Wye-Wye Transformers and Primary/Secondary Current Relationships

[wwhitney]

For a 1:1 wye-wye transformer, my understanding is that for each ungrounded leg, the secondary line current = the secondary coil current = the primary coil current = the primary line current. Is that correct, or have I overlooked some mixing or averaging that is happening when determining the primary currents given the secondary currents?

If that is correct, is the omission of wye-wye transformers from the list in 240.21(C)(1) of cases where the primary OCPD can provide protection for the secondary conductors an oversight?

Thanks,
Wayne

 

[LarryFine]

. . . for each ungrounded leg, the secondary line current = the secondary coil current = the primary coil current = the primary line current. Is that correct, or have I overlooked some mixing or averaging that is happening when determining the primary currents given the secondary currents?

First, a small correction: " . . . for each ungrounded leg, the secondary line current = the secondary coil current and the primary coil current = the primary line current."

A wye-wye system is a MWBC, three individual 1ph circuits with a shared neutral. There is no interaction of load division among phases like a wye-delta or a delta-wye bank.

If that is correct, is the omission of wye-wye transformers from the list in 240.21(C)(1) of cases where the primary OCPD can provide protection for the secondary conductors an oversight?

I will have to read it to respond. What does it say about a delta-delta transformer bank?

 

[wwhitney]

First, a small correction: " . . . for each ungrounded leg, the secondary line current = the secondary coil current and the primary coil current = the primary line current."

I did mean what I originally wrote: since I specified the transformer is 1:1, the secondary coil current = the primary coil current for corresponding coils. The 1:1 case is simplest, and it suffices to consider it, as for other turns ratios, you just get a consistent scale factor across all the currents on the primary.

Here's the text of 2017 NEC 240.21(C)(1):

Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device

Cheers, Wayne

 

[LarryFine]

I did mean what I originally wrote: since I specified the transformer is 1:1, the secondary coil current = the primary coil current for corresponding coils. The 1:1 case is simplest, and it suffices to consider it, as for other turns ratios, you just get a consistent scale factor across all the currents on the primary.

I completely overlooked the 1:1 mention. My apologies.

Here's the text of 2017 NEC 240.21(C)(1):

I do not know why it wouldn't apply to wye-wye, as long as neutrals are maintained.

 

[JoeStillman]

Isn't a wye-wye bank just three two-wire transformers with a common point (the neutral)?

 

[LarryFine]

Isn't a wye-wye bank just three two-wire transformers with a common point (the neutral)?

I think so, as I stated in post #2.

 

[electrofelon]

Isn't a wye-wye bank just three two-wire transformers with a common point (the neutral)?

Isn't a delta delta bank just three two wire transformers connected in series and then back to the beginning?