Feeder Neutral Counting as a Current Carrying Conductor for derating?

[kwired]

No, that is clearly two separate 2-wire circuits.

Agreed. A feeder tends to average out imbalances.

tends to but doesn't always have to.

 

[kwired]

Agreed that is plausible to say it is two different circuits. Yet we don't have a definition of circuit, so that is a judgement call.

A side effect of your point of view is that if you see a panelboard, and you see a 3 pole breaker, and you see 4 wires leaving in a cable or conduit, one neutral and 3 ungrounded conductors from the 3 pole breaker, you can not conclude from that information that you have an MWBC. You would need to further investigate the nature of the load.

And I would say it is also defensible to define circuit more broadly, so that you can determine whether a collection of conductors is an MWBC without having to look at the details of what the loading is. A different judgement call.

Cheers, Wayne

"Circuit" is a rather generic term and is not NEC defined.

Branch circuit, Branch circuit multiwire and a few specific branch circuit types like appliance, general purpose, motor, individual are defined.

OP's 208 volt two wire along with 120 volt two wire circuits may not had enough information to break down any further but likely would have been two different branch circuits of one the types that is defined. Put them on a single three pole breaker may or may not make them a multiwire branch circuit? It doesn't really specify that each ungrounded conductor must have load to the neutral conductor in the MWBC definition, though many seem to take it to intend that. The feeder side of this is essentially the same thing.

Keep in mind counting this as a CCC for ampacity adjustment doesn't physically change anything. What the CCC rule is about is determining heat contribution to the raceway/cable and doesn't have a whole lot of scientific method of approach to determining that heat, just a simple statement as to whether you consider the neutral as a CCC or not and is done in somewhat of a one size fits all manner.

 

[4thCuppaCoffee]

Thanks for such a spirited debate everyone. I certainly learned a lot. And honestly, I'm a bit star-struck seeing some of the forum OGs on this thread.
I won't count the neutral in the feeder to my 208V panel as a CCC. I really appreciate your time and feedback!

 

[LarryFine]

And honestly, I'm a bit star-struck seeing some of the forum OGs on this thread.

What's an OG? Original Gangsta'?

 

[CoolWill]

What's an OG? Original Gangsta'?

Oblivious Gits.

 

[GoldDigger]

To put it in one concise sentence, the neutral of a four wire wye circuit will carry current if the three line currents are unequal, but it will not be counted as a CCC for heating purposes because it only carries current to the extent that the current on one of the line conductors is reduced.

Triplen harmonics invalidate this result, since the neutral conductor can actually carry more current than any of the line conductors.

 

[LarryFine]

Understand that you rarely must include the neutral as a CCC, but you're certainly allowed to.

Triplen harmonics invalidate this result, since the neutral conductor can actually carry more current than any of the line conductors.

True, but the problem is waning like it arose, as that type of power supply is fading out.

 

[winnie]

To put it in one concise sentence, the neutral of a four wire wye circuit will carry current if the three line currents are unequal, but it will not be counted as a CCC for heating purposes because it only carries current to the extent that the current on one of the line conductors is reduced.

Triplen harmonics invalidate this result, since the neutral conductor can actually carry more current than any of the line conductors.

Unbalanced phase angles also invalidate the result, as shown by the 'single 208V load combined with single 120V load' scenario.

A scenario with different power factor loads could also put significant current on all 4 conductors.

I'm not sure how easy it would be to write a simple concise rule which always identifies the situations in which neutral balance doesn't work. For practical situations using normal loads (as opposed to contrived situations), a wye circuit should count as 3 CCCs. In rare outlier cases it should count as 4.

Perhaps the rule for counting the neutral could be replaced with a rule for figuring total conductor heating in the worst case.

For example: For purposes of derating, conductor count shall be the number of conductors carrying more than 10% of rated ampacity when current flow paths are maximally concentrated given circuit topology and load phase angle and supplied at the trip rating of the protecting OCPD.

Gah, that is convoluted.

 

[CoolWill]

It's never been a problem. This isn't some new technology that we have to rethink the code for. This is just conductors carrying current. For 100 years now the neutral hasn't needed to be counted as a CCC, and I'm sure many odd and non-normal use cases have been thrown on millions of wye-connected 4-wire feeders and MWBCs. Conductor ampacity is also conservatively over-rated, and NEC service calculations are ridiculously over-sized. There is just no rational world where not counting the neutral results in lava bubbling out of a conduit. It's what engineers think about when they get stoned and play Fortnite with their friends.

 

[winnie]

I don't need to be stoned to come up with wackadoodle scenarios.

Seriously, the neutral as a CCC wasn't a problem until it was, in a very few, very specific scenarios. Neutral overload because of harmonics really did happen.

My engineering fever dreams do have a bit of basis in reality.

The problem (rare as it truly is) is going away anyway because of harmonic mitigation requirements.

Jonathan

 

[jaggedben]

To put it in one concise sentence, the neutral of a four wire wye circuit will carry current if the three line currents are unequal, ...

In kwired's scenario, the neutral could carry the same current as the line conductors when the line currents are *equal*. No harmonics or phase angle shifts involved.