Parallel Ground Conductors

[wirestu]

We are connecting an 800 kW emergency generator to a 1600 Amp 4 pole automatic transfer switch. The consultants plans call for installing four sets of 4 500 MCM per phase in 4 - 4" rigid steel conduits. We are planning on installing them in a mixed phase configuration with 4 500 MCM phase conductors, 1 500 MCM neutral conductor and an equipment ground conductor in each conduit. We plan to loop the equipment ground conductor through the bond bushings at the switchgear side. The overcurrent device on the generator is 1600 amps. We would like comments on the following issues: 1) What size should the equipment ground conductor be? The bond bushings are rated up to a 1/O cu wire. Can we install a larger lug on the bond bushing or are there bushings available that would accept a larger conductor? Will installing a larger lug void the U/L rating of the bushing/lug combination? 2) We have no neutral load (only three phase motor and SDS through step down transformers). Are we required to install a neutral conductor from the generator (outside)to the ats (inside) or can we bond the ground and bond the neutral at the generator breaker?

Thanks in advance for you replies.

[ April 15, 2003, 11:48 PM: Message edited by: wirestu ]

 

[don_resqcapt19]

Re: Parallel Ground Conductors

The 1600 amp OCPD will require a 4/0 EGC in each raceway. Grounding bushings with lugs for 4/0 should be available but may not be commonly stocked at the supply house. Four sets of 500 kcmil is not enough copper for a 1600 amp OCPD. You need 4 sets of 600 kcmil or 5 sets of 400 kcmil. If there is no line to neutral load, you establish a grounding electode system at the generator, intall the generator as a SDS and bring the EGCs in to the transfer switch, you do not need the grounded conductor inside.
Don

 

[ron]

Re: Parallel Ground Conductors

It is always surprising when I see a feeder in excess of 800A, still using the next size circuit breaker. It happens a lot. Does anyone know when 240.4(C) came about (or some form of this article).

 

[charlie b]

Re: Parallel Ground Conductors

Originally posted by wirestu:We are planning on installing them in a mixed phase configuration with 4 500 MCM phase conductors, 1 500 MCM neutral conductor and an equipment ground conductor in each conduit.

Please tell me you meant 3 phase conductors in each conduit. Please tell me that this is what you meant by ?mixed phase configuration.? Please tell me that you did not mean that the 4 phase conductors in the first conduit are to be all Phase A, and the 4 in the next conduit are to be all Phase B, etc.

 

[gwz2]

Re: Parallel Ground Conductors

ron,

The 1959 NEC 240-5 has this Exception No. 1 " Rating of Overcurrent Protection. When the standard ampere ratings and settings of overcurrent devices do not correspond with the allowable current-carrying capacites of conductors, the next higher standard rating and setting may be used. "
Exception No. 2. "Adjustable-Trip Circuit-Breakers.
Adjustable-trip circuit breakers of the thermal trip, magnetic time-delay trip or instantaneous-trip types shall be set to operate at not more than 125 percent of the allowable current-carrying cpacity of the conductor. ".

The 1956 NEC 2403.
a. Fuses. had words to say not greater than 150 percent of the allowable current carrying capacity of the conductors - and -
b. Non-Adjustable-Trip Circuit-Breakers. had words - in accordance with the current-carrying cpacity of the conductor. - and -
c. Adjustable-Trip Circuit-Breakers. had words at not more than 150 percent of the allowable current-carrying capacity of the conductor.

The 1965 NEC , 240-5 Exception No. 1 had the 800 Amps or less of todays' code.

I do not have the 1962 NEC, if there was one.

Glenn

 

[sparky46514]

Re: Parallel Ground Conductors

Just wondering...wouldn't this installation be required to be installed per current codes?

 

[wirestu]

Re: Parallel Ground Conductors

The conduits will each contain a 3 phase conductors, neutral and ground, otherwise inductive heating would occur.

If the issue with current code is related to the size of the breaker, we notified the consultant and he has upgraded the plans to increase wire size to 600MCM.

Off topic, can you add a differnt U/L listed to lug to a U/L listed bond bushing or must the entire unit be an asssembly? This relates to another post that I have that has not gotten any respone. The largest wire that a 2 1/2" water ground clamp can take in the premade lug for the grounding electrode conductor is 1/0. We need a 4/0. Can we install a U/L listed lay in lug onto the water ground clamp. I assume the answer is no, but could you back it up with code sections.

Thanks,

Stu