120/240 1Phase MWBC

[LarryFine]

The neutral is a CCC when only carrying current that is unbalanced load from 1PH (310.15(B)(4)(a)). In other words it has current just unbalanced current and is a circuit conductor shared by 2 different phases. Is this correct thinking?

No, that's when it is not a CCC.

Infinity, how does the neutral carry approximately the same current as the lines of a 3PH wye system?

Picture a balanced 3ph Y MWBC carrying 10a on each line. With balanced loads, neutral carries no current. (Neutral only carries difference currents, not a CCC. That means only three CCC's.)

Now, let's say we reduce the load on one conductor by 2 amps. That means two lines carry 10a, one carries 8a, and the neutral carries 2a. Reduce the 8a to 5a, and the neutral carries 5a.

Reduce the 8a all the way to to 0a, and the neutral now carries 10a. For each amp not carried by the line conductor, the neutral current must carry the same amount of current.

Now, picture a 3-wire circuit supplied from a Y-system. Two line conductors and a neutral. As in the example above, since there's no third line conductor, the neutral sees current. Three CCC's.

How about a 3PH delta with 3W circuit from phase A & C sharing a common neutral, is this neutral a CCC?

Nope. The neutral and the two phases flanking it (i.e., not the high leg) are identical to a 120/240v 1ph system. And I mean identical. The high-leg open Delta started out as a 1ph service.

 

[tryinghard]

No, that's when it is not a CCC.

Oops I meant it the other way around, thanks[/QUOTE]

 

[tryinghard]

Now, picture a 3-wire circuit supplied from a Y-system. Two line conductors and a neutral. As in the example above, since there's no third line conductor, the neutral sees current. Three CCC's.

Okay, the neutral does not have to be counted as a CCC when only carrying current that is the unbalanced load from other conductors of the same circuit, (310.15(B)(4)(a)).

In other words it has current just unbalanced current and is a circuit conductor shared by up to 3 different phases.

A neutral is considered a non-CCC if it is part of a single-phase circuit from a 120/240 delta system or a three-phase circuit from a wye system (where harmonics do not exist), but not a single phase circuit from a wye system.


Is this correct thinking?

 

[LarryFine]

A neutral is considered a non-CCC if it is part of a single-phase circuit from a 120/240 delta system or a three-phase circuit from a wye system (where harmonics do not exist), but not a single phase circuit from a wye system.

Is this correct thinking?

Change the last part to "two circuits from a wye system" (or a 3-wire circuit - two hots and a neutral) and you have it.

(That's only because a 'single phase' circuit from a wye only has two wires, either two line conductors (208v) or one line conductor and one grounded conductor (120v).)

Three circuits sharing a neutral from a wye system have three CCC's.

Two circuits sharing a neutral from a wye system also have three CCC's.

 

[tryinghard]

Change the last part to "two circuits from a wye system" (or a 3-wire circuit - two hots and a neutral) and you have it.

(That's only because a 'single phase' circuit from a wye only has two wires, either two line conductors (208v) or one line conductor and one grounded conductor (120v).)

Three circuits sharing a neutral from a wye system have three CCC's.

Two circuits sharing a neutral from a wye system also have three CCC's.

Got it! I think nonlinear loads are prevalent in wye systems so it would be rare to have the neutral not count.

Thanks!

 

[LarryFine]

I think nonlinear loads are prevalent in wye systems so it would be rare to have the neutral not count.

Actually, nonlinear loads have to be rather substantial before counting the neutral really becomes necessary.

You're certainly allowed to count it even when it's not required.

 

[iwire]

Got it! I think nonlinear loads are prevalent in wye systems so it would be rare to have the neutral not count.

You can have non-linear loads on a Wye system and still not count the neutral as a current carrying conductor.

Per NEC 310.15(B)(4)(c) the neutral must be counted as a CCC when "the major portion of the load consists of non-linear loads"

To me a 'major portion' must be at least 50% of the total.