Need help with transformer OCPD and wire size calculations

[hidroela]

I have this Transformer 112.5kva 480 120/208 3phase
ILine Primary= 112,500VA / (480 ? 1.732) = 135.32A
ILine Secondary= 112,500VA / (208 ? 1.732) = 312.27A
According to 450.3(B)
Primary only protection
132.32 X 1.25 = 169.15A next size up OCPD 175A
Conductor?s size
Primary side:
135.12 X 1.25 = 169.15A
Conductor size 2/0 AWG at 75?C, from TABLE 310.15(B)(16)
Secondary side:
312.27 X 1.25 = 390.31A
More than three current-Carrying conductors
Conductor size 700 kcmil at 95?C, from TABLE 310.15(B)(16)
Primary and secondary protection

Primary side 132.32 X 2.5 = 338.3A next size up OCPD 350A

Secondary side 312.27 X 1.25 = 390.33A next size up OCPD 400A

Conductor?s size
Primary side:
Conductor size 500 kcmil at 75?C, from TABLE 310.15(B)(16)
Secondary side:
More than three current-Carrying conductors
Conductor size 700 kcmil at 95?C, from TABLE 310.15(B)(16)

Are this calculations right.

Best regards
HP

 

[david luchini]

There are a couple of issues with your calculations:

Primary Protection Only -

You don't indicate the OCPD that will protect the secondary conductors (per 240.21(C)) so it impossible to know if the conductors are sized properly. In addition, the neutral on the secondary might not be a current carrying conductor per 310.15(B)(5).

Primary and Secondary Protection -

You cannot use the "next size up" on the primary OCPD. You would need a 300A OCPD. Primary conductors could be 300mcm. (If the 350A OCPD was correct, you could use 400mcm instead of 500mcm.) Again, the neutral on the secondary might not be a ccc, so it could be 600mcm on the secondary with only 3 ccc's, or you are correct with 700mcm for 4 ccc's.

 

[hidroela]

There are a couple of issues with your calculations:

You cannot use the "next size up" on the primary OCPD

now i am confused because on 2011 NEC Table 450.3(B)
Notes: 1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that doesnot exceed the next higher standard rating shall be permitted.

Best Regards
HP

 

[david luchini]

now i am confused because on 2011 NEC Table 450.3(B)
Notes: 1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that doesnot exceed the next higher standard rating shall be permitted.

Best Regards
HP

You should see in Table 450.3(B) that Note 1 is indicated to apply to the primary in "primary only protection" for currents of 9 amps or more, and to the secondary in "primary and secondary protection" for currents of 9 amps or more.

Note 1 is NOT indicated to apply to the primary for "primary and secondary protection"

 

[hidroela]

You should see in Table 450.3(B) that Note 1 is indicated to apply to the primary in "primary only protection" for currents of 9 amps or more, and to the secondary in "primary and secondary protection" for currents of 9 amps or more.

Note 1 is NOT indicated to apply to the primary for "primary and secondary protection"

ok thanks for the explanations

Best Regards
HP

 

[hidroela]

There are a couple of issues with your calculations:

Primary Protection Only -

You don't indicate the OCPD that will protect the secondary conductors (per 240.21(C)) so it impossible to know if the conductors are sized properly.

i have been reading this examples from here i don't see where they make references to the type of OCPD used on that calculations

 

[david luchini]

i have been reading this examples from here i don't see where they make references to the type of OCPD used on that calculations

Its not the type of OCPD, is the size of the OCPD that matters. See Step 4 in your link.

Since in the Primary Only Protection example, you have no limit on the secondary OCPD size, you could use the 112.5kVA transformer to supply a 500A MCB panelboard, for example. In that case your secondary conductors would need to be two sets of 250mcm (for 3 ccc's) or two sets of 300mcm (for 4 ccc's.) The secondary conductor ampacity must be at least as high as the rating of the OCPD that protects them.

The point I was making is that it is impossible to say the 700mcm would be the correct size without knowing what size the OCPD is. They would be fine for a 400A or smaller OCPD, but undersized for a 450A or larger OCPD.

 

[hidroela]

Its not the type of OCPD, is the size of the OCPD that matters. See Step 4 in your link.

Since in the Primary Only Protection example, you have no limit on the secondary OCPD size, you could use the 112.5kVA transformer to supply a 500A MCB panelboard, for example. In that case your secondary conductors would need to be two sets of 250mcm (for 3 ccc's) or two sets of 300mcm (for 4 ccc's.) The secondary conductor ampacity must be at least as high as the rating of the OCPD that protects them.

The point I was making is that it is impossible to say the 700mcm would be the correct size without knowing what size the OCPD is. They would be fine for a 400A or smaller OCPD, but undersized for a 450A or larger OCPD.

thanks a lot for taken your time, i do really appreciated your help.
have a good day

Best Regards
HP

 

[hidroela]

(If the 350A OCPD was correct, you could use 400mcm instead of 500mcm.)

can you elaborated on this please

Best Regards
HP

 

[david luchini]

can you elaborated on this please

Best Regards
HP

Yes,

You selected a primary C/B of 350A on a transformer of 112.5kVA. Assuming your transformer is fully loaded or even a little bit overloaded, your primary current will be say in the range of 135-150Amps.

240.4(B) allows you to protect your conductors using the next standard higher size OCPD. 400mcm has an ampacity of 335, which is large enough for your load, and is allowed to be protected by the 350A c/b per 240.4(B). For that matter, you could even use 350mcm. 350mcm has an ampacity of 310, which is large enough for your load, and is allowed to be protected by the 350A c/b.

Because of 240.4(B), your conductor ampacity does not need to be greater or equal to your OCPD size on the primary. It does need to have sufficient ampacity for the load in accordance with 215.2(A)(1).

The "next standard size" up rule for protection of conductors does not apply to the secondary conductors, only the primary conductors.

 

[hidroela]

Yes,

You selected a primary C/B of 350A on a transformer of 112.5kVA. Assuming your transformer is fully loaded or even a little bit overloaded, your primary current will be say in the range of 135-150Amps.

240.4(B) allows you to protect your conductors using the next standard higher size OCPD. 400mcm has an ampacity of 335, which is large enough for your load, and is allowed to be protected by the 350A c/b per 240.4(B). For that matter, you could even use 350mcm. 350mcm has an ampacity of 310, which is large enough for your load, and is allowed to be protected by the 350A c/b.

Because of 240.4(B), your conductor ampacity does not need to be greater or equal to your OCPD size on the primary. It does need to have sufficient ampacity for the load in accordance with 215.2(A)(1).

The "next standard size" up rule for protection of conductors does not apply to the secondary conductors, only the primary conductors.

One more time thanks
Best Regard
HP