X0 conductor sizing

[pablodjv]

Hello everyone,

A 2000 kVA 34.5/480 V Delta - Wye transformer will carry mostly (>95%) three phase loads connected only by three phases, not using the neutral. By system design, some lighting panels will be connected L-N in this arrangement.

My question is: How do you properly size the conductors that go from to the transformer's X0 to the main switchgear neutral busbar?

Best regards and thanks in advance.

Pablo.

 

[Smart $]

Load calculation per 220.61.

(A) Basic Calculation. The feeder or service neutral load
shall be the maximum unbalance of the load determined by
this article. The maximum unbalanced load shall be the
maximum net calculated load between the neutral conductor
and any one ungrounded conductor.

Also see 250.24(C) or 250.30(A)(3) for minimum sizing, which for basic install will tell you it can't be any smaller than per Table 250.102(C)(1).

Welcome to the forum. :thumbsup:

 

[topgone]

Hello everyone,

A 2000 kVA 34.5/480 V Delta - Wye transformer will carry mostly (>95%) three phase loads connected only by three phases, not using the neutral. By system design, some lighting panels will be connected L-N in this arrangement.

My question is: How do you properly size the conductors that go from to the transformer's X0 to the main switchgear neutral busbar?

Best regards and thanks in advance.

Pablo.

You will compute for the maximum imbalance that your transformer will experience. Assume you loose one line and you will expect the neutral to have the same amount of current as the lines.

In your case, if your transformer load is 1900 kVA (95%), your line current will be 5962 amperes. If one of your supply lines gets broken, you will expect to have a 5962 amperes going thru your neutral back to your transformer!

 

[Carultch]

Hello everyone,

A 2000 kVA 34.5/480 V Delta - Wye transformer will carry mostly (>95%) three phase loads connected only by three phases, not using the neutral. By system design, some lighting panels will be connected L-N in this arrangement.

My question is: How do you properly size the conductors that go from to the transformer's X0 to the main switchgear neutral busbar?

Best regards and thanks in advance.

Pablo.

If there is no possibility of an imbalanced current, then the smallest size for the neutral conductor is to be no smaller than the equipment grounding conductor, as you would calculate it per 250.122.

 

[Smart $]

If there is no possibility of an imbalanced current, then the smallest size for the neutral conductor is to be no smaller than the equipment grounding conductor, as you would calculate it per 250.122.

Not for a transformer secondary conductor. See my earlier post.

 

[infinity]

Not for a transformer secondary conductor. See my earlier post.

I agree and I think that he meant GEC which T250.66 is mirror of 250.102(C)(1).

 

[Smart $]

I agree and I think that he meant GEC which T250.66 is mirror of 250.102(C)(1).

In this case, it would be not smaller than the SBJ or SSBJ... which is essentially the same size as the GEC, as you say.

 

[infinity]

In this case, it would be not smaller than the SBJ or SSBJ... which is essentially the same size as the GEC, as you say.

Agreed, until you get into parallel raceways where the SSBJ's can be sized based on the ungrounded conductors within the raceways and SBJ's that are subject to the 12.5% rule.

 

[lauraj]

If the SBJ is located at the transformer, 2014 NEC 250.30(A)(3) has no minimum sizing requirement for the grounded conductor, so it could just be sized based off of max unbalance load. Am I missing something?

 

[don_resqcapt19]

If the SBJ is located at the transformer, 2014 NEC 250.30(A)(3) has no minimum sizing requirement for the grounded conductor, so it could just be sized based off of max unbalance load. Am I missing something?

If the system bonding jumper is installed at the transformer, you will have a supply side bonding jumper between the transformer and the first OCPD. I see no reason for the neutral to be larger than required for the maximum possible line to neutral load.

 

[Smart $]

If the system bonding jumper is installed at the transformer, you will have a supply side bonding jumper between the transformer and the first OCPD. I see no reason for the neutral to be larger than required for the maximum possible line to neutral load.

What about a bolted line to neutral fault after the first OCPD and before any branch-circuit OCPD? I believe that justifies a transformer secondary grounded conductor being at the very least not less than the size required for the EGC on the load side of the first OCPD.

 

[Smart $]

BTW, this viewpoint falls under 215.2(A)(2) and 215.2(B) general statement.

The hard part of interpreting Code as such is correlating transformer secondary conductors as feeders, especially when 240.21(C) seems to establish them as a separate type of conductor.

 

[don_resqcapt19]

BTW, this viewpoint falls under 215.2(A)(2) and 215.2(B) general statement.

The hard part of interpreting Code as such is correlating transformer secondary conductors as feeders, especially when 240.21(C) seems to establish them as a separate type of conductor.

Yes, I was not seeing the transformer secondary conductors as feeder conductors, but looking at Article 100, it appears that they are.

Feeder. All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.