conductors in parallel

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alberto

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goog afternoon, i hope some of you can help me, i have a big question.
i am installing an electrical subestation, the transformer is 300 kva, 13200-220/127 volts, 3 phases, the nominal current on secondary side is 787.29 amp, there are only two 3" pvc conduits, i am considering to install 9 conductors of 350 kcm (3 per phase), 3 conductors of 350 kcm for neutral and 2 conductors of 1/0 awg for equipment grounding.
6 conductors of 350 in one conduit (2 of each phase)and one equipment grounding conductor,
6 conductors of 350 in the other conduit (1 of each phase and 3 for neutral) and one equipment grounding.
am i correct?
if not
what do i have to do?
i am sorry because my english is not good.
 
Re: conductors in parallel

I assume you are paralleling all of the phase conductors (i.e. they all feed the same piece of equipment). Then all parallel conductors have to be physically ran the same. For example, they all must be the same length, and all must follow the same route. It has been debated on this forum, and I think most of the regulars here agree that having 2 phase conductors in one conduit and 1 phase conductor in another conduit is a violation.

Steve
 
Re: conductors in parallel

Running conductors in the manner you described would be a violation of 310.4. Each conduit must have the same number of conductors. In addition, each conduit must have a Phase A conductor, a Phase B conductor, a Phase C conductor, a neutral, and a ground. You could put two of each in a single conduit, but then you would have to put two of each in all the conduits. The conduits have to have the same contents. However, your 3? PVC conduit only has room for 6 conductors of size 350. So you can?t get two sets of 350 MCM conductors into each of two conduits.

Now let?s talk ampacity. You suggest using 350 MCM. Each of those is good for 310 amps. But when run with more than 3 current-carrying conductors in a single conduit (which is what you are suggesting), you must derate them to 80% of 350 (the 90C rating), or 280 amps. A pair of such conductors will only give you 560 amps. That is far short of the transformer?s 787.29 amps. So as far as that goes, you are better off using 2 sets of 350?s, with only one set per conduit. This will give you 610 amps. That at least is closer to the 787.29.

You did not say which type of PVC you are dealing with, nor what type of conductor. A schedule 40 PVC has room for 10 conductors of size 4/0 THHN. That would give you an ampacity of 4 times 260 (the 90C rating for a 4/0) times 80% derating factor, or 832 amps.

But to rain on that parade, you also did not say whether you expect a significant percentage of non-linear loads. If you do, then you must count the neutrals. That makes for more than 6 current-carrying conductors per conduit, and makes the derating factor 70%, instead of 80%. The configuration I described (with the 4 sets of 4/0) would then only be good for 728 amps. You would have to upsize to 250 MCM. But a 3? schedule 40 PVC can only hold 8 of those. Under ?engineering supervision,? you might be able to squeeze in a 1/0 EGC, but I have not done that math.

You do have a serious design problem on your hands. I might wonder how it came to be that only two conduits are available, and that they are 3 inches in size. That might not be enough for your application. It is hard to have to swallow, but you need more room for conductors. If that means you must dig again, or pour more concrete, or abandon what is there and start over, then so be it. Or if that means you have to replace the transformer (or restrict it to a lower rating), then so be it.
 
Re: conductors in parallel

alberto -

You English is fine. My lovely bride (born and raised in El Paso) would be ecstatic if my Spanish were anywhere near as good as your English.

From your question you appear to clearly understand Ampacity, Parallel Conductors, Conduit Fill. steve and charlie b are right. You want each conduit to have the same number of each phase conductor and a neutral (EGC may be optional).

You have a tough one here. With only 2 each, 3" conduits, I don't see any way to get all of the power the transformer is capable of. I'm guessing you don't have the option to dig it up and add another conduit - because that would be the best solution.

With only the two existing conduits, the best option I see is to maximize the available power from the transformer with out over-heating the secondary conductors.

With that in mind, I have a couple of questions:

Are you constrained to follow the NEC? For example, a utility does not have to follow the NEC. They tend to run smaller wire, and let it run hot.

What is the conduit spec? I'm guessing type A since that matches with 40% fill on your proposed pulls.

So, assuming;
1. You have to follow the NEC.
2. Conduit is type A PVC installed such that you can use the 90C ampacity table.

here are some suggestions:

1. Do not install an equipment grounding conductor. Bond and ground the neutral at the transformer. Bond and ground the neutral at the building. This only works if you know there is no continuous metallic path between the transformer and the building. If you have to have the EGC, adding the 1/0 to the 250kCMIL will put you over 40% fill - about the same as the 6-350kCMIL you proposed.

2. Install 8 - 250kCMIL Xhhw-2 per conduit. If the neutrals are not considered current carrying, this gives an ampacity of
4 x 290 x .8 = 928A. You would have to set your overcurrent protection under that.

If the neutrals are considered current carrying, the the?ampacity is
4 x 290 x .7 = 812A

3. Tell the customer about the transformer derate. If they don't like it, prepare to start digging.

good luck

carl
 
Re: conductors in parallel

Sometimes 4/0's will give you more "bang for the buck" & may be the answer. 3" Schedule 40 PVC can handle (9) 4/0 THWN/THHN in each conduit.
Do you need the full capacity of the 300KVA xfmr? If not then possibly you could see to it that the sec. conductors leaving the xfmr terminate in an OCD that would limit the load on the conductors so as not to exceed their ampacity.
 
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