ECN Forum Forums Code Related Discussion NEC & other Code issues Branch circuits from different panelboards in same conduit

Yeah... 250.30 1-6


[This message has been edited by e57 (edited 06-27-2005).]

e57,

I belive that your read of 210.4 is not correct.

'Nominal Voltage' is clearly defined in article 100, and if you happen to have two delta-wye transformer sets, 30 degrees out of phase (fed from different MV distribution), one with 206V line to line and 119V line to neutral, the other with 212V line to line, 122V line to neutral, these both would be 208Y/120V systems.

Furthermore, 210.4 only applies to multiwire branch circuits.

210.5 speaks to Branch Circuits in general, and doesn't mention any coding requirements for the ungrounded conductors of different systems.

IMHO your interpretation of the code is 'how it should' read Furthermore, I think that the coding we are discussing here is good workmanship.

But as I read things there is no requirement that the ungrounded conductors be any color at all, as long as they are not white, grey, or green. All of the ungrounded conductors from all of the systems could be black.

Note that if we go into this a bit further, if there are _any_ systems anywhere in this building, even ones totally not connected to the services in question, that operate at a different nominal voltage, then 210.4D would apply. So if these lights are supplied by multiwire circuits, then explicit coding/identification would be required for all conductors, which might be more trouble than simply running the circuits in separate conduit.

-Jon

Jon,
"how it should' read" Why Thank You... [swagger]

"I think that the coding we are discussing here is good workmanship." What do you mean by that?

Anyway, 210.4d says, "Where more than one nominal voltage system exists..." As opposed to a system not of "Nominal Voltage". It does not say they have to be two different nominal voltages, it says "more than one". Even though that would be its most common use. There are a few inspectors here who read it that way too. Then again they have a tendancy to read things differently here, its the water. Sorry, Jon, I'm sticking to my guns here.

And, yes it would only apply to multi-wire branch circuits. But you might as well make it easy on yourself, cause it says when they "exist in a building", it doesn't say in the same conduit, just that they "exist in a building".

Anyway, it would be funny, depending on how you look at it, if a hot and neutral got connected by accident. You could sync the generating sources from there. As they could be anywhere from 0-180 dregrees out of phase at anytime. Variations in freq from either, and you'd have that eery dimming effect. Actually, you might have some of that anyway, as the lights will stobe differently. I've seen it when doing large multiple generator installations in the military. Over a distance you get that road glare type effect. That building is going to be spooky. This is one job that needs an engineer, as it might need one in the future.

If it were me... I would take both sources to the same location, and have a simple transfer switch.

The lighting circuits in question will indeed be multi-wire branch circuits. Specifically, there will be 3 circuits from each of the two panels, so 3 phase wires plus one neutral and one ground conductor (something we generally require regardless of type of conduit used) for each panel. 10 wires in all, 6 ckts, 2 neutrals, 2 grounds, all in one common conduit.

The separate electrical systems will each be bonded at the source according to code. I see no indication that there is any intent to bond the two systems together, other than the obvoius path of the common conduit run.

One last question: What is the probability of there being a significant difference in ground potential between the two service points which are about a mile apart? We are dealing with different service points with a good deal of separation, but the same utility, and maybe or maybe not fed from the same utility substation.

If there were a difference of ground potential, then connecting the two points via a conduit path would result on a chronic small current flow through the conduit (at least until one end ran out of electrons ).

Radar

I'd say that the two separate systems' conduits and conductors should remain totally separate, to completely avoid the possibility of either an accidental or unwittingly-intentional interconnection, fault, or shock hazard.

In addition, the source of each circuit should be very clearly marked at each and every access point. It would be way to easy for someone to expose themself to a hot conductor, thinking they have deenergized a circuit.