or 3) hot conductors (presuming the loads are linear).

 

[charlie b]

Re: derating help

Originally posted by killer76:maybe some could show me were we get the 70 degree. . . .

It's not "70 degree," it's a "seventy percent derating factor." It comes from Table 310.15(B)(2)(a). If you have 7, 8, or 9 current-carrying conductors in the same raceway, you derate the ampacity of each by a factor of 70%.

 

[1793]

Re: derating help

Charlie:

Thanks for your post. I have been looking at various threads, posts, on topics like this and as I'm sure you know after reading different replies and thoughts on topics we start to confuse ourselves. The more I think about topics the worse it gets.

Again thank you for your reply.

 

[roger]

Re: derating help

Norb here are some examples that may help you visualize this.

In a Two Wire Circuit, both conductors are Current Carrying.

If you took an ammeter and read both conductors separately, you could have 40 amps accumulatively between the two conductors.

In a Three Wire Single Phase Circuit, the neutral carries only the unbalance current of the two legs and therefor does not have to be counted as another Current Carrying Conductor.

If you took an ammeter and read all three conductors separately, you would never have more than 40 amps (the sum of both circuits if loaded 100%) accumulatively.

See 310.15(B)(4) for Wye connected neutrals.

Roger

[ February 10, 2006, 05:32 PM: Message edited by: roger ]

 

[celtic]

Re: derating help

Originally posted by killer76:


maybe some could show me were we get the 70 degree, is that just the ambient conditions of the location

Originally posted by celtic:

Table 310.15(B)(2)(a):
Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable

Has nothing to do w/ambient temp. Ambient temp is another derate - IF applicable.

Originally posted by celtic:

Table 310.16:
Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60?C Through 90?C (140?F Through 194?F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30?C (86?F)
-

If need be, you must also derate for ambient temperature. That derate factor is located at the bottom of T310.16

 

[1793]

Re: derating help

Roger:

I do appreciate your reply. I have a tendency to get bogged down in the wording of the NEC and begin to doubt myself.

I did not think my post was correct but I have been proven wrong before. The diagrams are helpful and thank you

 

[bjp_ne_elec]

Need some clarification regarding Roger's post - wouldn't this be the case in a three-phase system, as opposed to a single phase (that's what Roger's post talks to) situation?

Never mind - I re-read Roger's post - and he specifically says single phase three wire circuit - which would have two hots and a neutral.

Thanks

Brett