Ampacity of larger cord

[thunder15j]

I have a customer that needs more electrical power to his portable plant. It is not practicle to run new heavier conductors to the plant. The existing feed consists of a paralled 535 MCM diesel locomotive single conductor(6 hot conductors + 2 grounds) feeding a portable plant from a 1200 amp breaker. Conductors are laying on the ground or in some places they have been covered with clean rock free native soil. The single conductor cords also have either USE or RHW stamped one them (memory lapse).

I cannot find cable/cord ampacities in NEC art 400, but cable manufactures show ampacity of 737 amperes @ good old 90 degrees C. Obviously I cannot land these wires under a breaker and use that rating. However, since the wires (535 MCM) are landed on lugs (most lugs are 90 C) bolted to busbar and then other higher rated wires are cabled from busbar to breaker on both ends, I feel I can use the 90 degree C.

I was thinking of installing a 1600 amp 100% continuous duty breaker with a 1400 amp trip unit to replace the 1200 amp breaker. Wire ampacity would be 2 x 737 = 1474 amps. There are other considerations such as voltage drop, ground wire sizing, and last but not least costs.
Please hit me with some comments. thnx

 

[bphgravity]

Re: Ampacity of larger cord

I have never heard of 535-kcmil conductors? The NEC specifies 500-kcmil and 600-kcmil, but nothing in the middle. Type RHW and USE are only 75 degree rated insulations, and 737-ampere sounds awefully high unless the conductors are in "Free Air." You describe some sort of direct burial?

 

[don_resqcapt19]

Re: Ampacity of larger cord

All of the DLO cable sizes above 4/0 are "oddball" sizes. The ampacities provided by most of the DLO cable manufacturer's are based on 90?C in free air (Table 310.17). Another DLO link. I didn't find any listed for direct burial in my quick search, but it may exist. Even without the listing, I might permit it to be burried for temporary use based on 525.20(G). I know that this is not the same application, but it is the same type of usage. If the cable is not in free air, I would limit the ampacity to that shown in Table 310.16.
Don

 

[thunder15j]

Re: Ampacity of larger cord

More info on this job: most of the single conductor on this job is burried at 4 feet underground. The facility consists of equipment sitting on bare ground. In conversations with the plant owner and also with cable mfgs, it has been pointed out to me that the earth at this level is rather cool and moist, hence heat dissipation possibilities. However, because of experience in submersible water pump motors (Franklin Electric) I know that an electric motor that is improperly installed in a body of water( lake or large diameter well that doesn't afford proper water flow going by the motor) will become hot and fail. Shall we use the 90 degree charts or use just the 75 degree charts (if I can find 75 degree charts for this cable)? Another question is that why does the portable cable charts for type W, G , and DLO allow a much greater ampacity for the same wire sizes as THHN of similiar? My bet is it is because of the skin effect of such cable.