grounding conductor size

spskas

Member
Location
Iowa
Occupation
Electrician
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?
 

tom baker

First Chief Moderator
Staff member
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
 

infinity

Moderator
Staff member
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
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?
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.
 
Last edited:

spskas

Member
Location
Iowa
Occupation
Electrician
Thanks for the input, makes total sense. In regards to the 10 sets of 500's, I misspoke, there will be ten sets of 600's. Thanks again!
 

spskas

Member
Location
Iowa
Occupation
Electrician
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
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?
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
another head scratcher. My conduit is rigid steel
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 :D

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
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.
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.
 
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