I'm dealing with 120/208 system wye

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Brian S.

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I'm dealing with 120/208 system wye. Can you a have a MWBC and share a neutral? If the two receptacles are 20 amps wouldn't the common wire have 40amps. Am I missing something stupid? I know a Delta would cancel each other and the neutral would have only unbalanced load.
 
Brian S. said:
I'm dealing with 120/208 system wye. Can you a have a MWBC and share a neutral? If the two receptacles are 20 amps wouldn't the common wire have 40amps. Am I missing something stupid? I know a Delta would cancel each other and the neutral would have only unbalanced load.
Click to expand...

The common wire would have 20 amps (assuming a 120deg phase angle difference,) not 40 amps.
 
david luchini said:
The common wire would have 20 amps (assuming a 120deg phase angle difference,) not 40 amps.
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Thank you
I found this made me think I was doing something stupid.
(5) Neutral Conductor.
(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).
 
Brian S. said:
I know a Delta would cancel each other and the neutral would have only unbalanced load.
Click to expand...
In a delta system there can be no neutral current because there is no connection to a neutral.

That is, if the source is delta, there can be no line to neutral loads in the first place because there is no neutral.
If the source is wye and the loads are wired delta (line-to-line) there can be no neutral current regardless of balance because none of the loads are connected to the neutral.

For an MWBC, by code there cannot be any line to line loads attached to the circuit, so the issue of neutral loads becomes very relevant.
The four wire wye MWBC may have neutral current, but only to the extent that one or more of the lines is not carrying full current, and for that reason the neutral is not a CCC.
In your case of a three wire partial wye MWBC, the neutral will always carry some current regardless of the balance (unless there is no load at all, of course.)
 
GoldDigger said:
In a delta system there can be no neutral current because there is no connection to a neutral.

That is, if the source is delta, there can be no line to neutral loads in the first place because there is no neutral.

i


If the source is wye and the loads are wired delta (line-to-line) there can be no neutral current regardless of balance because none of the loads are connected to the neutral.

For an MWBC, by code there cannot be any line to line loads attached to the circuit, so the issue of neutral loads becomes very relevant.
The four wire wye MWBC may have neutral current, but only to the extent that one or more of the lines is not carrying full current, and for that reason the neutral is not a CCC.
In your case of a three wire partial wye MWBC, the neutral will always carry some current regardless of the balance (unless there is no load at all, of course.)
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I was referring to a delta with a center tap
 
infinity said:
A 3?, 4W Delta system would have a neutral.
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Which would be the center tap of one 240 secondary, and in any case is a large step from the OP's starting point of a 208Y/120. :)
And does bring up the interesting point that an {A,C,center tap} circuit on a high leg delta would be an MWBC, but a circuit containing {A,B,C and center tap} would not be an MWBC under the code definition. (...a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit) regardless of the type of loads connected to it.
 
GoldDigger said:
Which would be the center tap of one 240 secondary, and in any case is a large step from the OP's starting point of a 208Y/120. :)
And does bring up the interesting point that an {A,C,center tap} circuit on a high leg delta would be an MWBC, but a circuit containing {A,B,C and center tap} would not be an MWBC under the code definition. (...a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit) regardless of the type of loads connected to it.
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I agree, the OP kind of pivoted from WYE to Delta. :D
 
I am referring to a wye 120/208.
my second statement in my op I was referring to a delta a-c centertap as having to carry only the unbalanced load.I only stated delta. sorry for the confusion.
 
GoldDigger said:
Just in the interest of standardization, the NEC refers to that as 208Y/120 (even though the 120 is the wye part.)
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yes the system voltage is 208 sometimes my brain goes faster than my hands. system voltage is listed first, we are suppose to know that 208 is phase to phase and 120's a phase.
 
Brian S. said:
I'm dealing with 120/208 system wye. Can you a have a MWBC and share a neutral? If the two receptacles are 20 amps wouldn't the common wire have 40amps. Am I missing something stupid? I know a Delta would cancel each other and the neutral would have only unbalanced load.
Click to expand...
With a wye system (voltage doesn't matter) you can have a MWBC with two "phase" conductors and a neutral conductor, in which case the current on the neutral is approximately same as on the phase conductors if the phase conductors are the same current level. This is because of the phase angle being 120 degrees instead of 180 degrees. This is the reason for the information you quoted a few posts back.

You can also have a MWBC that uses all three "phase" conductors and a neutral and if the three phases are at same amps the neutral will carry the unbalance which will be zero amps.
 
ActionDave said:
Where does the code say that? I thought it was okay if you had a two pole breaker.
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You are both right.:)

Goldy has the rule and you have an exception.

210.4(C) Line-to-Neutral Loads. Multiwire branch circuits
shall supply only line-to-neutral loads.

Exception No. 1: A multiwire branch circuit that supplies
only one utilization equipment.

Exception No. 2: Where all ungrounded conductors of the
multiwire branch circuit are opened simultaneously by the
branch-circuit overcurrent device.
Click to expand...
 
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