EGC sizing

[hhsting]

I have feeder breaker1 that is 1200AF (amps frame)/1000AT (amps trip) and feeder breaker2 that is 1200AF (amps frame)/800AT (amps trip). Question is equipment grounding conductor size is it based on amps frame or amps trip NEC 2014 Article 250? Which code section 250.122?

 

[infinity]

250.122 based on the size of the the overcurrent protection not the frame size.

 

[charlie b]

Yea. What he said. :happyyes:

 

[hhsting]

Lets say somebody changes trip unit setting from 1000AT to 1200AT then would not that be a problem EGC was sized based on 1000AT? Assume phase feeders are sized 1200Amps.

 

[infinity]

Lets say somebody changes trip unit setting from 100AT to 1200AT then would not that be a problem EGC was sized based on 1000AT? Assume phase feeders are sized 1200Amps.

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Depends what the EGC size is based on Table 250.122 for 1000 amps and 1200 amps. If the ungrounded conductors are already sized for 1200 amps then the EGC should already be upsized even with a 1000 amp OCPD.

 

[Dennis Alwon]

Depends what the EGC size is based on Table 250.122 for 1000 amps and 1200 amps. If the ungrounded conductors are already sized for 1200 amps then the EGC should already be upsized even with a 1000 amp OCPD.

To clarify for the poster-- you are saying that if the trip level is set lower than the breaker max then the equipment grounding conductor can be sized to the lower trip ampacity. Correct?

 

[charlie b]

Lets say somebody changes trip unit setting from 1000AT to 1200AT then would not that be a problem EGC was sized based on 1000AT?

The simple answer is that that "problem" belongs to the "somebody" who does the future revision. We don't have to design today for changes that someone else might make later.

 

[charlie b]

To clarify for the poster-- you are saying that if the trip level is set lower than the breaker max then the equipment grounding conductor can be sized to the lower trip ampacity. Correct?

That is what Rob said in his post #2. His post #5 is bringing 250.122(B) into play. If the conductors are sized for 1200 amps, and the OCPD is set to 1000 amps, then the conductors must have been "increased in size" beyond what was needed for ampacity purposes. Therefore, the EGC will have also been increased in size proportionally, so that the EGC will already be big enough to support a change to a 1200 amp setting.

 

[infinity]

To clarify for the poster-- you are saying that if the trip level is set lower than the breaker max then the equipment grounding conductor can be sized to the lower trip ampacity. Correct?

Yes, it's no different than having a 125 amp fuse in a 200 amp disconnect switch you would size the EGC based on the OCPD size, in this example 125 amps. His other hypothetical question regards changing the trip setting later because the conductors are already sized for 1200 amps. In that scenario the EGC would already need to be oversized {250.122(B)} because the conductors have been increased in size from 1000 amps.

 

[kwired]

Lets say somebody changes trip unit setting from 1000AT to 1200AT then would not that be a problem EGC was sized based on 1000AT? Assume phase feeders are sized 1200Amps.

If you have 1000A OC protection but have 1200 amp conductors, 250.122(B) is going to require an increase in EGC size anyway over what it would be for 1000 amp conductors. (did not check to see if end result is same/greater size EGC than required for a 1200 amp device).