Parallel conductors

[tacomafc]

I have to feed a 400 amp main disconnect switch. The job has to be done with parallel feeders. Should I use 3/0 or 4/0 or 250kcmil? The conductors will be in one 4 inch sch 80 pvc pipe. Should they be derated? Thank you for your time Frank

 

[petersonra]

Re: Parallel conductors

can you get 10 250MCM cables in a 4 inch conduit?

 

[iwire]

Re: Parallel conductors

What is the actual load?

How many current carrying conductors?

Are you sure you want to run them all in one raceway?

 

[alberto]

Re: Parallel conductors

good evening frank,
in my opinion the conductors have to be derated, because the conduit will have more than 3 conductors, then you have to apply a conduit fill factor, i supose that it will not need neutral conductor so:
the ampacity for cable 250 kcm is 255 amp.
the conduit will have 6 conductors (2 per phase)
the conduit fill factor is: 0.8
255 x 0.8 x 2 = 408 amp.(considering the terminal temperature 167?F (75?C)).
the EGC will be #3 A.W.G.
there are other factors to be consider, like:
1.- voltage drop
2.- if you need to install the neutral conductor the conduit fill factor will change because you would have 9 conductors in the conduit (the factor would be 0.7)

 

[charlie b]

Re: Parallel conductors

Alberto posted his comment while I was typing mine. My approach is different, so I'll offer it for your consideration.

- - - - - - - - - - - - - - - - - - - -

A 4/0 copper THHN is good for 260 amps at its 90C rating. If you put 6 current-carrying conductors of this size and type in the same conduit, you take 80% of the parallel rating of 2 x 260. The result is an ampacity of 416 amps. You can easily fit six #4/0 THHN phase conductors, plus two #4/0 neutral conductors, plus one #1/0 Grounding Electrode Conductor into a 4 inch schedule 80 PVC.

If that is not your choice of conductor type, then follow this process to get your answer.

 

[alberto]

Re: Parallel conductors

charlie you are right, if the terminals or lugs are for 90?C, but if terminals or lugs are for 75?C, you can't use the ampacity of the conductor for 90?C, that is my opinion.
why do you select 1/0 A.W.G. for Electrical Grounding Conductor?, can you explain me?
may be i am wrong because i am from mexico and the code that we use is a bad copy of the NEC

 

[iwire]

Re: Parallel conductors

Charlie and Alberto how can we even attempt to answer this question with the info given?

This raceway could have four to eight current carrying conductors. The adjustment factor could be 70% or 80%.

On top of that if the load is only 351 amps we only have to provide conductors rated at least 351 and we could still have a 400 amp breaker.

If I was presented with a single 4" raceway to provide a 'full' 400 amps I would run one set of 600 Kcmil CU.

Getting eight 4/0s in a 4" raceway is more labor than four 600 Kcmils in the same raceway.

Both will pull hard but less spools to set up, strip, attach to the puller etc.

Alberto if the conductors are rated 90 C we can derate from the 90 C rating as long as the resulting ampacity does not exceed the 75 C ampacity of the terminals.

I am sure Charlie just misread table 250.122 as a 1 AWG CU is a large enough EGC for 400 amps.

 

[George Stolz]

Re: Parallel conductors

Unless Frank's feeding a service...

 

[steve66]

Re: Parallel conductors

Why do you need parallel conductors? I would consider using one set of 500's or 600's, depending on the actual load.

Steve

 

[charlie b]

Re: Parallel conductors

Originally posted by alberto: charlie you are right, if the terminals or lugs are for 90?C, but if terminals or lugs are for 75?C, you can't use the ampacity of the conductor for 90?C, that is my opinion.

You are right, as far as you have taken it. But you can take it one step further. Take a look at the last sentence in 110.14(C). You can look up the value in the 90C column, multiply that value by the de-rating factor, and come up with the ampacity of the conductor. But before you use that calculated value, you have to compare it with the number in the 75C column. You have to use the smaller of the two values. In my example, I calculated 416 amps for a pair of 4/0. The 75C column value is 230 amps, so a pair of them would be 460 amps. The smaller of these two values (416 versus 460) is 416, and that was my answer.

 

[charlie b]

Re: Parallel conductors

Originally posted by iwire: Charlie and Alberto how can we even attempt to answer this question with the info given?

You are right, it can?t be done, and I agree with your reasons why it can?t. That is why my response was in the form of an example. That is why I closed my post with the statement that if the actual configuration was different, then Frank could use that process to come up with a solution that fits the actual configuration.

Originally posted by iwire: I am sure Charlie just misread table 250.122 as a 1 AWG CU is a large enough EGC for 400 amps.

Bob: I?ll respond to your comment by addressing Frank.

Frank: Your original question used the term ?feed? to get power to a ?main? disconnect. From a purely technical language perspective, those two terms are incompatible. The conductor to a ?main? is properly called a ?service,? and a ?feed? conductor ( ?feeder?) provides power to a panel downstream from the main. I interpreted your question as being a related to a service. That is because you described yourself as a ?utility power engineer,? from which I infer that you would not be concerned about anything downstream of the main service disconnect. Was I wrong about that?

 

[john m. caloggero]

Re: Parallel conductors

Hi Frank: I assume that the supply consists of a 3-phase, 4-wire sevice system because of the term used (main disconnect switch). Therefore, 8 conductors in one raceway. The equation for this is: conductor ampacity = (load divided by 0.70)(0.5)= conductor ampacity = (400 divided by 0.70) x (0.5)= 285.7. You are allowed to apply the correction factor to the ampacity of the conductor. If 90 degree C conductors are used, and the new current carrying capacity, after application of the correction factors is at least that for which the terminations are rated, ie., 75 degrees C, it will satisfy the NEC. Therefore, 250 Kcmil Cu conductors rated for 90 degrees C will do the job. The next step is to determine the size of the raceway based on the type conductor insulation. If 250 Kcmil, XHHW is used, each conductor has an area of 0.3904 sq in. Four in PVC has a area of 4.503 sq in. 8 x 0.3904 = 3.123 sq in. The grounded system conductor will serve to ground the 400 amp main switch and a grounding electrod conductor must be installed to a grounding electrode system.

 

[john m. caloggero]

Re: Parallel conductors

Frank: sorry a typo in the quantity of conductors in pvc conduit should be Eight not Four.
John