At an industrial site, I'm installing a new 2500kva pad mount 13,800v/480 volt pad mount transformer which will feed a 4000 amp magnum drawout breaker. I plan to feed the 4000 amp gear with ten parallel conduits with 500mcm copper feeders in it, no neutral to this system. The question is can I downsize my grounding conductor in each of the conduits or does the grounding conductor have to be the same size as the feeders?
grounding conductor size
- Thread starter spskas
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zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
This is a feeder from a SDS (not a service)? I only ask because a service would usually require bringing the neutral in.
Correct, SDS, not a service.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
this is a supply-side bonding jumper, see definition and 250.30 A 2 1, which sends you to 250.102 C, where its based on the size of the derived conductors. Take a look at note 3
- Location
- New Jersey
- Occupation
- Journeyman Electrician
You base the SSBJ in each raceway on the size of the secondary conductors in that raceway so each SSBJ is sized for 500 kcmil, however 10 sets of 500 kcmill is too small for 4000 amps. (380*10=3800 amps)
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
As Tom noted, this would be a SSBJ. Parallel SSBJs are sized to the conductors in each raceway per 250.102(C)(2), not the sum of all conductors of each phase. And assuming this transformer outside and using PVC conduit you can just install a 1/0 in each raceway and install a SBJ at the transformer and also at the 4000 amp OCPD. You can then also use it as neutral but it is also your SBJ. See 250.30(A)(1) Exception No. 2.
As a side note, the 10 sets of 500 are not large enough as this is only 3800 amps and you can't use the next size up rule.
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- Location
- New Jersey
- Occupation
- Journeyman Electrician
Then you're good with a #1/0 SSBJ in each raceway.
I did some more reading on this and it leads me to another head scratcher. My conduit is rigid steel, so the way I read the code is I could bond my ten conduits together at the transformer with a 1/0 and do the same thing in my Magnum gear. The way I read the code book is I can bond the conduit on both ends and not pull a grounded conductor (250.102(C)(2). Thoughts on this?
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
If you have a qualified SSBJ per 250.30(A)(2) that is the steel conduit you just need to bond each end of each raceway. You will need to use separate jumpers for each raceway (10 at each end) in order to use 1/0. If you use a single jumper at each end it would have to be sized for the total of the phases which is not practical.
WasGSOHM
Senior Member
- Location
- Montgomery County MD
- Occupation
- EE
FWIW,
look up the conductivity of steel,
use the conduit OD to figure the area of that outer circle,
use the ID to figure the area of that inner circle, subtract both areas.
This cross sectional area should let you figure the resistance of your conduit length by comparing it to the CSA of copper wire, which everyone knows
Gawd only knows how close this will come to the other resistances, but the NEC has reasons for their rules.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
I should have mentioned that I am assuming that these raceways are terminating open ended at the gear and transformer due to the sizes involved. If they where connected to the enclosures thru clean punched holes no jumpers would be required.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
Then you will need 20 jumpers. Ten 1/0 on each end.
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