Table 430.52 Maximuim

[wireguy8169]

A discussion in class went something like this:

Sizing TDF's for a 60Hp 460V Three Phase Motor. If you go to the tables the FLC is 77 amps then table 430.52 its a 1.75 max adjustment.

So 77 * 1.75 = 134.75A

So if that is the maximum protection it was discussed that you should go to the next lower OCPD based on 240.6 which would be 125A TDF.

Now if you look at one of the exceptions in 430.52 if the value after applying Tabel 430.52 does not correspond to a standard OCPD you are permitted to use the next standard higher OCPD based on 240.6.

Which is 150A TDF.

Now I realize in the real world there are other issues, but this is a sample question the book answer key gives 125A and that is what some came up with and others 150A.

One reason for the 125 TDF's is that table 430.52 says max so you should not exceed that, but then you have the exception.

Also in the diagram in the book its showing that the fuses are not the sole overcurrent protection device there is a feeder to the Fuseable disconnect and then that feeds the starter.

I am asking the forum to just give some insight on to how they see it or if we are way off base and are wrong well that too.

Thanks

 

[texie]

A discussion in class went something like this:

Sizing TDF's for a 60Hp 460V Three Phase Motor. If you go to the tables the FLC is 77 amps then table 430.52 its a 1.75 max adjustment.

So 77 * 1.75 = 134.75A

So if that is the maximum protection it was discussed that you should go to the next lower OCPD based on 240.6 which would be 125A TDF.

Now if you look at one of the exceptions in 430.52 if the value after applying Tabel 430.52 does not correspond to a standard OCPD you are permitted to use the next standard higher OCPD based on 240.6.

Which is 150A TDF.

Now I realize in the real world there are other issues, but this is a sample question the book answer key gives 125A and that is what some came up with and others 150A.

One reason for the 125 TDF's is that table 430.52 says max so you should not exceed that, but then you have the exception.

Also in the diagram in the book its showing that the fuses are not the sole overcurrent protection device there is a feeder to the Fuseable disconnect and then that feeds the starter.

I am asking the forum to just give some insight on to how they see it or if we are way off base and are wrong well that too.

Thanks

IMO the correct answer is 150 amp per 430.52(C)(1) exception 1. As I recall, this is a pretty common type of exam question.