(2017) NEC 240.21(C)(1)

[paddycakes]

Hey everybody,

I'm trying to determine whether or not overcurrent protection is required for the secondary conductors of a delta-wye transformer. Without getting into too much detail the installation meets all requirements specified in 240.21(C)(2)-(C)(6), but i'm unsure about 240.21(C)(1) where the second paragraph states:

"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."

It seems to imply that all delta-wye transformers need secondary overcurrent protection. Is this true?

 

[infinity]

Yes and if the transformer is feeding a panel that would have a separate requirement for an OCPD either integral or ahead of the panel.

Welcome to the forum.

 

[paddycakes]

Awesome, thank you!

 

[infinity]

You're welcome. Here's the code reference for panelboards.

408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.

 

[Carultch]

The idea is that, in certain qualifying topologies of transformers, the fault currents and overload currents will line up winding-to-winding, and not distribute onto windings of other phases. Delta-to-Delta 3-wire on both sides and single phase 2-wire on both sides are what qualifies. The primary OCPD can effectively protect the secondary conductors at a rating you get after rescaling it by the transformer's ratio. E.g. 50A at 480V becomes 100A at 240V. "Protect by proxy" as I like to call it.

In WYE systems on either side, or anything with a center-tapped secondary, you don't get to take credit for this, and therefore need secondary OCPD. An overload on the secondary side could go "unnoticed" by the primary OCPD, which is why you also need the secondary OCPD to catch this kind of fault and overload.

 

[infinity]

The idea is that, in certain qualifying topologies of transformers, the fault currents and overload currents will line up winding-to-winding, and not distribute onto windings of other phases. Delta-to-Delta 3-wire on both sides and single phase 2-wire on both sides are what qualifies. The primary OCPD can effectively protect the secondary conductors at a rating you get after rescaling it by the transformer's ratio. E.g. 50A at 480V becomes 100A at 240V. "Protect by proxy" as I like to call it.

In WYE systems on either side, or anything with a center-tapped secondary, you don't get to take credit for this, and therefore need secondary OCPD. An overload on the secondary side could go "unnoticed" by the primary OCPD, which is why you also need the secondary OCPD to catch this kind of fault and overload.

Nice explanation.