Voltage Drop?

[dude454]

We have a inspector saying that you are only aloud to put three conductors in a conduit do to voltage drop, regardless if you use de rating factor.

He says the only way you can put more is to have your wires pre twisted do to impedance loss.

He says the caculations for this is in the code book. The only table I can find relates to 3 wires or less.
Is there a table for more than three wires?

Could he be he right, and it is against code?

Thanks for any replies
Ron

 

[bennie]

Re: Voltage Drop?

I was wondering what happened to that guy who designed the Edsel.

I would schedule a meeting with the chief inspector, and reach an agreement. This person is dangerous. I hope the chief inspector is not his father.

Good luck; Bennie

[ October 18, 2003, 04:32 PM: Message edited by: bennie ]

 

[iwire]

Re: Voltage Drop?

No it is not against code and it is done daily.

I do not know what you can tell him to make him see the light.

 

[roger]

Re: Voltage Drop?

No, he is not right. Ask him why the NFPA would waste money and ink to include table 310.15(B)(2).

Then ask him where you can buy 500 mcm twisted pair, and speaking of twisted, never mind.

Seriously, he is responsible for citing the violation, not just saying it's in the code book.

Of course just making this bogus of a statement indicates he wouldn't understand the NEC.

Roger

 

[dude454]

Re: Voltage Drop?

Thanks everybody for such a quick response!

Just kind of wondering why the don't have a table for more then to conductors, to figure voltage drop.
Since I think as you add conductors the impedance in each wire would have to change.

Is there a way to figure this as the amount of wire increase in a pipe.

So all have some formula on paper to show him when I set up the meeting?

Thanks
Ron

 

[iwire]

Re: Voltage Drop?

Others may be able to expand on this, but as far as I know Voltage drop is affected by the current, voltage and resistance.

I have never heard of voltage drop being affected by the number of conductors.

The number of conductors effects the heating of the conductors.

 

[ryan_618]

Re: Voltage Drop?

I read this right after it was posted. I was going to write: "this is the stupidest thing I've ever heard". I haven't thought of anything better so:

THIS IS THE STUPIDEST THING I HAVE EVER HEARD!!!

 

[iwire]

Re: Voltage Drop?

Originally posted by roger:
Then ask him where you can buy 500 mcm twisted pair,

Come on Roger just pull 3 or 4 of them out across the floor, chuck them up in a cordless drill and twist em up.

It does not get difficult until 1000kcmil.

 

[roger]

Re: Voltage Drop?

Bob, I'm showing my age I guess, the 500' spools of 500 MCM I used to carry around on my shoulders seem to be a little more dificult now.

Roger

 

[bennie]

Re: Voltage Drop?

I hate to be the bearer of bad news, but everyone is correct in some parts of this issue.

Inductive reactance will reduce with twisted conductors, however capacitive reactance will increase. The sum total is the capacitive reactance will depend on the applied voltage, inductive reactance will depend on the intensity of the current. Capacitive reactance does not produce an EMF field of self inductance.

Notice all cable assemblies other than NM sheathed building wire have twisted conductors.

It is not practical to twist the individual conductors and thankfully it is not required.

 

[Guest]

Re: Voltage Drop?

Originally posted by iwire:
<snip> Or ask him why there is a table for derating for more than three conductors in a raceway. <snip>

Bob,
Isn't this the table that Dude can't find?

[Edit add: Whoops! Dude wants a voltage drop table not a derating table for 3+ conductors?].

[ October 18, 2003, 08:38 PM: Message edited by: awwt ]

 

[steve66]

Re: Voltage Drop?

As far as voltage drop is concerned, the code has no requirements. Only suggestions for keeping losses in the wire to a reasonable level. For the most part, voltage drop isn't really a safety issue.

Steve

 

[stevebrown]

Re: Voltage Drop?

As far as Code requirements go, of course there are none.

Just to make sure, this is 60 Hz power, correct? We're not looking at 400 Hz aircraft power or something else out of the normal realm?

At 400 Hz, accepted practice down here is you should be twisting the conductors.

Tell your inspector that you are really concerned over excessive voltage drop, and to do your part you are going to add spacers to keep the conductors perfectly centered in the conduit and will also get the HVAC contractor to supply refrigerated, dry air to the conduits.