Parallel Conductors

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I have a question for all you gurus: (iwire somewhat touched on this)

Is the tolerence different for conductors of different phases? (Before you say this I know per code the answer's "No"--I'm looking for field experience.) One person posted in one of sandsnow's posts that their difference in conductor length was corrected by swapping the phases in a run which makes me think they were referring to conductors between phases, not within a phase. (That swapping phase thing is not allowed by code though either.) Consider buswork in MCC's. If phase A, B, and C are top to bottom the bus to the individual cans will be different too. It's acceptable there. . .

Second question: How important is bundling in a enclosed metal wireway?

OK I just noticed that I'm rambling and the power is set to go out any time now. Thanks for any responses.

Edit: so it made some sort of sense!
 
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The original post did say a "maximum future load of 3200 amps". If that's the case, and assuming the wire is copper, I don't think it should be a problem. Everyone has been doing the calculations with 4000 amps, so the 800 amps difference should give you an additional safety factor.

Of course, someone will probably come along in the future and decide the 11 sets of 500 is good for 4000 amps. Then they will probably load it up.

Steve
 
The absolute two toughest design conditions an engineer goes through:

1. "We've always done it this way."
2. "We've never done it that way."
 
DGrant,
Second question: How important is bundling in a enclosed metal wireway?
Click to expand...
The raceway itself will keep the conductors close to each other. There have been problems with cable tray installations where the conductors were not grouped.
Don
 
don_resqcapt19 said:
DGrant,

The raceway itself will keep the conductors close to each other. There have been problems with cable tray installations where the conductors were not grouped.
Don
Click to expand...

Don-

Thanks for your response. The applications I'm referring to are 1500 - 2500 amps using 500 MCM RHH/RHW-2 running between a Generator and its switchgear. For >2500 we usually use A,B,C,N bundles on ladder cable tray. This is usually a short span around 15 feet.

As you can see, keeping the conductors all the same length is quite a chore and all the slack cable is pulled to one side or another--it does NOT look pretty! BUT it is code compliant.

Now I'm wondering if it would be better to keep the cable within a phase all the same length but set a tolerance for cables phase to phase? Or is the NEC only referring to those cables with in a phase because the cables of OTHER phases can have different properties (e.g. different conductive material)?

-Dale
 
Bob
I agree it's virtually impossible to comply with the Code on the length requirement. Based on what I learned in the thread I started a while back and referenced in my first post, I'm looking at a couple feet as not to worry in most long runs. The shorter the run, then the length differential must be down to a foot or less not to worry about it.

My first thought is proposal to allow tolerance would have to be carefully worded. If you are using 3/0cu in parallel for 400amps, then any imblance at all will senfd the conductor over it's allowable ampacity. If you are using 600cu in parallel for 800 amps, then you have a built in fudge factor since your parallel ampacity is 840 amps(420 each parallel run).

Like DGrant, I'm looking for real world experience in any problems in parallel conductor failures. Besides Charlie Tuna's, I have not heard of any.
 
rbalex said:
The absolute two toughest design conditions an engineer goes through:

1. "We've always done it this way."
2. "We've never done it that way."
Click to expand...

As an inspector, I am so tired of hearing those words.

I just say (but I do it nicely) let's go back to EMT instead of MC in the walls, RMC in the ground instead of RNC. Metal boxes in the walls instead of plastic for houses. Strap everything to the structure instead of ceiling wires. Isn't that the way we always used to do it??
 
DGrant041 said:
Don-

Thanks for your response. The applications I'm referring to are 1500 - 2500 amps using 500 MCM RHH/RHW-2 running between a Generator and its switchgear. For >2500 we usually use A,B,C,N bundles on ladder cable tray. This is usually a short span around 15 feet.

As you can see, keeping the conductors all the same length is quite a chore and all the slack cable is pulled to one side or another--it does NOT look pretty! BUT it is code compliant.

Now I'm wondering if it would be better to keep the cable within a phase all the same length but set a tolerance for cables phase to phase? Or is the NEC only referring to those cables with in a phase because the cables of OTHER phases can have different properties (e.g. different conductive material)?

-Dale
Click to expand...


The conductors on a PER PHASE basis must be (nearly) identical. Between phases within reason doesn't matter.

I have a job in house now where the continuous load is 1220A. We're using 3-600MCM (1260A) and the distance is no more than 15 ft. Appearance is not the issue, but will it survive! Keep them equal and ride the backs of the guys who do the measuring and terminating.
 
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