Service Conductor Size

[texie]

I treat the grounded conductor of a 4 wire high leg delta just like I treat the grounded conductor of a 3 wire 120/240 volt single phase system. For all practical purposes, it is the same electrically.

Agreed.

 

[Dennis Alwon]

What I said above applies here, too. For example, you can feed a 200a 1ph MB panel and a 100a 3ph panel, the high leg only needs to be sized for 100a, and the neutral only needs to be sized for the 1ph neutral load.

That is for downsizing the neutral-- I agree, you can size it as lsmall as the grounding electrode conductor if the calcs work. I am saying that you cannot downsize the high leg conductor as the op asked

 

[texie]

I wonder what the OPs type of loads are on this. I have never seen a POCO supply a high leg delta service this large unless they only have 2 primary phases available from a Y system and that is the best they can do.

 

[LarryFine]

I have seen 4 wire delta systems with 3 conductor sizes many times where the service OCPD was provided by fuses. The two ungrounded conductors that serve the single phase loads are the largest, then the neutral, and the high leg the smallest.

Exactly. Remember that the high-leg delta started out as a 3ph modification (usually open-delta) to an existing 1ph service. The added conductor was usually sized only large enough for the added load, often only one circuit.

Remember the "delta" breaker?

A friend's parents' house had a high-leg delta that supplied only the outside disconnect for the A/C compressor unit, with what appeared to be a #10 red wire. It didn't even pass through the meter, if I remember correctly.

 

[texie]

That is for downsizing the neutral-- I agree, you can size it as lsmall as the grounding electrode conductor if the calcs work. I am saying that you cannot downsize the high leg conductor as the op asked

Down sizing the high leg on small commercial 4 wire delta service when the 3 phase load is limited is commonly done with a fusible main disconnect. I used to live in an area where this was very common and they had these high leg services by the bucket load in light commercial areas and the major 3 phase load is A/C.

 

[LarryFine]

That is for downsizing the neutral-- I agree, you can size it as lsmall as the grounding electrode conductor if the calcs work. I am saying that you cannot downsize the high leg conductor as the op asked

You certainly can, as long as it's sized for its calculated load and protected accordingly. You can either use fuses or more than one main breaker.

 

[texie]

Exactly. Remember that the high-leg delta started out as a 3ph modification (usually open-delta) to an existing 1ph service. The added conductor was usually sized only large enough for the added load, often only one circuit.

Remember the "delta" breaker?

A friend's parents' house had a high-leg delta that supplied only the outside disconnect for the A/C compressor unit, with what appeared to be a #10 red wire. It didn't even pass through the meter, if I remember correctly.

Yep, this was very common in sunbelt states like Florida and Arizona in the early days of A/C in homes.

 

[texie]

Dennis, the question I raised, the thing I don't know, is whether the neutral conductor of a high leg delta would carry only the unbalanced current from the three phases. To me, it is not obviously a yes or a no. On one hand, whatever current leaves on Phase A and doesn't return on either Phase B or C must necessarily return on the neutral. So that tends to make the answer "yes." But the math is not as simple as it is on a 4-wire from a wye secondary. I once proved, mathematically, that the total heat generated by balanced current in three phases will be higher than the total heat generated by lower currents in the phase conductors and the remaining unbalanced current in the neutral, assuming the same power is fed in both circumstances. But I don't have that proof handy anymore, and I don't know whether it would work the same for a high leg delta.

To that I will add that310.15(B)(5) gives us two cases in which the neutral would count as a CCC. The high leg delta is not one of those cases. So it is left up to us to decide whether the neutral only carries unbalanced current from the phases.

In a 3 phase 4 wire delta the neutral is electrically just like a neutral in a 120/240 single phase service and can be adjusted for the load. However, in a 3 phase 3 wire corner grounded service it would be a grounded conductor as opposed to a neutral and must be sized the same as the other 2 phases. Kind of moot as a corner grounded service today would be a real rarity although some oldies are still running.

 

[Dennis Alwon]

You certainly can, as long as it's sized for its calculated load and protected accordingly. You can either use fuses or more than one main breaker.

How would you protect it properly. That was my point. 3 phase breaker--- one phases with a different amperage....haha... If you are going to say you lower the overcurrent protective device then I thjnk that is obvious and not what I think the op was talking about.

 

[LarryFine]

As I at least hinted to, tap A and C phases from the large 1ph conductors, and feed B directly to the smaller 3ph main breaker. Taps and high-leg protected.

 

[petersonra]

in the case of a 4 wire delta system, the grounded conductor is not a neutral. I know it is being a little picky.

 

[LarryFine]

in the case of a 4 wire delta system, the grounded conductor is not a neutral. I know it is being a little picky.

Disagree. Defend your position.

 

[charlie b]

If I were to run a circuit from a breaker on Phase B (the high leg), through the load, and back to the grounded bus, I would have a single phase 208 volt circuit. In that circumstance, the "neutral" (if that is the correct term) would not just be carrying unbalanced load from the other phases. So it would have to be counted as a CCC. But is that a legal installation?

 

[LarryFine]

Charlie, I believe it still falls under the "any current the neutral is carrying is current not having to be carried by another conductor" theory.

 

[texie]

If I were to run a circuit from a breaker on Phase B (the high leg), through the load, and back to the grounded bus, I would have a single phase 208 volt circuit. In that circumstance, the "neutral" (if that is the correct term) would not just be carrying unbalanced load from the other phases. So it would have to be counted as a CCC. But is that a legal installation?

No, you can't use the high leg to feed line to neutral load. It can only be used to supply line to line single phase 240 load that does not need a neutral (with non slash rated breaker) or 3 phase load.

 

[charlie b]

No, you can't use the high leg to feed line to neutral load.

That would have been my guess. But I don't like to guess. Can you give me a code citation?

 

[texie]

That would have been my guess. But I don't like to guess. Can you give me a code citation?

Oh, now your going get technical on me. You engineers are so picky.:happyyes:
This has been discussed over the years about where it says that you can not supply line to neutral loads from the high leg. AFAIK there is no direct code citation. However one code issue would be there is no single pole breaker rated for the voltage if you are 120/240 panel. Also most US 208 rated loads are considered to be line to line, not line to neutral.
Another issue comes up when attempting to supply 120/240 2pole loads (in other words a 240 single phase load that needs a neutral). Bad idea as one never knows which items within the load are connected line to neutral.

 

[synchro]

in the case of a 4 wire delta system, the grounded conductor is not a neutral. I know it is being a little picky.

Disagree. Defend your position.

The grounded conductor at the center-tap is a neutral for single phase loads that are connected between it and phase A or C (where B is the high-leg). It is a neutral N with respect to A and C because the sum of the A-N and C-N voltages (the vector sum of phasors, or sum of time-domain waveforms) is essentially zero. I think this is what Larry was getting at.

The center-tap between phases A and C is not a neutral for 3-phase loads that are connected to A, B, and C. That's because the vector sum of the voltages at A, B, and C relative to the center-tap point is not zero, but is in fact the phasor voltage at the high-leg B terminal relative to the center-tap (that's because the sum of A and C voltages relative to the center-tap is zero as mentioned above). I believe this might be the perspective that petersonra expressed in his post.

 

[LarryFine]

My response is that nothing that happens on the high leg contributes to the neutral current; it will only affect the other two lines.

Only a line-to-neutral difference between the two 120v lines will cause neutral current, just like on any 120/240v 1ph service.

A high-leg service can be treated like a separate 1ph 120/240v service and a 3ph 240v delta service that happen to share some wires.