LV/MV Mixed

Status
Not open for further replies.
B

Brandon Loyd

Senior Member
Hello all,
I've come across a strange installation where the client has run MV 4800V conductors through an existing 480V switchboard, and through the XFMR connected to the SWBD, to serve as the line side feed to an LIS that feeds the same SWBD and XFMR.

So two questions: 1) Can 4800V conductors pass through 480V SWBD and 4800V-480V XFMR? 2) If you open the LIS to de-energize the XFMR and SWBD, the 4800V conductors that pass through are still energized, is this allowed?
 
Maybe for both questions. You might want to check sections 408.3(2) and its exception, 408.4(B), 408.5, and 408.19.
Note: Just because you can, doesn't mean you should.
 
It's a little tricksy...

Technically, if you are looking at LV equipment, 300.3(C)(1) would come into play, requiring that ALL conductors have a rating equal to the highest voltage available. So you COULD technically do it if EVERY wire were run with 5kV rated cable, which is totally impractical for 480V wiring.

But then 300.3(C)(2) goes on to say circuits over 1000V shall NOT occupy the same space as LV wiring, UNLESS allowed by following sections, one of which says:
(c) In motors, transformers, switchgear, switchboards, control assemblies, and similar equipment, conductors of different voltage ratings shall be permitted.
Click to expand...
This is where it gets a bit nebulous. In all cases where this has come up for me, it has been LV wiring WITHIN MV switchgear, controllers, etc., because in most cases that is unavoidable, i.e. the 120V control wiring on a MV motor starter or the wiring on a PT circuit of an MV switchboard.

In your case, you are looking at MV wires run into LV gear. Most inspectors I know would not accept that under 300.3(C)(1)
 
Jraef said:
It's a little tricksy...

Technically, if you are looking at LV equipment, 300.3(C)(1) would come into play, requiring that ALL conductors have a rating equal to the highest voltage available. So you COULD technically do it if EVERY wire were run with 5kV rated cable, which is totally impractical for 480V wiring.

But then 300.3(C)(2) goes on to say circuits over 1000V shall NOT occupy the same space as LV wiring, UNLESS allowed by following sections, one of which says:

This is where it gets a bit nebulous. In all cases where this has come up for me, it has been LV wiring WITHIN MV switchgear, controllers, etc., because in most cases that is unavoidable, i.e. the 120V control wiring on a MV motor starter or the wiring on a PT circuit of an MV switchboard.

In your case, you are looking at MV wires run into LV gear. Most inspectors I know would not accept that under 300.3(C)(1)
Click to expand...

Interesting point, however why wouldn’t you apply 300.3(C)(2)(c) given that there is no such thing as a MV switchboard? 300.3(C)(2)(c) is obviously in reference to circuits rated over 1000 volts but in LV equipment. It’s a bad idea all around, but that’s besides the point now.
 
...there is no such thing as a MV switchboard...
Click to expand...
Not generally, but it does exist...
www.se.com

Country Selector | Schneider Electric Global

Country Selector Page
www.se.com www.se.com

I don't consider the issue to be obvious to anyone, hence my saying it's "tricksy... It will be up to an AHJ to accept it or not, and in my experience, you could try to make your argument of it being "obvious" by virtue of the word "switchboard" in the NEMA sense of the word, but I think you would be losing that argument if the AHJ chose to enforce 300.C.1. The INTENT (as I see it) of 300.C.2's opening statement is to NOT allow them to be mixed, except in, as I said, MV equipment where it is unavoidable, because it is inherently dangerous to have MV cables running through LV gear.
 
Jraef said:
Not generally, but it does exist...
www.se.com

Country Selector | Schneider Electric Global

Country Selector Page
www.se.com www.se.com
Click to expand...
That is switchgear. Am I missing something? (the documentation does make matters confusing since the marketing material seems to use switchboard and switchgear interchangably)

Jraef said:
I don't consider the issue to be obvious to anyone, hence my saying it's "tricksy... It will be up to an AHJ to accept it or not, and in my experience, you could try to make your argument of it being "obvious" by virtue of the word "switchboard" in the NEMA sense of the word, but I think you would be losing that argument if the AHJ chose to enforce 300.C.1.
Click to expand...
I think anybody would lose any argument with an AHJ having that mindset, so I don't think that makes what I said any weaker of an argument. What I mean by obvious is that I am making no inferences, its verbatim. Maybe apparent would be the better choice of word. Either way, the manner in which the rule is currently written, allows it. Whether it's approved or not is another story.

Isn't the NEMA/ANSI/UL/IEEE product safety standard sense of the word what matters here?

Jraef said:
The INTENT (as I see it) of 300.C.2's opening statement is to NOT allow them to be mixed, except in, as I said, MV equipment where it is unavoidable, because it is inherently dangerous to have MV cables running through LV gear.
Click to expand...
I agree its a bad idea all around, no issue there.
 
Last edited:
Ideally, if the intent was to allow incidental LV control wiring in a space primarily fo MV wiring, the code should read "LV wiring shall not occupy the same space as wiring over 1000 V except....."
The way it is actually written suggests that it is aimed specifically at MV wiring occupying a primarily LV space. But that is probably not what is actually meant.
 
Status
Not open for further replies.
Top