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).]
