Parallel Conductors

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paulgarett

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I installed a temporary back-up generator and installed the emergency conductors to the Automatic Transfer Switch. It was a 3 Phase, 277/480 Volt system, a 600 kW Generator, with a 1000 Amp Circuit Breaker. Three of the conductors I used for each of the 3 phases were 444.4 MCM RHH outdoor rated, "freight-train" cable. I only had 4/0 cable for the neutral, so I installed two 4/0 cables in parallel for the neutral conductor. My question is, do all the phases have to be in parallel or single since I used parallel conductors for the neutral and single conductors for the 3 phases?
 
So you had three raceway each with a set of 444kCMIL for phases and two 4/0 paralleled grounded conductors?
Did you have a set of grounded conductors in each of the three raceway or only in one?
 
Parallel conductors

Parallel conductors

I used no conduit. The cables are temporary outdoor rated cables that I laid on the cement from the back-up generator to the ATS. I ran one 4/0 grounding conductor from the back-up generator to the ATS grounding lug.
All of the conductors for the circuit are laid next to each other on the ground.
The 3 phases use single cables. The neutral uses 2 cables in parallel. The grounding conductor uses a single cable. Because I used 2 cables in parallel for the neutral, do all of the phases have to be run in parallel, or vice-versa?
 
ron said:
So you had three raceway each with a set of 444kCMIL for phases and two 4/0 paralleled grounded conductors?
Did you have a set of grounded conductors in each of the three raceway or only in one?
Click to expand...

Ron I believe he has one raceway with 3 non paralle l circuit conductors and the grounded conductors are in parallel. I don't think this is a violation but I am not certain.
 
paulgarett said:
My question is, do all the phases have to be in parallel or single since I used parallel conductors for the neutral and single conductors for the 3 phases?
Click to expand...

Each phase, including the neutral can be different from the other phases.

For example a 600 amp feeder

Phase A could be 3 sets of 250 AL

Phase B could be 3 sets of 3/0 CU

Phase C could be 2 sets of 350 CU

Assuming the max neutral load is 400 amps the Neutral might be 1 600 CU

Also the above would all have to be in the same raceway or in the case of a temp, all laid together.
 
paulgarett said:
I installed a temporary back-up generator and installed the emergency conductors to the Automatic Transfer Switch. It was a 3 Phase, 277/480 Volt system, a 600 kW Generator, with a 1000 Amp Circuit Breaker. Three of the conductors I used for each of the 3 phases were 444.4 MCM RHH outdoor rated, "freight-train" cable. I only had 4/0 cable for the neutral, so I installed two 4/0 cables in parallel for the neutral conductor. My question is, do all the phases have to be in parallel or single since I used parallel conductors for the neutral and single conductors for the 3 phases?
Click to expand...
400 and 500 RHH is rated about 525 and 620 amps in free air. I do not know the rating of the cable you are using, but I think your breaker ampacity is too large. To answer your question
do all the phases have to be in parallel since I used parallel conductors for the neutral and single conductors for the 3 phases?
Click to expand...
they do not.
 
Right. As Bob detailed, for each paralleled-conductor group acting as a single conductor, the individual conductors must match, so the current division is equal. The closer to maxing the conductor ampacity you get, the more critical the equality of the division must be.

However, since you're not grouping the conductor sets, i.e., not using multiple raceways, there is no requirement to use the same quantity of conductors in each paralleled-conductor group. Notice that Bob's example has two sets of three conductors and one set of two.
 
Temp. conductors

Temp. conductors

Thank you for all of your responses. If I remember right the cables' insulation is RHH or RHW-2. The conductors are Nickel-Coated Copper. The ampacity of 444.4 MCM of this cable is about 800 Amps. The cable has very thick insulation and is used, I have heard, for freight trains, run by diesel generators. The NEC is a little vague about this discussion. Section 310.4 (2005). I think there should be a dedicated section on back-up generator's electrical requirements for the emergency feeders. Does anyone know where I can find legal requirements about back-up or temporary generator's emergency feeders. This is an important issue since hospitals and other important buildings need emergency stand-by generators when their permanent generator becomes non-operable due to mechanical reasons.
 
Paul,
The manufacturer's amapcity of ~800 amps is in free air at 90?C. While you may be able to use the free air part for this application, you can't use the 90?C part. Also you can't terminate that conductor on standard terminations as they are not suitable for use with the fine strands in the DLO cable.
 
That is interesting about the lugs. I used normal lugs. What do you call the type of lugs to use with fine stranded wire, so when I ask for them at the electrical supply house, they would know what I was talking about or how to look them up.
 
paulgarett said:
What do you call the type of lugs to use with fine stranded wire, so when I ask for them at the electrical supply house, they would know what I was talking about or how to look them up.
Click to expand...
Uh, how about "the type of lugs to use with fine stranded wire"? :cool:
 
paulgarett said:
That is interesting about the lugs. I used normal lugs. What do you call the type of lugs to use with fine stranded wire, so when I ask for them at the electrical supply house, they would know what I was talking about or how to look them up.
Click to expand...

T&B catalog tables list lugs for CODE cables and FLEX cables. Example compression connector # 58180. Steve
 
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