Transformer secondary ground wire size

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sportybear13

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engineer
I'm looking for clarity on grounding, especially NEC 250.102 and 250.122 and when/where to use them related to XFRs and secondary wire sizes.
EX: Confirm ground wire for 75KVA XFR secondary with 250A OCPD and #4/0H copper. It's either #2G (NEC 250.102(c)(1)) or #4G (NEC 250.122) copper for the ground wire.
 
You said ground wire size which is too generic to find it in the NEC. If you're taking about the conductor run between the transformer secondary and the 250 amp OPCD then that would be a supply side bonding jumper (SSBJ). For the SSBJ you would use Table 250.102(c)(1). T250.122 is for the EGC, that would come after the OCPD.

Also if the secondary is feeding a 250 amp OCPD then the conductors would need to be rated for a minimum of 250 amps.

Welcome to the Forum. :)
 
I agree with Rob. But let me help clear up (or is it add to?) the confusion:
  • Table 250.122 is for equipment grounding conductors (EGCs). These provide a low impedance path back to the source during a fault, causing the upstream breaker to trip. An EGC is sized based on the rating of the upstream breaker. But there is no breaker between the transformer secondary and the downstream panel. So 250.122 is the wrong table to size the wire between them.
  • Table 250.66 is for grounding electrode conductors (GECs). These are run from the transformer secondary to planet Earth. They are sized based on the cross-sectional area of the secondary conductors. Their functions would take too long to discuss here. The wire between the transformer secondary and the downstream panel does not connect to planet Earth. So 250.66 is the wrong Table to size these wires.
  • Table 250.102(c)(1) is for the SSBJ that Rob mentioned. They are sized on the same basis as the GECs. It is worth noting that tables 250.66 and 250.102(c)(1) are identical until you get to the very large sizes of the secondary conductors.
 
charlie b said:
I agree with Rob. But let me help clear up (or is it add to?) the confusion:
  • Table 250.122 is for equipment grounding conductors (EGCs). These provide a low impedance path back to the source during a fault, causing the upstream breaker to trip. An EGC is sized based on the rating of the upstream breaker. But there is no breaker between the transformer secondary and the downstream panel. So 250.122 is the wrong table to size the wire between them.
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Doesn't the code say that in the absence of an upstream OCPD that the GEC should be sized to the OCPD that would have been necessary to protect the conductors?
 
ggunn said:
Doesn't the code say that in the absence of an upstream OCPD that the GEC should be sized to the OCPD that would have been necessary to protect the conductors?
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The GEC is sized according to the secondary conductors and T250.66. There are no OCPD sizes in that table only conductor sizes.
 
ggunn said:
Doesn't the code say that in the absence of an upstream OCPD that the GEC should be sized to the OCPD that would have been necessary to protect the conductors?
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It does not. You may be confusing GEC and EGC. Please reread my first two bullets above, and let me know if you have any additional questions.
 
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