Short circuit calculation per NEC 2020 408.6

[anbm]

We replaced one existing electrical panel -225A, 208/120V, 3PH with a new panel at very much same location, re-use existing conduit but pull new wires. We went to site and did not find where its source panel as all existing conduit ran above ceiling, we had note on drawing for EC to verify its source panel later in construction. The replaced panel does not have manufacture name tag, we guess it is 10 kAIC... We noted new panel to be 22KAIC on print to be safe.

The plan inspector fails the project and ask for the short circuit calculation per NEC 2020 408.6 for the new panel, we can do the estimate SC to satisfy him but have no clue where the source is because the building is so old, main elec. room is 300 feet plus away from panel location. BTW, it takes us 5 hours driving to site from out office. What can we do under this situation to satisfy the inspector and still comply with code? Are there any exemptions in codes? One thought is power the new panel from existing main elec. room which we knew all the sources and can calculate the SC, but it will cost more because of long run... I am an elec. designer.

 

[mayanees]

If you can find the upstream transformer that feeds the panel, then do an infinite primary calculation using the transformer's nameplate impedance, that would be a worst-case SCCR. I don't know how else you'll be able to show it short of a fault current calculation that goes back to the source.

 

[xptpcrewx]

We replaced one existing electrical panel -225A, 208/120V, 3PH with a new panel at very much same location, re-use existing conduit but pull new wires. We went to site and did not find where its source panel as all existing conduit ran above ceiling, we had note on drawing for EC to verify its source panel later in construction. The replaced panel does not have manufacture name tag, we guess it is 10 kAIC... We noted new panel to be 22KAIC on print to be safe.

The plan inspector fails the project and ask for the short circuit calculation per NEC 2020 408.6 for the new panel, we can do the estimate SC to satisfy him but have no clue where the source is because the building is so old, main elec. room is 300 feet plus away from panel location. BTW, it takes us 5 hours driving to site from out office. What can we do under this situation to satisfy the inspector and still comply with code? Are there any exemptions in codes? One thought is power the new panel from existing main elec. room which we knew all the sources and can calculate the SC, but it will cost more because of long run... I am an elec. designer.

No way around it, you will need to know where/how this equipment is being fed and have an understanding of the motor loads. As suggested above, the infinite bus method might be a sufficient approach provided the equipment won’t be driven into an overdutied condition.

 

[petersonra]

I doubt the inspector cares all that much how long it takes you to drive to the site. he needs to know that the new panel AIC rating exceeds the available short circuit current. There are several ways you could make a calculation that would probably suffice. The panel might be far enough away from the supply that just the resistance in the wires might keep the SCC to an acceptable level.

It should not be real hard to backtrack where the panel is fed from and figure out how long of a run it is, and what size wires are used.

Just out of curiosity, how did you come up with a 22kAIC panel? You can't just wave your hands around and make up a number, even if it seems reasonable.

 

[anbm]

I doubt the inspector cares all that much how long it takes you to drive to the site. he needs to know that the new panel AIC rating exceeds the available short circuit current. There are several ways you could make a calculation that would probably suffice. The panel might be far enough away from the supply that just the resistance in the wires might keep the SCC to an acceptable level.

It should not be real hard to backtrack where the panel is fed from and figure out how long of a run it is, and what size wires are used.

Just out of curiosity, how did you come up with a 22kAIC panel? You can't just wave your hands around and make up a number, even if it seems reasonable.

Hahaha, if a 208/120 panel is fed out of 150kVA xfmr or larger, SC typically greater than 10KA, of course this also depends on distance and xfmr impedance. So next level for panel standard AIC rating is 22KA. Saw couple newer panels nearby are rated at 22KA as well.

 

[don_resqcapt19]

There are a number of circuit tracers that will help you identify the power source. Some of them even work through a transformer, so you can find the feeder breaker or transformer in the main electrical room and work from there.