Transformer Primary Conductor Sizing

[richwaskowitz]

A customer wants to use a J class fuse at 250% of the nameplate FLA to provide short circuit protection for the primary circuit of a 14 kVA 3 phase 480 volt power transformer. This will result in a LPJ45 fuse. We would have normally used RK5 fuses at 125%, but they want Js. There will be secondary protection at 125% also. My question is, what dictates the size of the primary circuit conductors? Are they based on 125% of the FLA (21 amps / #10) or on the 45 amp fuse (#8)?

 

[don_resqcapt19]

I can't think of any code rule that permits you to protect the transformer primary conductors with an OCPD that exceeds the ampacity of the conductors.

 

[yired29]

I can't think of any code rule that permits you to protect the transformer primary conductors with an OCPD that exceeds the ampacity of the conductors.

Why would we not be able to use the rules in 240.4 B on the primary conductors of a transformer?

Lets say we have a 3 phase 45 kva transformer 480/120-208 (75 deg. Term.)


Primary FLC 45 x 1000 / (480 x 1.73) = 54.19

Table 450.3 B Primary and Secondary protection
54.19 x 125% = 67.73 amps
(Note 1 allows next standard size 240.6 A 70 amps)

#6 THHN = 65 amps @ 75 deg.
65 amps is equal to or greater than the FLC of the transformer
240.4 B allows a #6 @ 65 amps to be terminated on a 70 amp OCPD


Maximum size primary OCPD
54.19 x 250% = 135.47 amps
(No note 1 next standard size down 240.6 A 125 amps)

#2 THHN = 115 amps @ 75 deg.
115 amps is equal to or greater than the FLC of the transformer
240.4 B allows a #2 @ 115 amps to be terminated on a 125 amp OCPD


Secondary FLC 45 x 1000 / (208 x 1.73) = 125.06

Table 450.3 B Primary and Secondary protection
125.06 x 125% = 156.32 amps
(Note 1 allows the next standard size 240.6 A 175 amps)

240.21 C 2 (10 feet)
2/0 THHN = 175 amps @ 75 deg.
175 amps is equal to or greater than the OCPD

 

[don_resqcapt19]

240.4(B) applies, but the question was, can you protect the #10 primary supply conductor with a 45 amp fuse and the answer is no. The size of the transformer primary supply conductors is based on the rating of the primary OCPD and not the full load current of the transformer.

 

[yired29]

240.4(B) applies, but the question was, can you protect the #10 primary supply conductor with a 45 amp fuse and the answer is no. The size of the transformer primary supply conductors is based on the rating of the primary OCPD and not the full load current of the transformer.

I believe the conductors need to meet both FLC and OCPD if the primary OCPD is larger than the FLC of the transformer. The primary conductors can have an ampacity less than the OCPD as long as the rules in 240.4 B are followed and the ampacity of the conductors meet the load.

 

[yired29]

I can't think of any code rule that permits you to protect the transformer primary conductors with an OCPD that exceeds the ampacity of the conductors.

240.4(B) applies, but the question was, can you protect the #10 primary supply conductor with a 45 amp fuse and the answer is no. The size of the transformer primary supply conductors is based on the rating of the primary OCPD and not the full load current of the transformer.

first post and second post don't agree with each other.

 

[don_resqcapt19]

first post and second post don't agree with each other.

Maybe I should have said the conductor must be protected per the rules in Article 240, but again the question was if you can use a #10 on a 45 amp OCPD for this application, and the answer is still no.

 

[don_resqcapt19]

I believe the conductors need to meet both FLC and OCPD if the primary OCPD is larger than the FLC of the transformer. The primary conductors can have an ampacity less than the OCPD as long as the rules in 240.4 B are followed and the ampacity of the conductors meet the load.

If the conductor is protected per the rules in Article 240, I don't see a requirement that the conductor have an ampacity that is equal to or greater than the full load current of the transformer. If the transformer is oversized for some reason, I don't see a need to feed it with a conductor sized to the FLA of the transformer, however an undersized conductor will require a smaller OCPD and that may not hold the inrush.

 

[yired29]

If the conductor is protected per the rules in Article 240, I don't see a requirement that the conductor have an ampacity that is equal to or greater than the full load current of the transformer. If the transformer is oversized for some reason, I don't see a need to feed it with a conductor sized to the FLA of the transformer, however an undersized conductor will require a smaller OCPD and that may not hold the inrush.

The only point I have is that if you size the primary OCPD at 125% of FLC than the conductors need to have an ampacity that could handle the FLC and follow the rules in 240. With OCPD sized at 125% of FLC there is a potential for currents equal to the FLC of the transformer. If OCPD is sized under FLC than the conductors only need to be sized based on OCPD.

For example

3 phase 480 / 120-208 225 kva transformer (75 deg termination)

Primary FLC = 225 x 1000 / (480 x 1.73) = 270.95 amps
270.95 x 125% = 338.69 amps
If by design installer choses not to use Note 1 to table 450.3 than the OCPD would be 300 amps. Note 1 is permissible language not required.

The smallest conductor that can be terminated on a 300 amp OCPD following the rules in 240.4 B is a 250 kcmil which has an ampacity of 255 amps at 75 deg. With a 300 amp OCPD we could use the transformer to its FLC of 270 amps this is where we need to be sure the conductors can handle the load and OCPD selection. In this case we would need to use a 300 kcmil conductor which has an ampacity of 285 amps at 75 deg.

This situation doesn't happen often but it does. Hope that makes sense.

 

[don_resqcapt19]

I don't see how that is any different than what I said in post #8. My post said you could use a smaller conductor if the transformer is oversized. If you are using the transformer at its full load current, it is not oversized.