Transformer Secondary Protection

[timm333]

I am trying to figure out the NEC rules for the protection and sizing of transformer secondary cable in supervised location. The transformer is 500kVA 4160V-480V/277V (delta-wye grounded). The length of the transformer secondary cable is 100 feet. NEC table 450.3(A) states that the secondary protection is not required in the supervised location. However, NEC 240.21(C)(3) states that if the secondary protection is not provided, then the length of the transformer secondary cable cannot be more than 25 feet.

This transformer feeds a 480V switchboard which has a 400A main breaker. This 400A breaker is on the secondary side of transformer, but it is 100 feet away from the transformer. Can this 400A main breaker of switchboard be used as the secondary protection of the transformer? Thanks

 

[david luchini]

I am trying to figure out the NEC rules for the protection and sizing of transformer secondary cable in supervised location. The transformer is 500kVA 4160V-480V/277V (delta-wye grounded). The length of the transformer secondary cable is 100 feet. NEC table 450.3(A) states that the secondary protection is not required in the supervised location. However, NEC 240.21(C)(3) states that if the secondary protection is not provided, then the length of the transformer secondary cable cannot be more than 25 feet.

This transformer feeds a 480V switchboard which has a 400A main breaker. This 400A breaker is on the secondary side of transformer, but it is 100 feet away from the transformer. Can this 400A main breaker of switchboard be used as the secondary protection of the transformer? Thanks

You are conflating two separate issues... Transformer Secondary Protection and Secondary Conductor Protection.

The allowance to not have transformer secondary protection in Table 450.3(A) is dependent on the transformer primary protection size.

Even if you permitted to not have transformer secondary protection, you would still need secondary conductor protection. See 240.92(C) for transformer secondary conductor protection requirements in Supervised Industrial Installations.

Yes, the 400A main breaker of the switchboard can be uses as the secondary protection of the transformer, but there is not enough information to determine if it is providing proper protection of the secondary conductors.

 

[timm333]

240.92(C) has good information. In our case the transformer primary fuse is 100E, transformer impedance is 5.75%, and the ampacity of the cable (on the secondary side of the transformer) is 420A.

240.92(C)(3) states that the overcurrent device has to provide short circuit and ground fault protection. The main breaker of the switchboard is at the ending point of cable, so it will only provide the overload protection and it will not provide the short circuit and ground fault protections. Because in order to provide the short circuit and ground fault protections, the breaker has to be at the starting point (not at ending point) of the cable. Is it correct?

 

[david luchini]

240.92(C)(3) states that the overcurrent device has to provide short circuit and ground fault protection.

What Code cycle? 240.92(C)(3) says "Physical Protection" in the 2017 and 2020 Codes.

 

[timm333]

I was looking at NEC-2014, but NEC-2020 is the same. It is not about physical protection.

240.92(C)(3) says: """The conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors within recognized time vs. current limits for all short-circuit and ground-fault conditions.
It is not about physical protection."""

Basically my question is that if the breaker is at the ending point (instead of starting point) of the cable, will it still provide the short circuit and ground fault protections?

 

[david luchini]

I was looking at NEC-2014, but NEC-2020 is the same. It is not about physical protection.

In the 2020 NEC, 240.92(C)(3) says "Physical Protection. The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

Basically my question is that if the breaker is at the ending point (instead of starting point) of the cable, will it still provide the short circuit and ground fault protections?

240.92(C)(1) tells you how to protect the secondary conductors against short circuits and ground faults. Comply with one of the three methods listed there.