Derating Woes

Status
Not open for further replies.
Why would that be?

If a MWBC was taken from the 1ph subpanel the neutral currents would still cancel out would they not?

What changes just because the panel is fed from a 3 phase panel?


Now I could see where the feeder to the 1ph panel from a 3ph source would need to be counted but why the branch circuit neutrals from the 1ph load center if MWBC's are used?


JAP>
 
jap said:
Why would that be?

If a MWBC was taken from the 1ph subpanel the neutral currents would still cancel out would they not?

What changes just because the panel is fed from a 3 phase panel?


Now I could see where the feeder to the 1ph panel from a 3ph source would need to be counted but why the branch circuit neutrals from the 1ph load center if MWBC's are used?


JAP>
Click to expand...

They are not 180° out of phase therefore do not cancel each other out the same way as a single phase
 
jap said:
Why would that be?

If a MWBC was taken from the 1ph subpanel the neutral currents would still cancel out would they not?

What changes just because the panel is fed from a 3 phase panel?


Now I could see where the feeder to the 1ph panel from a 3ph source would need to be counted but why the branch circuit neutrals from the 1ph load center if MWBC's are used?


JAP>
Click to expand...

310.15(B)(5)(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye connected
system, a common conductor carries approximately
the same current as the line-to-neutral load currents
of the other conductors and shall be counted when applying
the provisions of 310.15(B)(3)(a).
 
jumper said:
310.15(B)(5)(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye connected
system, a common conductor carries approximately
the same current as the line-to-neutral load currents
of the other conductors and shall be counted when applying
the provisions of 310.15(B)(3)(a).
Click to expand...

Yes, but that depends where in the circuit you intend to apply this rule.

You dont have a 3 wire circuit consisting of two phase conductors and the neutral conductor of a 3ph 4 wire connected system once your past the subpanel.
You'd have a 3 wire circuit consistiting of two phase conductors and the neutral conductor of a 1ph 3 wire connected system past the subpanel.

JAP>
 
jap said:
Yes, but that depends where in the circuit you intend to apply this rule.

You dont have a 3 wire circuit consisting of two phase conductors and the neutral conductor of a 3ph 4 wire connected system once your past the subpanel.
You'd have a 3 wire circuit consistiting of two phase conductors and the neutral conductor of a 1ph 3 wire connected system past the subpanel.

JAP>
Click to expand...

Nope. Your single phase panel is not a system. It is just a single phase panel fed from a three phase wye system.
 
I would think this would apply to the feeder from the 3ph panel to the single phase panel but not so much from the 1ph panel to the branch circuits.

jAP>
 
luckylerado said:
If this were a single phase sub panel off of a 3 phase multi gang service pack, the MWBC neutral would need to be counted correct??

I am thinking condos or other similar.
Click to expand...
More specifically when the source is three phase wye, because of the phase angles.

If you ran a single phase sub from a three phase high leg delta (using the two 120 volt legs) you still have 180 degree phase angle across this one side of the delta.

But you probably hardly ever would run into this in condos or similar as the high leg is hardly ever usable in such an application.
 
Status
Not open for further replies.
Top