Transformer secondary protection

[Dan S]

Can You install multiple panels on the secondary side of a 480-208/120v transformer
if the total of individual panel main c.b. is not over 125% of the secondary amperage?

 

[charlie b]

It can be done, but there are more restrictions than just the rating(s) of the overcurrent device(s). Take a look at 240.21(C).

Welcome to the forum.

 

[don_resqcapt19]

If the primary is protected at no more than 125% then there is no restriction on the size and number of secondary OCPDs. In that case the only issue would be the one Charlie mentioned...the required protection of the secondary conductors.

 

[david luchini]

And if the primary were protected at not more than 250%, then it would be more accurate to say that the total of the multiple panel main c/bs cannot exceed the allowable value for a single OCPD (rather than just 125% of the secondary amperage.)

For example, with a 45kVA xfmr (124.9A secondary, 1.25*124.9A=156.1A) you could install two 50A mcb panels and one 75A mcb panel. The total of the mcbs would exceed 125% of the secondary current, but would not exceed the allowable value of a single OCPD (175A).

 

[jim dungar]

I believe that the ability for the main device to exceed 125% is only available if there is a single secondary protective device.

240.21(C) is about conductors.
450.3 does not mention multiple secondary devices.

 

[david luchini]

I believe that the ability for the main device to exceed 125% is only available if there is a single secondary protective device.

240.21(C) is about conductors.
450.3 does not mention multiple secondary devices.

I don't believe this is correct.

Look at Table 450.3(B): Primary and Secondary protection (assume over 9 Amps)

Primary protection is not to exceed 250%

Secondary Protection is not to exceed 125% (except next standard higher size allowed.)

Note 2: Where secondary over current protection is required, the secondary OCPD shall be permitted to consist of not more than six circuit breakers...grouped in one location. Where multiple OCPDs are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device.

 

[jim dungar]

I don't believe this is correct.

Thanks.

Note to self. Do not post after 11:00PM