Paralleling Secondary Conductors of Transformers

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user-1129882's picture
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Joined: 2017-01-22

Hey guys, I want to know if there's a limit on how many sets of paralleled cables are allowed on the secondary side of a 1,500kVA transformer. I know four sets are allowed.

We're designing for a 2,000A, 3phase, 480V service, but after adjustment factors and plus 25% continuous load, I came up with 1,750kcmil cable. This size cable is kind of difficult to locate and expensive so I was planning on using five or six sets of smaller conductor per phase. Is this acceptable to do?

user-1015872's picture
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Joined: 2016-08-28

I could not find a specific number of sets spelled out in the 2011 Code. It does say that paralleled conductors shall be permitted when done in accordance with Sec. 310.10(H)(2) through (6). Must also comply with Sec. 310.15(B)(3)(A) adjustment factor. Paralleled also mentioned in Sec. 250.102 and 300.12.

The company I work for is installing new emergency generators at a hospital and all the feeders are run in parallel - up to 5 sets per feed. The largest being 500kcmil.

user-1015872's picture
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Joined: 2016-08-28

I just did a quick search and found a picture of 8 sets coming into a switchgear. They look to be 500 or 600kcmil.

kathirasan's picture
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Joined: 2013-12-07

What type of load is going to be fed?

user-1129882's picture
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Joined: 2017-01-22

It's for a small factory, the biggest loads are the cooling, and the others are just conveyors with motors (producing juices and packaging). The calculated load is around 1,700A, so we going with the next higher which is 2,000A. Due to cable and conduit sizes we planning to go with 1 more set and reduce the cable and conduit size entering the switchgear.

user-1129882's picture
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Joined: 2017-01-22

Its for a small factory that will be producing orange juice, lime juice and coconut water. The largest loads are the cooling, it's requiring 1,000A and the other loads such as motors, lights are around 750A, so we going for the next higher fuse rating, 2,000A. What we're doing is trying to minimize the size of cable and conduits entering the switchgear, that is why we're trying to downsize the cable and increase the parallel sets to at least 5 or 6.

AllanWStevens's picture
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Joined: 2013-10-22

The biggest issue is how practicable the installation becomes, given the desired routing. Since the parallel sets need to be all the same length and run closely together in order to allow for the equal sharing of current, the real limitation is how ungainly or not you can make the installation. Given the difficulty in weight and bending, you are probably needing to try no larger than 750kcmil or maybe 1,000kcmil if you have large radius bends or strong pulling equipment. You also need to see how the manufacturer configured the cable termination points to gauge how they are thinking you'll be connecting up to the source/load.

Depending on the routing path you're taking can also be an issue -- if underground, you quickly get into the 'death spiral' of adding a paralleled set which adds mutual heating and reduces the conductor current rating, thus requiring another set, and this goes on until you throw in the towel and try to route in a more exposed fashion to avoid the mutual heating issue.

user-1129882's picture
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Joined: 2017-01-22

Thanks. Well, I was thinking to use 6 sets of 600kcmil after applying adjustment factors, so to reduce size of conduits entering the switchgear.

Vagabond's picture
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Joined: 2017-02-07

I have personally connected 8 sets of 500 MCM CU in 1,500kVA transformers. As a field guy, I'd say that conduit is cheap and usually cemented in place. Don't downsize it when you downsize your conductor. It another circuit is needed in the future you might just appreciate the larger conduit being in place.

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