Article 240 and Grounded Conductors

[wwhitney]

Say I have a 240V 3P3W corner grounded system, and I use panelboards with just 2 busbars for the ungrounded conductors along with a terminal bar for the grounded conductors. OCPD is appropriately rated 240V to ground.

Now I install a 2 pole 40A breaker with (2) #8 Cu ungrounded conductors, a #10 Cu grounded conductor, and a #10 EGC to supply a small panelboard that supplies a 30A 3-phase load plus a 10A 2-wire load supplied between the two ungrounded conductors.

Does this comply with the NEC? If not, what code section is violated? [Note that an ungrounded conductor could fault to the #10 grounded conductor and carry 40A without tripping the OCPD, but the same is true of the #10 EGC.]

Cheers, Wayne

 

[jaggedben]

Did you deliberately pick a corner grounded system because 220.61 only applies to neutrals? Because I think it's still an interesting question if it was 120/240 split phase.

I think one could possibly say that 240.4 is violated if there is no express permission to reduce the size of the grounded conductor. But I think that's reading between the lines, as long as the branch circuit OCPDs for the loads connected to the grounded conductor still effectively protect the grounded conductors from overload.

You could make your example even more interesting by making it a 100A feeder and the 40A breaker a 70A breaker, in which case the feeder EGC needs to be 8awg and it's still an open question if the grounded conductor can be 10awg. At one time I thought there was a blanket requirement that grounded conductors be as large as the required EGC size for the same circuit, but I think I was mistakenly extrapolating from something that used to be in Article 705.

 

[wwhitney]

Did you deliberately pick a corner grounded system because 220.61 only applies to neutrals? Because I think it's still an interesting question if it was 120/240 split phase.

Yes, that's the followup question, decided to keep the OP shorter and wait to bring that up.

I think one could possibly say that 240.4 is violated if there is no express permission to reduce the size of the grounded conductor.

This arose because I submitted a PI for a new subsection to 240.4, 240.4(I) which said "Neutral conductors sized according to their load calculated per 220.61 shall be permitted to be protected against overcurrent by the overcurrent protective devices for their associated ungrounded conductors."

The CMP response was "The protection of grounded conductors is dependent on the protection of the ungrounded conductors as specified in section 240.22." Which doesn't track, 240.22 is related to whether or not there's an OCPD in the grounded conductor, while 240.4 covers the size of the OCPD relative to the size of the grounded conductor.

So I agree, 240.4 on its face appears to say that the grounded conductor must be sized to be protected at its ampacity the OCPD at its supply, and as far as I can see, 240.4 is lacking any permission to downsize any grounded conductor. Even though it is common to downsize neutrals, and 220.61 tells you how to calculate a reduced neutral load.

Here's a followup question on the OP: if the configuration proposed is OK, then suppose we instead use a 3-phase panel for our corner grounded delta and use 3-pole breakers. Can I connect the #8 Cu ungrounded conductor and the #10 Cu grounded conductor to a 3-pole 40A breaker?

Obviously this provides no less protection than the configuration in the OP. Yet it seems more "wrong" and I would think that many would say it violates 240.4. But how so, if 240.4 doesn't require OCPD in the grounded conductor? [Which is the usual assumption, but doesn't seem to be stated anywhere in 240.4].

Cheers, Wayne

 

[tortuga]

I'll add another one, a collector has a custom three phase to two phase transformer bank creating 240V/340V 2P3W (90 DEG phase angle) separately derived system for demonstrating historic 2-phase motors and powering a resistive heater in his wood shop, the panelboards have 2 busbars for the ungrounded conductors along with a '200%' neutral terminal bar for the grounded conductors. OCPD is appropriately rated 240V to ground (480V rated).

His electrician installed a 2 pole 40A breaker with (2) #8 Cu ungrounded conductors, a #10 Cu grounded conductor, and a #10 EGC to supply a small sub-panel that supplies a 30A 3 wire 2-phase motor plus a 10A 2-wire 340V heater load supplied between the two ungrounded conductors.
Does this comply with the NEC?

Spoiler: Note

The neutral of the two phase system will see 1.41 X the current of the ungrounded conductors.

 

[PaulEd]

Say I have a 240V 3P3W corner grounded system, and I use panelboards with just 2 busbars for the ungrounded conductors along with a terminal bar for the grounded conductors. OCPD is appropriately rated 240V to ground.

Now I install a 2 pole 40A breaker with (2) #8 Cu ungrounded conductors, a #10 Cu grounded conductor, and a #10 EGC to supply a small panelboard that supplies a 30A 3-phase load plus a 10A 2-wire load supplied between the two ungrounded conductors.

How do you get a 30amp 3phase load from a 2pole 40amp breaker ? Or is this something possible w a corner grounded system?

 

[Tulsa Electrician]

How do you get a 30amp 3phase load from a 2pole 40amp breaker ? Or is this something possible w a corner grounded system?

Yes, with a corner grounded delta.
If it's a 240v corner grounded delta system. You would have line to line of 240 volts even on the grounded leg.

Edit, pic from webb.

 

[don_resqcapt19]

How do you get a 30amp 3phase load from a 2pole 40amp breaker ? Or is this something possible w a corner grounded system?

The only issue is most two pole breakers for 240 volts are slash rated at 240/120 and cannot be used on a 240 corner grounded system. A slash rated breaker can only be used on a system where the voltage to ground does not exceed the lower number.

 

[kwired]

How do you get a 30amp 3phase load from a 2pole 40amp breaker ? Or is this something possible w a corner grounded system?

to supply a small panelboard that supplies a 30A 3-phase load plus a 10A 2-wire load

It is a feeder supplying a 30 amp three wire load and a 10 amp two wire load.

 

[jaggedben]

.... At one time I thought there was a blanket requirement that grounded conductors be as large as the required EGC size for the same circuit, but I think I was mistakenly extrapolating from something that used to be in Article 705.

Update: This requirement does exist. It's in 215.2(B). Thanks @Dennis Alwon (in another thread).

 

[wwhitney]

Any thoughts on the OP? Is it simply true in general that a grounded conductor in a feeder is protected against overload only by load calculation? 215.2(B) ensures a minimum size to protect against SCGF.

Cheers, Wayne

 

[jaggedben]

Any thoughts on the OP? Is it simply true in general that a grounded conductor in a feeder is protected against overload only by load calculation? 215.2(B) ensures a minimum size to protect against SCGF.

Well, in your example, it's also protected by the branch circuit breaker for the 30A circuit. Although this would not be true in all cases.

But note this is not the only case where the code allows a calculation to protect a conductor from overload, assuming that a sum of breakers may be somewhat larger. See also 230.90 Exception 3.

In other words, yes, it is true.

 

[wwhitney]

Well, in your example, it's also protected by the branch circuit breaker for the 30A circuit. Although this would not be true in all cases.

Maybe, maybe not, I didn't specify the branch circuit OCPD. The 3 phase 30A load could be a motor with 50A OCPD. In which case the OCPD in the supplying panelboard could be 60A instead of the 40A I originally specified.

I guess I'm convinced that the NEC rules suffice for protection of a feeder grounded conductor against overload (by calculation) and SCGF (by 215.2(B)'s minimum size). That leads to a followup question which merits a new thread, to follow shortly.

Cheers, Wayne

 

[kwired]

Any thoughts on the OP? Is it simply true in general that a grounded conductor in a feeder is protected against overload only by load calculation? 215.2(B) ensures a minimum size to protect against SCGF.

Cheers, Wayne

That's my thoughts.

Note there is no similar rule for branch circuits. I have run say a 14 AWG neutral with a motor circuit many times when the motor needed much larger conductors and overcurrent devices but the only load on said neutral was a contactor coil and maybe a timer and/or indicator light as part of the controls.

 

[jaggedben]

That's my thoughts.

Note there is no similar rule for branch circuits. I have run say a 14 AWG neutral with a motor circuit many times when the motor needed much larger conductors and overcurrent devices but the only load on said neutral was a contactor coil and maybe a timer and/or indicator light as part of the controls.

I was going to say that since there's no similar rule for branch circuits that means you have to rule a full size neutral. Because since there is no permission to use a calculation instead, how do you otherwise comply with 240.4?

I mean, I don't think it was crazy of you to run a smaller neutral if you really knew the neutral load, but where is the code justification?