Tap Conductors Art 240(B)(3) vs (B)(5)

[pisani168]

Fellow Sparkys,

I have an overdutied chiller and applied UL508A Supplement SB. Unfortunately, the parts / devices I ordered to apply Supplement SB are delayed and the summer is coming closer. I was thinking of remediating the overduty issue by using an isolation transformer. Here is my problem.

I have a tap conductor (outside of unlimited (90ft) length Art 240(B)(5)) feeding the chiller. (B)(3) clearly says that taps feeding transformers are limited to 25ft in combined length (PRI + SEC). (B)(5) outlines outside taps of unlimited length if routed outside of a building. Which rule applies?

In my opinion, if the CMP wanted transformers to be fed via taps of unlimited lengths, they would have mentioned it. But I'd like to hear your opinion.

I will probably place a fusible disconnect on the PRI side of the transformer. I can then feed the transformer without running into any code issues. But I am still interested in your opinion.

 

[Dennis Alwon]

If the tap is outside then you have unlimited length on the secondary providing you have fulfilled the requirement in 240.21(B)(5)

 

[pisani168]

Hi Dennis,

the secondary is clear to me. The primary is what concerns me. I have an 85ft tap conductor that would feed the transformer. I believe I am limited to 25ft primary and secondary if I’d install a OCPD on the secondary only. Or is my thinking wrong?

 

[don_resqcapt19]

The outside tap does not apply where the circuit originates within a building or structure. Where is the tap connection made?

 

[pisani168]

The tap originates on the roof on the line side of a fusible disconnect.

 

[augie47]

IMO, If they originate outside (on the roof) and remain outside (B)(5) would be the applicable Section.

 

[wwhitney]

IMO, If they originate outside (on the roof) and remain outside (B)(5) would be the applicable Section.

Which would require that the taps terminate on an OCPD, rather than supply the transformer directly as (B)(3) permits, correct?

But if the total primary and secondary lengths were 25 ft or under, then (B)(3) would apply, allowing the taps to supply the transformer directly. The fact that it is outside doesn't preclude the use of (B)(3).

Seems clear cut to me.

Cheers, Wayne

 

[pisani168]

Which would require that the taps terminate on an OCPD, rather than supply the transformer directly as (B)(3) permits, correct?

But if the total primary and secondary lengths were 25 ft or under, then (B)(3) would apply, allowing the taps to supply the transformer directly. The fact that it is outside doesn't preclude the use of (B)(3).

Seems clear cut to me.

Cheers, Wayne

Good thinking. You called out the OCPD. The question is if the the secondary conductors is considered a tap or not. And the only reason why I’m throwing this out here is because the wire size won’t change. Just the impedance.

However, I am almost certain that you are correct.


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[wwhitney]

Good thinking. You called out the OCPD. The question is if the the secondary conductors is considered a tap or not. And the only reason why I’m throwing this out here is because the wire size won’t change. Just the impedance.

Once the primary side 210.24(B)(5) outdoor tap of unlimited length terminates on an OCPD as required, then downstream of that OCPD is a separate situation. The standard rules on transformer, primary feeder, and secondary conductor protection would apply. [Rules I'm not familiar with.]

Cheers, Wayne

 

[pisani168]

Once the primary side 210.24(B)(5) outdoor tap of unlimited length terminates on an OCPD as required, then downstream of that OCPD is a separate situation. The standard rules on transformer, primary feeder, and secondary conductor protection would apply. [Rules I'm not familiar with.]

Cheers, Wayne

We are saying the same thing but you found a better way to describing it. Terminate the tap in an OCPD and then feed the transformer off the OCPD.

 

[don_resqcapt19]

And if you use a two wire to two wire transformer, or a 3 wire delta-delta transformer, no protection of the secondary conductors will be required. Any other type of transformer will require that the secondary conductors be protected in accordance with one of the rules in 240.21(C).

 

[pisani168]

It’s 3 wire delta / wye. I have slash rated equipment so I’ll need 277V against ground.

 

[don_resqcapt19]

It’s 3 wire delta / wye. I have slash rated equipment so I’ll need 277V against ground.

Then you will be required to provide protection for the secondary conductors in accordance with one of the 240.21(C) rules. That will also require a GEC and a connection to a grounding electrode, which may be difficult for a roof installation.

 

[wwhitney]

That will also require a GEC and a connection to a grounding electrode, which may be difficult for a roof installation.

In which case maybe this is an example where it would be useful to install the transformer as a non-SDS? If the feeder tap to the chiller has a neutral conductor (which I would think it would, as I understand it is currently supplying the chiller), then with the delta-wye transformer, the primary neutral could be connected to the secondary neutral, as I understand it. Then there would be no SBJ and no GEC.

Of course, I don't really understand how using an isolation transformer resolves an "overduty issue," so I'm not sure if it would need to be an SDS to do that.

Cheers, Wayne

 

[don_resqcapt19]

In which case maybe this is an example where it would be useful to install the transformer as a non-SDS? If the feeder tap to the chiller has a neutral conductor (which I would think it would, as I understand it is currently supplying the chiller), then with the delta-wye transformer, the primary neutral could be connected to the secondary neutral, as I understand it. Then there would be no SBJ and no GEC.

Of course, I don't really understand how using an isolation transformer resolves an "overduty issue," so I'm not sure if it would need to be an SDS to do that.

Cheers, Wayne

As far as the overduty issue the impedance of the transformer reduces the available fault current to a value that does not exceed the SCCR of the equipment.

Maybe a better choice would be an inductor and not a transformer to reduce the available fault current.