Generator Cable Load Hogging

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mcmanumi

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Question about the best way to layout temporary diesel generator cables from generator to load...

Scenario:
  • 2MW mobile diesel generator (480V) connected to a rental transformer (480V Delta, 12.47Wye)
  • Transformer connected to electrical distribution system for summer peak shaving operations (ambient air temps >90 deg. F)
In the past, we would lay out the 4/O cam lock cable flat on the ground, open air, 10 cables per phase laid parallel to each other with no gap between cables. Over time we realized that this would cause uneven load distribution ("load hogging") among the cables of each phase which in turn would cause 1 or 2 of the cables per phase to overload (>400amps) and potentially burn up.

What is the best way to lay out the cables to prevent this?
  • Leave a gap between ALL the cables on the ground?
  • Instead of running 10 cables per phase (i.e.-10 A-phase cables next to each other, 10 B-phase cables next to each other, etc.), run the phases alternately (i.e.-ABC, ABC, ABC, etc.)?
Assumptions:
  • The total cable run between the generator & the transformer is less than or equal to 50' (we're using 50', 4/O cables)
  • Cam lock connections are all good
  • Connections at the generator & transformer are good
 
I would intermixed the phases, but that is just me. They are often staggered,if as you have them, in cable tray.

Look into Proximity Effect. This causes current to flow unevenly inside a conductor.
 
I would be inclined to intermix them as well, but sometimes that is not real convenient.

Over time we realized that this would cause uneven load distribution ("load hogging") among the cables of each phase which in turn would cause 1 or 2 of the cables per phase to overload (>400amps) and potentially burn up.
Click to expand...

Is this just speculation on your part or did you measure the current and find one of the conductors was hogging all the current.

2 MW at 480 V is about 2400 Amps. If you get an even split at full load that is 240 Amps per cable. But realistically, the cables are probably locomotive cable or something similar and can likely take a lot more current than the NEC tables would suggest for typical wiring in conduit. I think in free air 4/0 locomotive cable is rated for 400 Amps so even if you got a cable that went up to 400 Amps it would not hurt the cable any. Likely you are using 400 Amp camloks so they probably don't care either.

If you are using the double setscrew version of the 400 A camloks they are rated for 400 Amps at 90 deg C.

I would think your worst case might be the conductors in the middle of the bundle, and they would have less current then the others and you would get more current as you went outward from the center conductor. But I doubt it could be as high as 400 A. And even if it was, it probably is not going to hurt anything.
 
hello...Im sizing a transfer switch for a backup generator and, as per NEC695.4 ba1, I need to size the overcurrent device and therefore the transfer, to carry the LRA of the fire pump (125HP, code letter G) this will mean a current size or more than 2000A. This is 208 V, 3ph.

Is it correct?
 
jim dungar said:
I would intermixed the phases, but that is just me. They are often staggered,if as you have them, in cable tray.

Look into Proximity Effect. This causes current to flow unevenly inside a conductor.
Click to expand...
Assuming sets of paralleled conductors that installation method is not permitted.
392.20(C) Connected in Parallel.
Where single conductor cables comprising each phase, neutral, or grounded conductor of an alternating-current circuit are connected in parallel as permitted in 310.10(G), the conductors shall be installed in groups consisting of not more than one conductor per phase, neutral, or grounded conductor to prevent current imbalance in the paralleled conductors due to inductive reactance.
Single conductors shall be securely bound in circuit groups to prevent excessive movement due to fault-current magnetic forces unless single conductors are cabled together, such as triplexed assemblies.
Click to expand...
 
petersonra said:
I would be inclined to intermix them as well, but sometimes that is not real convenient.



Is this just speculation on your part or did you measure the current and find one of the conductors was hogging all the current.

2 MW at 480 V is about 2400 Amps. If you get an even split at full load that is 240 Amps per cable. But realistically, the cables are probably locomotive cable or something similar and can likely take a lot more current than the NEC tables would suggest for typical wiring in conduit. I think in free air 4/0 locomotive cable is rated for 400 Amps so even if you got a cable that went up to 400 Amps it would not hurt the cable any. Likely you are using 400 Amp camloks so they probably don't care either.

If you are using the double setscrew version of the 400 A camloks they are rated for 400 Amps at 90 deg C.

I would think your worst case might be the conductors in the middle of the bundle, and they would have less current then the others and you would get more current as you went outward from the center conductor. But I doubt it could be as high as 400 A. And even if it was, it probably is not going to hurt anything.
Click to expand...

DLO is typically not used for this. Either types W or Type SC/SCE. Both are Article 400 cord products and have a NEC 90deg ampacity of 405 amps for 4/0. The single pole camlock connectors used with this are listed for 400 amps continuous with 4/0 type SC/SCE or type W

Edit: as far as how to lay them out, we go ABCNG-ABCNG no engineering behind that here, its just how everyone has always done it in our industry.
 
Todd0x1 said:
DLO is typically not used for this. Either types W or Type SC/SCE. Both are Article 400 cord products and have a NEC 90deg ampacity of 405 amps for 4/0. The single pole camlock connectors used with this are listed for 400 amps continuous with 4/0 type SC/SCE or type W

Edit: as far as how to lay them out, we go ABCNG-ABCNG no engineering behind that here, its just how everyone has always done it in our industry.
Click to expand...
Its about the same thing. The point was not to specify a specific type of cable but to point out the amp rating of the cable. I think a lot of these cables are dual or more than dual rated.
 
petersonra said:
Its about the same thing. The point was not to specify a specific type of cable but to point out the amp rating of the cable. I think a lot of these cables are dual or more than dual rated.
Click to expand...

Sorry for being pedantic, but theyre not the same thing. DLO is usually dual rated as RHW or something similar -a Chapter 3 wire type. Great for when you need flexible conductors for a permanent installation. The types SC/SCE and W are Article 400 portable cord and are the correct thing to use for portable applications like in OPs post.
 
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