EGC for 2000kw Temporary Generator

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PE-noy

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2000kw temporary generator.
The generator is NOT a separately derived system as the neutral is not bonded at the generator. It will connect to the 4000A switchboard via 3000A/3P CB by 8 sets of 4/0 flex cables to be laid on the ground (temporary). I have a couple of questions:
1) The required EGC per 250.22 is 400kcmil. Can I parallel this EGC using (2) 4/0 instead?
2) Can the EGC be terminated on the main ground bar in the Main Elect Room or does it have to be terminated in the Main Switchboard main ground bus.

Thanks in advance.
 

iwire

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Staff member
Location
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You have not run enough power cable to be NEC compliant so worrying about the EGC size seems a bit odd. :grin:

IMO the NEC requires a full size EGC.
 

iwire

Moderator
Staff member
Location
Massachusetts
Question: How many 4/0 cables per phase? It look's like the cables are undersized for the OCPD protection that you have.

Pretty much standard practice for generator rental companies to use 4/0 portable power cable at 400 amps, although the NEC does not allow it.

They are going by the cable manufactures ratings instead of the NECs ratings.

That said it usually works fine as a 3000 amp service will not normally draw more than 1600 - 2000 amps.
 
Does the utility company have 12 to 24 months of kwd on file?

On these type jobs, we would convert the kw to kva with power factor, upsize 25%, and size the feeders and egc accordingly.

We always go by NEC tables. I believe there is an ampacity table for welding cable which is what the generator suppliers down here use.

Does the Main Line Generator CB have an adjustable trip setting?

This has worked safely and effectively for us in the past.

JMO
 
Pretty much standard practice for generator rental companies to use 4/0 portable power cable at 400 amps, although the NEC does not allow it.

They are going by the cable manufactures ratings instead of the NECs ratings.

The NEC rating for 4/0 copper types W or SC is 405a @ 90 deg (2008 table 400.5(B) when basically in free air). The cable is fine, most SC and the camlok connectors are rated for 105 deg.

The terminations, on the other hand, probably aren't rated for 90 deg, so you either need 90 deg terminations, larger pigtails to make the transition, or to use the 75 deg column. I've also seen a fan used to keep the terminals cool in a short-term well-supervised situation (12-14 hours), however I can not recommend it.
 

PE-noy

Member
8 sets of 4/0 welding cable is good for 3240A (90deg C). sizing of the current carrying conductor is not the question. It is wether the EGC can be a parallel run of (2) #4/0 which should have a total area larger than #400kcmil that is required.
the other is wether EGC has to be terminated on the main ground bar in the main ER or if it has to be terminated at the Main Swbd main ground bus.

thank you again.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
8 sets of 4/0 welding cable is good for 3240A (90deg C). sizing of the current carrying conductor is not the question. It is wether the EGC can be a parallel run of (2) #4/0 which should have a total area larger than #400kcmil that is required.
the other is wether EGC has to be terminated on the main ground bar in the main ER or if it has to be terminated at the Main Swbd main ground bus.

thank you again.
Table 250.122 calls out the minimum sizes allowed for EGCs. I have never found anything that allows for smaller conductors in parallel.

In fact, 250.118 (1) uses the phrase "A copper, aluminum, or copper-clad aluminum conductor". That seems to be quite clear that it is a single conductor.

It can however be a raceway or busbar of some sort.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
8 sets of 4/0 welding cable is good for 3240A (90deg C). sizing of the current carrying conductor is not the question. It is wether the EGC can be a parallel run of (2) #4/0 which should have a total area larger than #400kcmil that is required.
the other is wether EGC has to be terminated on the main ground bar in the main ER or if it has to be terminated at the Main Swbd main ground bus.

thank you again.

Is welding cable a NEC recognized wiring method?
 

iwire

Moderator
Staff member
Location
Massachusetts
8 sets of 4/0 welding cable is good for 3240A (90deg C). sizing of the current carrying conductor is not the question.

The equipment those conductors terminate to will be rated 75 C max. (See 100.14)

Table 400.5(B)s column 'D' can only be used where the conductors do not touch each other other than up to 24" where they enter the enclosure.

There is no 'free air' rating of portable power cable as it is not installed as overhead spans, it is laid out on the ground and I have yet to see a rating that takes into account the heat sink effect of what the cable is laying on.

At best the 4/0 is NEC rated at 360, more likely 277 amps per column F of the 75C rating.

And the answer to the EGC stands, I see nothing that allows you to parallel smaller cables to reach the required size for EGCs.

250.122(A) 'Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122,'
 
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