Concerning the requirement to not mix voltages in conduit, I tend to think that it is OK to run, say a dryer circuit with a laundry circuit in the same conduit (assuming all other codes requirements are met), being all the same potential to ground, but another opinion by another EC was that it would be a violation.

Besides, 300.3(C)(1) says its OK, don't it?

What requirement not to mix voltages? You are always permitted to mix power circuits as long as all conductors are rated for the maximum voltage in the raceway and the maximum voltage is 600 volts or less. It is not a code violation to have 120/240 circuits, 120/208 circuits and 480/277 circuits in the same raceway with each other. When you have control or signaling circuits, you can't, in general, mix them with power circuits. You need to apply Article 725 for those circuits.
Don

Another non-existant code I was brought up on...

I wonder how it got started? The EC in question wasn't trained by Jim Hixson as Joe and I were... I even "remember" a clause on voltage to ground to include all poles of multiwire circuits and the like...

Weird.

Thanks for clearing me up, Don.

There is a code restriction against having devices in the same undivided box with voltages over three hundred between devices. This is often what folks are remembering when they think that different voltage systems can not be in the same conduit or raceway.

In my opinion it is best practice to keep different voltage systems separated in different raceways were practicable but the code does not require it.
--
Tom

404.8(B)

Thanks, Tom. That very well may be the confusion as well as the control circuit angle. Both Mr Hixson and the EC that stated that it was a violation have had lots of industrial experience, and probably thought (wrongly) that the 725 requirements were for all circuits, probably learned by failing an inspection rather than reading it themselves. Just guessing a theory... (and wasting more cyberspace!)

I have heard this several times in the past.
I can't say for sure,but I have a couple of opinions as to why this comes up.

Opinion #1 Mis-interpretation

In article 300 there is a code:
Conductors of different systems- Conductors of high and low voltage shall not occupy the same wiring enclosures.
This code is actually under the sub-heading:
Requirements for Over 600 Volts,

Wiring over and under 600volts shall
(must) be seperated with a few exceptions.

I think maybe some have read this thinking it reffered to 120/208 as low voltage and 277/480 high voltage.

Conductors occupying the same raceway must have insulation rated at the highest voltage contained in that raceway. Therefore 120-208-240v would be covered by insulation rated at 300v, but if you read the writing on a piece of THHN it is rated for 600V.

Opinion #2 Because

My other idea is that this is something that has been taught and carried on as many things we are taught in the trade, without reference to actual codes or requirements. Often we do work in a certain way because we were told to do it that way, and sometimes never question why. Which brings me to my thought as to why many teach/preach not to mix voltage. They are typically refering to 120/208 in the same conduit with 277/480. I will not offer my opinion as to whether it's a good practice,
but I'm sure others will.
And that would be safety concerns. Which could probably lead to a whole new thread of discussion.

[This message has been edited by TE (edited 09-03-2002).]