Panel Conductor

[kevin458]

I am not able to find which NEC rule addresses the size of conductor feeding the panel-board. I think for 3-phase panel-boards with mostly lightning loads, the ampacity of phase conductors should be 125% the rating of the main bus of the panel-board, and the neutral conductor should be 200% the rating of the main bus of panel-board. What do you guys think? Thanks.

 

[don_resqcapt19]

There is no need for the conductors to have an ampacity greater than the rating of the supply side overcurrent protective device. It is a rare case where you would ever need a 200% rated neutral.

 

[infinity]

I agree with Don. We've installed 3?, 42 circuit panels with 60 amp feeders.

 

[kevin458]

Thanks. So if the main bus of lighting panel-board is rated as 100A, should the incoming 3-phase feeder conductors be sized for 100A or 125A?

 

[augie47]

sounds like a notion based somewhat on Code, somewhat on engineering and somewhat on rules of thumb and/or old wives tales
I don't believe the Code ever specifically notes, or than by examples, that commercial lighting is often address as "continuous load" and as such 210.20 would require your OCP device to be not less than 125% of the continuous load, but your buss rating of a panel can not be less than the OCP device protecting it so, other than adjsuting for voltage drop or derating, your phase conductors are seldom larger than you panel buss and OCP device.
As far as the neutral goes, the oversized neutral is often a result of engineering where harmonic currents are involved. This thread might prove interesting to you:
http://forums.mikeholt.com/showthread.php?t=141989&highlight=oversize+neutral

 

[augie47]

Thanks. So if the main bus of lighting panel-board is rated as 100A, should the incoming 3-phase feeder conductors be sized for 100A or 125A?

your conductors should be sized to carry the load with the size of the overcurrent device protecting them taken into account.
As show in earlier posts, you could have a 100A buss protected by a considerable smaller breaker.
Take a UPS or generator output for instance. Your supply might be limited to 30 or 60 amps but for diversification you might want (12) 20 amp circuits. in that case a 12 circuit panel would likely have a 100 amp or greater buss.

 

[infinity]

Thanks. So if the main bus of lighting panel-board is rated as 100A, should the incoming 3-phase feeder conductors be sized for 100A or 125A?

The bus is rated for 100 amps so the OCPD protecting it needs to be rated for 100 amp or less.

 

[electrofelon]

I am not able to find which NEC rule addresses the size of conductor feeding the panel-board. I think for 3-phase panel-boards with mostly lightning loads, the ampacity of phase conductors should be 125% the rating of the main bus of the panel-board, and the neutral conductor should be 200% the rating of the main bus of panel-board. What do you guys think? Thanks.

I think what you are thinking of is the "125% rule" for continuous loads. Note this applies to service entrance conductors, feeders, branch circuits, and OCPD's (sorry dont feel like looking up the code references now) and NOT generally to other equipment. For example a 200A MLO panelboard can serve 200 amps forever. However, when you consider 408.36 (panelboards to be protected at or below their rating) it is a bit tricky to come up with a way to load a panel to its full rating continuously because it will have an OCPD upstream protecting it and that OCPD must be rated at 125% of the continuous load. So if you do that, you violate 408.36. One would need an OCPD and assembly listed for 100% operation to pull this off. The other possible case I can think of is if you have a MLO service rated panel with 2-6 disconnecting means. This case is a bit odd and there is an unsettled debate on whether this panelboard can be loaded continuously to its rating. IMO it can, even though the service entrance conductors must be sized to 125%. Others have argued that, based on careful dissection of the wording and definitions, that the busing of the panelboard are service entrance conductors and thus you need the 125% rating on the panelboard too.

 

[kwired]

There is no high or low limit on size of conductor as it relates to the rating of bus bars. One must also consider there needs to be enough conductor and high enough level of overcurrent protection in relation to the calculated load, and that there are overcurrent protection limits on the panelboard as well as on conductors.

You could feed a 600 amp panelboard with 12 AWG conductor if there is only going to be 20 amps maximum load connected to it, and of course that 12 AWG would need 20 amp overcurrent protection on it. Might cost much less to use a lower amp rated panelboard though.

 

[infinity]

There is no high or low limit on size of conductor as it relates to the rating of bus bars. One must also consider there needs to be enough conductor and high enough level of overcurrent protection in relation to the calculated load, and that there are overcurrent protection limits on the panelboard as well as on conductors.

You could feed a 600 amp panelboard with 12 AWG conductor if there is only going to be 20 amps maximum load connected to it, and of course that 12 AWG would need 20 amp overcurrent protection on it. Might cost much less to use a lower amp rated panelboard though.

Yup, it's really that simple.

 

[iwire]

I ran into a 225 amp three phase panel supplied by 10 AWG from a very small transformer, maybe 2kVA.

 

[kwired]

I ran into a 225 amp three phase panel supplied by 10 AWG from a very small transformer, maybe 2kVA.

Unless it was a 24 volt system, 10 AWG was overkill

 

[iwire]

Unless it was a 24 volt system, 10 AWG was overkill

It may have been the smallest wire they had that the 250 kcmil lugs could grip.

I was more than a bit disappointed, I had already run a 30 - 50 amp 208 volt single circuit for a split system to it before I had opened it. DOH! :rant: