Here's a situation maybe you all can help me with. On one particular design / build project, the electrical subcontractor is proposing to place branch circuit wiring from two different panelboards in the same conduit run. The idea here is to feed a string of light fixtures from alternate panelboard circuits and save a little on conduit in the process.

The first light would connected to a circuit from panel A, the second light on panel B, the third on A, and so on alternating back & forth. The string of lights is between the two panelboards, so the branch circuit conduit would run from panel to panel with a J-box at each light fixture location. The light fixtures themselves are surface wall mount fluorescents mounted just below the J-box, and have a short piece of conduit to connect to the afore mentioned J-box.

Back when I was a field foreman, we never dreamed of doing something like this. The reasons seem so obvious, but I am unable to find anything specific in the NEC that precludes it. In fact, NEC 300-3(c)(1) would appear to support their proposed design.

Thanks for any clarification,
Radar

Radar I am not sure what you mean by


but code wise this installation is fine.

It is just a design issue.

Of course that is assuming other code articles are followed like derating etc.


Bob

The only reason I wouldn't do something like that is if the came from different meters. I don't like having metering mixed, even if they are for the same customer. And seeing that they are from panels from different locations, I would probhably use striped wire for one. Just to make sure they don't get connected by the neutral by accident, although that would only be required if they were from different transformers, but still a wise idea.

Don't really understand why, the guy is doing it. Does this serve some purpose? Like say one was from a UPS, or generator.

Yeah, my question is also,why? It's legal. After all, circuits of different voltage can be in the same conduit so it stands to reason that two sources is OK. It's just a strange design.

One code issue that comes to mind is the requirement that neutrals from different systems be identified; you are required to have all the conductors of a circuit come from the same source. Mixing them up in the same pipe can lead to all sorts of interesting problems.

reno keep in mind different panels are not necessarily different systems.

Check out e57s post. ;}

IWire- nice smile! I don't want to get bogged down in what is a 'different system.' It just doesn't seem like a good thing if the "hot" wire comes from one panel, and the neutral goes to another- even if the two are simply separate load centers from the same service!

I believe the code says all conductors for a circuit must be in the same raceway....I would think that meant the neutral has to go back to the panel the hot comes from. If you don't identify the neutrals, it is all too easy to get them mixed up....and that's where the trouble starts.

So how do you identify a nuetral? My personal favourite way is to run a colored "Sharpie" along the wire, leaving a SINGLE stripe. That way, I know the red stripe goes with the red wire, etc.

Thanks for all your input. To answer Bob's first question, and as addressed in other posts, this concept seems repulsive to me from a workmanship point of view, the possibility for errors by installers and/or future workers is just too great and very dificult to troubleshoot. I and another foreman once spend a few days in a new highrise in LA tracking down and correcting a half dozen or so crossed neutrals in exit lights (emer & norm). I can't tell you how much fun that was!

Without being specific concerning location, the two panels in this case are far enough apart that they are most probably on different meters, although the same customer and same utility. I'm thinking they're about a mile apart. Voltages are the same, but different utility meters.

The reason for the alternating panel circuits in the first place is that's what is indicated in our preliminary (as-bid) design. We do this so that in the event that an entire auxiliary power station bites the big one, we don't loose all the lighting. Every other light is still powered from the neighboring power station. Again, I am avoiding specific locations here.

I believe the main reason for considering the single conduit is to economize; minimize costs on a firm fixed price design / build contract. After all, a single conduit is less expensive to provide and install than two are, even if it is a size bigger.

Thanks again for the great responses,
Radar

Radar,
Oh! Wait!!! The "separate meter" part is BIG. That gets back to the reason for 230-2, especially (e)Signage.
Think of the Fire Dept Responder who thinks he just de-energized a power to the room before they pump 125 GPM of fire water all over, only to find half the circuits in the pipe still energized.
Got to have separate conduits, signage/labeling, "billboards, red flags, pom-poms".
Sorry, I got carried away!

Well, we would fall under the 'By Special Permission' claus in exception #4 of 230-2(a). Besides, I'm only reasonably, not entirely, certain the panels will be metered separately.

However, this illustrates my concern. I think the fire responder issue would apply to anyone who may have to work on the system in the future. Imagine getting into a J-box thinking you've killed the circuits and discovering there are others from who knows where that are still live?

I think this is a really poor design, but am not able to locate a code section to prevent it. Thanks again for your help,

Radar

If you are worried about power from one meter going down, why not run power from both to a transfer switch and then hit the lighting. I think this would be a better solution.

I know that this sounds somewhat unorthodox, but what about using NM-B inside the conduit?

Since you are going to be alternating fixtures, you could pull the cable _through_ every other junction box, and never have conductors from different systems exposed in the same junction box. This would seem to eliminate the problem of accidentally mixing conductors from different panels.

-Jon

Yep this would be a 230.2E requirement if you are allowed to do it. (If it is two services!? You should check that "Special permission" before you build it...) And if it is allowed, still a bad idea, (from two services, especially if not located together.) its not like two different systems of different voltages. You would need the signage to let you know there were two sources... See 225.37

Although these would be two different systems, and need to be identified as such. That means Neutrals and Grounded conductors to identify "system" if in the same conduit, or boxes. Although they might be the same voltage, if they are on different transformers, they are different "systems".

A while back I did a resturaunt with 4 different transformers to 8 panels, all same voltage. It was easier to run all the circuits together. So I got rolls of all my colors... ABCN with black stripe, ABCN with red stripe, ABCN with blue stripe, and no stripes. (The black, red, and blue were slightly different than main color with contrasting boarder.)

My wife works at a disaster control center, and it has several services, and generators all right next to eachother. And a really interesting array of transfer switches. All can be switched to different loads. And it can shed non-critical, all the critical stuff is on UPS. If the door wasn't always locked, I'd get a picture... But it has a huge warning on the door, "Authorized personell only - This facility powered by multiple sources"

Are you marking your own neutrals in some way? Is white THHN available in striped varieties? Not in my area, at least off the shelf. The mistakes happen when care is not taken in labeling each end of wires. This thread has me wondering what is available for neutrals from the factory.

The Romex in the conduit idea seems to me like it would add more difficulties than it would help, since space and code issues limit such installations severely.

One point worth noting here: This is a design / build contract, and the bid documents described basically what we want (the alternating panel circuits on adjascent fixtures idea) without much guidance on just how to design it as long as the design complies with all codes & ordinances. In other words, the contractor is free to design this any way he wants, as long as it meets the performance criteria and is code compliant.

Any design interference from us regarding specifics of his design would result in a contract change. If we now tell him to install separate conduits, or install NM inside the conduit, or whatever, we are changing the contract from the original condition, and affecting his cost base.

OK - so here's another question: NEC 700-9(b) states that wiring from emergency source to emergency load shall be kept entirely separate of all other wiring, other wiring shall not enter the raceway, etc (paraphrased). The obvious exceptions are inside transfer switches, exit or emergency fixtures, light outlets for exit or emergency lighting, etc.

The question is this - in this example, if the branch circuits feeding these lighting units are emergency circuits (at both panels), which are on different meters, and therefore different systems, can they be mixed as proposed above, or must they be kept separate according to 700-9(b)?

Thanks again for all the great replys,
Radar

Radar,

I am just going to repeat you to make sure that I understand where you are coming from.

You (your company) has a contract with a sub, and the sub has the leeway to do 'anything' as long as it meets both the performance criteria _and_ code. The sub has come to you with a proposal which, as far as you know, meets both the performance criteria and code, but does so in a fashion that you are leery of. I presume, then, that you want to achieve one of the following:

1) Find a code reference that makes this proposed design untenable, so that the sub simply can't do it the way that you don't like.
2) Find a method that is the equivalent of what the sub is proposing, but addresses your safety concerns.
3) Find a method that is almost the equivalent of what the sub is proposing, with a slight change in performance criteria that will be easy to negotiate.
4) Decide that you can't force this issue and give up, but at least you know that you tried.
5) Decide that you can't force this issue and re-negotiate a different set of performance criteria with the sub so that they have no reason to use the design that you don't like.

IMHO, your best bet is option 3. I do not see a _significant_ hazard in this installation, as long as the wires from the two different systems are very well segregated. If the conductors from system A simply pass through the JBs for system B, and vice-versa, and if the colors for system A and system B were easily differentiable, then the chance of error will be very slight. Since the sub has no reason to cut the wires from one system in the JB of the other system, this is not going to be a significant performance requirement. However I think that you could only make a code case for identifying separate neutrals.

Identifying _all_ of the wires is IMHO a small performance change. I suggested using romex as a _cheap_ way of identifying all of the wires; but perhaps one could use E57's technique of using custom colored wires, or perhaps pre-twisted wire sets. This sounds like a large enough job that the cost of custom colored wire would be a small premium; in fact you might consider doing the _entire_ job with different colors from each supply. Since this is a change in the performance criteria, I'd presume that you would have to negotiate it, and would have eat the materials cost increase. But I'd think that the cost increase is probably slight.

These latter approaches would result in essentially the same labor as currently proposed. Wire manufacturers can easily make different colors or standard colors with custom striping, and can also easily duplex/triplex/quadruplex your conductors. If you absorb the change in materials cost, this might be very easy to negotiate with the sub.

-Jon

Just to clarify a bit, I currently work for a public agency (quagmire, doonboggle, etc) and we are the customer in this case. We contract with a GC for the design/build work, and the GC's electrical subcontractor is the topic of this thread. They're not our subcontractor per se'.

Our preference would be for the contractor to run one conduit with it's associated wiring from one of the panels, with J-boxes at every other light fixture; and another conduit with it's associated wiring from the other panel, with J-boxes at the remaining every other fixture location. Our problem is that we did not communicate our preference, only the requirement that every other light be connected to one panel, the remaining to the other panel.

Radar

A clarification: If the wiring from the two different panels, are indeed from seperate system transformers, then ALL of the wiring needs to be identified to each system. Neutrals, Phase, and control/switching too.

See 200.6D for neutrals, and 210.4D


The way I have understood this, is that identification is required for each system, not just when they are of a different voltage. Like if you have 120/208, and 277/480 in the same conduit, the same would be true for two different systems of 120/208, or two different 277/480's. This reduces the chances of connecting them, and coming up an 8-wire some weird voltage system. As they are two seperate voltages even if they are the same! What voltage do you get from A1 - A2?

And yes, striped wire is the best way I know of doing this. I get it for the same price as I do regular wire, just have to wait longer for it. If in a rush I get NAILED on shipping though.

Now no one has brought this up... How are these two systems or even if the are just different panels bonded together? The conduit with the lights on it? A water line?

I beleive they need to be grounded at the same place and be sized from Table 250.66.

Now wait... The grounds for each service, from seperate power stations, are MILE APART?! Hope the guy putting the coupling for this together has a WILL. As reguardless of bonding, your lighting conduit will be a parralel path to ground for each service, and likewise the bare-handed guy putting it together.


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

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

Good point, e57!

As you suggest, having them bonede at the same point is a way to do it.
So would having them tied to the same "ufer" or grounding grid.
Or, I think simply running a #8 bonding wore panel-to-panel would meet the requirement. Or- maybr not! Do we need to protect both services from lightning if they are far enough apart?

Reno you chimed in there wile I added/edited that last parragraph.

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.