Circuits in Raceways

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champion

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I am attempting to lay out circuits on the electrical schematic for a small commercial job.
One of the raceways that I am laying out will consist of a 208v 3ph ckt for a rooftop A/C unit and one 120v ckt for a maintenance receptacle to be located near the A/C unit.
A registered PE is indicating that I can't run the 120v ckt in the same raceway as the 3ph ckt to the air conditioner.
Also---I am planning to power parking lot lighting from 208v 3ph supplies connecting 208v lights between phases a-b, b-c, a-c.
Each ckt will connect to six lights and each parking lot pole mast consist of 4 1080 watt fixtures which means that on some of the pole mast, two lights will be supplied from part of one ckt and the remaining two on that mast will be supplied from a second ckt.
The PE would like me to re-design so each pole or mast only has one ckt tied to it.
Confused---or does this PE know something I don't understand.
A little reassurance would be appreciated.

Thanks in advance for feedback

Jim
 
Re: Circuits in Raceways

As far as the roof top and the maintenance circuit in the same raceway, I see no problem. The PE is out to lunch on this one. However I do agree with his recomending only taking one circuit up each pole. There are other posts regarding this issue. The bottom line if you go to service daid light and do not know there are 2 circuits bad things could happen. I know we are always supposed to check and never assume. But that is why they call them accidents
 
Re: Circuits in Raceways

I agree with the other Jeff, as well. I saw something in the handbook last night that might be helpful here. There was an illustration showing two different circuits of different voltages being run into the same junction box. The point of the illustration was to show how the high-voltage and the lower-voltage grounded conductors must be identified to make sure they are not confused between systems. I've never seen anything in the code that said these different circuits must be run in separate raceways. It seems to me that if they have rules about having to mark the grounded conductors to keep them from being confused, it must be OK for the conductors to be run together. Otherwise, why would they need such a rule? I believe I saw it in article 200 or 210 (it was the article about identification of conductors). I'd ask the PE for a code reference if he thinks running the two circuits together is not legit.

As for the circuit splitting, I also agree that while your design might be legal, I think it would be better practice to have only one circuit on each pole.
 
Re: Circuits in Raceways

300.3(C)(1) allows you to put any ac or dc circuit rated 600 volts or less in the same raceway. It requires that all conductors have insulation systems that are rated for the highest voltage system in the raceway. The 120 and 208 volt conductors you described are both rated for 600 volts, so this is legal.

As to your second question, it is a design issue, not a code issue.
 
Re: Circuits in Raceways

If we follow the code to the letter, wouldn't putting the 208V, 3 phase and the 120V 2 wire in the same conduit require derating the conductors?

Chances are that the receptacle will only be used when the AC unit isn't, and vice versa. And what if the AC unit is 400A, and the receptacle is only 20A. Do we really need to derate 400A wire by 20% (80A) just because it shares a conduit with a 20A circuit.

Steve
 
Re: Circuits in Raceways

Originally posted by champion:
I am attempting to lay out circuits on the electrical schematic for a small commercial job.
One of the raceways that I am laying out will consist of a 208v 3ph ckt for a rooftop A/C unit and one 120v ckt for a maintenance receptacle to be located near the A/C unit.
A registered PE is indicating that I can't run the 120v ckt in the same raceway as the 3ph ckt to the air conditioner.
<rest snipped>
Jim
Click to expand...
I am inclined to agree that you do not "have" to do what he wants to meet code, but there are other issues involved such as derating of the conductors, conduit size, and being able to have only a single power source in a conduit. Thats some times a usefull thing to do. I personally do not like running multiple power sources in the same conduit. If you need to change something later in the conduit, its not always clear what wire is what circuit.
 
Re: Circuits in Raceways

Originally posted by steve66: Do we really need to derate 400A wire by 20% (80A) just because it shares a conduit with a 20A circuit?
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Good point. But I think the answer should be ?yes.? Not that there would be a significant threat to the health of the 400 amp wire. Rather, putting a full-rated current through the 400A wire would raise the temperature of the conduit enough to threaten the 20A wire. If both wires are derated to 80%, then neither would be at risk of overheating.
 
Re: Circuits in Raceways

Jim,

One of the open items we have here is the relationship the PE has to the project. If he is the client or client?s agent; well, he can have whatever he?s willing to pay for.

If he?s simply looking over your shoulder as the client?s consultant or acting as an AHJ ? for example, for an insurance company; he needs to clearly justify any assertions of NEC violations of your proposed design with applicable citations. And there?s no reason you shouldn?t ask for it, PE or not.

If he is the engineer of record and has designed it the way he is asking for now and you are attempting a bit of ?value engineering,? he gets what he wants and doesn?t need to justify a Code compliant design to anyone installing it.
 
Re: Circuits in Raceways

As for the circuit splitting, I also agree that while your design might be legal, I think it would be better practice to have only one circuit on each pole.
Click to expand...
However I do agree with his recomending only taking one circuit up each pole. There are other posts regarding this issue. The bottom line if you go to service daid light and do not know there are 2 circuits bad things could happen
Click to expand...
It is not uncommon to have more than one circuit in a site pole, at least on the ones I work.

One reason would be two levels of lighting, all lights on when the business is open, then when the business closes only some lights on for security.

Another reason, one that I consider good design is two circuits per pole just for the fact if a circuit trips you still have lighting.

This is as Charlie pointed out a design issue. :D

This whole issue of more than one circuit in a site pole being inherently dangerous makes no sense to me.

When we open a junction box do we assume it only contains one circuit?

I often work on commercial refrigeration equipment with both 150 amp 480 3 phase feeders and 60 amp 208 3 phase feeders in the same control cabinet.

If you assume anything in this trade assume you will die if you do not test for voltage before you touch things.

[ September 15, 2004, 06:09 PM: Message edited by: iwire ]
 
Re: Circuits in Raceways

Originally posted by iwire: If you assume anything in this trade assume you will die if you do not test for voltage before you touch things.
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I have never heard that expressed better. Bravo!
:cool:
 
Re: Circuits in Raceways

Help me understand the "2 circuit" issue. The way I'm reading it we're dealing with a single 3-pole breaker serving multiple single-phase loads. I would be more worried about balancing that circuit than I would about safety issues.
 
Re: Circuits in Raceways

Originally posted by sdbob:
The way I'm reading it we're dealing with a single 3-pole breaker serving multiple single-phase loads.
Click to expand...
We can not assume that it is a common trip breaker or that it is a breaker at all.

240.20(B)(3) allows single pole breakers with handle ties for this circuit. If a line to ground fault happens on phase A, handle ties can not be counted on to open phase B or C.

This circuit could also be fed from a fused disconnect, we still install a lot of fused disconnect switch gear.

Handle ties or fused disconect, either way the light(s) might be out with power still at the fixture(s).

Always check for voltage. :)

Bob
 
Re: Circuits in Raceways

As always, the knowledge and advise from Mike Holt site has come through.
Thank you all for your input and reasurance.
SAFETY FIRST.
I will be considering definite individual supplies for the power poles and will be looking into other options on the parking lot lighting.
By the way, the PE is looking over my shoulder at my request and is not the designer.
Thank you for all advice and reminding me of overlooked options.
Humans like myself can make mistakes.
Extra eyes on design never hurts a thing.

Jim
 
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