parallel 1/0 conductors in PVC

Status
Not open for further replies.
S

sokkerdude

Member
Location
Arkansas
This is not a coed question. If I was to parallel 13- 1/0 CU THWN conductors all the same phase let say "A" phase in a 4 in. PVC conduit, that does not pass through any metal. Would there be any reason to derate according to table 310-15. Other than the coed say to. Where or how would the heat be produced? What would be the electrical nature for the conductor to heat up, if any? Thank You..
 
Re: parallel 1/0 conductors in PVC

The heat is caused by the I squared R losses in the conductors and that does not change if all of one phase is in the same raceway. In fact there will be some additional heat in your proposed installation do to the inductive reactance caused by separating the phase groupings.
Don
 
Re: parallel 1/0 conductors in PVC

Sokkerdude, The code doesn't say you have to derate these conductors, all you have is one current carrying conductor.

Read the last ten words in parenthesis in 310.4 before the exceptions.

Roger

[ September 07, 2003, 01:06 PM: Message edited by: roger ]
 
Re: parallel 1/0 conductors in PVC

Roger, are you saying that if I have to run a 400 amp 3 phase feeder and I choose to use 2 sets of 3/0 THWN, I can put all 6 conductors in one raceway and not have to derate?

This is a puzzler to me, why would we ever bother to run multiple raceways?

I understand what the code says in 310.4, but does the conductor? ;)

They are all still carrying current and producing heat.

Bob

[ September 07, 2003, 02:15 PM: Message edited by: iwire ]
 
Re: parallel 1/0 conductors in PVC

Hello Bob, I should quit doing this, but I'm just picking on another wording flaw in the code book. :(

You and Don are correct.

Sokkerdude, I appologize, but then again. ;)

Roger
 
Re: parallel 1/0 conductors in PVC

Originally posted by roger:
Hello Bob, I should quit doing this, but I'm just picking on another wording flaw in the code book. Roger
Click to expand...
No you should not quit doing this, but I should recognize when you are.

You made me think about this and I agree if we take what 310.4 says literally,(which is how we are supposed to take a code article aren't we?) we do not have to derate the conductors.

What's up with that? :)

Bob
 
Re: parallel 1/0 conductors in PVC

The NEC should be rewritten in simple, plain, non-contradictory, specific English.

The NEC is a great thing and I am not belittling the hard work that goes into making it, but it's too bulky and too confusing for the average electrician.

The NEC (and not to belittle electricians) should be written to no more than a sixth grade intellect. Understanding the code in its current form is a burden and a challenge.

I say this with the utmost respect for the book and for those who authored it. I'm just saying that it's out of hand.

Please don't be a hater and please don't slay the messenger.

Most respectfully,
../Wayne

PS: I just tried the Spell Check. It caught a few words. It's a very different spell-check format than I am used to but it worked! So much for ieSpell !
 
Re: parallel 1/0 conductors in PVC

Wayne: I agree with your fraustration. I don't agree with writing the code at a sixth grade level. Too me that would be up.

I feel it should be written by professional English linguistics, using accepted words and format. This should be at least in the high school level of reading comprehension.

There is a lot of ground swell, about the NEC being a monopoly. I expect a lot of changes in the near future, or the complete crash of the code as we now know it.
 
Re: parallel 1/0 conductors in PVC

The cable will produce heat just because of resistance loss But

don
additional heat in your proposed installation do to the inductive reactance caused by separating the phase groupings.
Click to expand...
inductive reactance heating should not happen if the phase is run in plastic as sokkerdude said

sokkerdude
PVC conduit, that does not pass through any metal.
Click to expand...
But I don't see where the code allows for the instlation of the same phase in the same conduit other than underground?
I have always have thought that they must be installed so that all conductors of a circuit are in each raceway?
Look at 300.3 (B) (1)

(1) Paralleled Installations. Conductors shall be permitted to be run in parallel in accordance with the provisions of 310.4. The requirement to run all circuit conductors within the same raceway , auxiliary gutter, cable tray, trench, cable, or cord shall apply separately to each portion of the paralleled installation, and the equipment grounding conductors shall comply with the provisions of 250.122. Parallel runs in cable tray shall comply with the provisions of 392.8(D).

Exception: Conductors installed in nonmetallic raceways run underground shall be permitted to be arranged as isolated phase installations. The raceways shall be installed in close proximity, and the conductors shall comply with the provisions of 300.20(B).
 
Re: parallel 1/0 conductors in PVC

hurk,
Anytime the conductors of an AC circuit are separated, the inductive reactance of the conductors goes up resulting in additional I squared Z (R) losses in the conductor. This is the source of the additional heat, not the inductive heating effect that would be observed if the raceway were ferrous.
Don
 
Re: parallel 1/0 conductors in PVC

hurk,
But I don't see where the code allows for the instlation of the same phase in the same conduit other than underground?
Click to expand...
I don't know why the exception to 300.3(B)(1) even exits. I think that 300.3(B)(3) permits this type of installation anywhere.
don
 
Re: parallel 1/0 conductors in PVC

Wow that is a good question. as it does seem that 300.3 (3) would allow for this type of install. Do you think this exception is just one more erta that got over looked?
 
Re: parallel 1/0 conductors in PVC

Originally posted by don_resqcapt19:Anytime the conductors of an AC circuit are separated, the inductive reactance of the conductors goes up?.
Click to expand...
This is true. And it becomes worse if (1) You add more parallel conductors to each phase, and (2) You keep the conductors of each phase close to the other conductors of the same phase, and (3) You keep the three phases far separated from each other. That is the basis for the requirement that we run a set of A, B, and C in each paralleled raceway.

Originally posted by iwire:Are you saying that if I have to run a 400 amp 3 phase feeder and I choose to use 2 sets of 3/0 THWN, I can put all 6 conductors in one raceway and not have to derate?
Click to expand...
With all six conductors close to each other (specifically, with any one of the Phase A conductors being about as close to a Phase B conductor as it is to the other Phase A conductor), the mutual inductance tends to vanish. All you are left with is the I2R heating. But since each of the six conductors will be inflicting its own I2R heating on each of the other five, the cumulative heating effect forces us to derate the lot.

Originally posted by sokkerdude:Would there be any reason to derate according to table 310-15.
Click to expand...
Back to the original question, and please allow me to rephrase the original question as follows: ?If you install something in violation of one code section, do you still have to comply with the rest of the code?? The correct answer is ?Go fix the installation, and ask us again.? But we all realize that what you are really asking is whether parallel conductors of the same phase will produce the mutual heating effect that is the reason for 310.15(B)(2) derating. That answer is ?Yes,? as others have said.
 
Status
Not open for further replies.
Top