Service to transformer

[mclassen]

I am bidding a job to install a new service to a step-up transformer (240/460). This transformer feeds a industrial machine. How do I size the conductors? Like a service to the disconnect and then like a motor after the disconnect? These are my calculations: 30KVA transformer, 175amp fuses at the service, 2/0 CU to the disconnect, #3 CU to the transformer, #6 CU to the machine (The machine disconnect has 50amp, 600v fuses. (1)20HP, (1)1-1/2HP, (2)1HP motors ).

 

[RUWired]

This transformer feeds a industrial machine. How do I size the conductors?

Conductors are sized per 409.20. That is also in line with 430.24.

These are my calculations: 30KVA transformer, 175amp fuses at the service, 2/0 CU to the disconnect, #3 CU to the transformer, #6 CU to the machine

I'm not sure why you are installing a primary disconnect and reducing to # 3 awg. The 175 amp fuses with 2/0 cu. primary is good and the secondary # 6 awg is good, but there is a problem with the secondary O.C.P.D. for the tranformer.

(The machine disconnect has 50amp, 600v fuses. (1)20HP, (1)1-1/2HP, (2)1HP motors ).

The maximum protection for the 30 kva is 45 amps. You would need an O.C.P.D. rated at 45 amps. This might not be enough to start the 20 hp motor. IMO you would need a 45 kva transformer.

Rick

 

[augie47]

it appears you are sizing your transformer primary conductor by load and not by overcurrent protection. Without secondary protection (assuming you have a delta-delta transformer) the max ocp on the 240 primary would be 90 amps. The #3 would be fine, of course, but your OCP would need to be 90 not 175.

 

[RUWired]

With a 2-1 ratio the maximum 90 amp primary breaker still might not be enough to start the 20 hp motor.

 

[augie47]

With a 2-1 ratio the maximum 90 amp primary breaker still might not be enough to start the 20 hp motor.

Rick, I agree, but I could not substanuiate so I addressed the NEC problem.

I think if I were going to try and get by with the 30, I'd add secondary protection and increase my primary OCP.

 

[RUWired]

I think if I were going to try and get by with the 30, I'd add secondary protection and increase my primary OCP.

Gus, He has primary protection at 175 amps and secondary protection at 50 amps, providing the control panel meets the 240.21 requirements. But IMO the 50 amps is 5 amps over the 125%.

So the question is, would most inspectors allow the 50 amp or stick to the 45. The math works out to 45.15.

Rick

 

[gbudke]

Most inspectors that I have dealt with do not even look at the size of the transformer and the OCP for it. But the way it was described to me by an Electrical Engineer when I questioned the prints / spec book for a job, he said 125% then the next largest size commercially available breaker is how it is sized.

 

[augie47]

Gus, He has primary protection at 175 amps and secondary protection at 50 amps, providing the control panel meets the 240.21 requirements. But IMO the 50 amps is 5 amps over the 125%.

So the question is, would most inspectors allow the 50 amp or stick to the 45. The math works out to 45.15.

Rick

Rick, Thats an interesting question since you have a motor load involved.
Using 430.52 he would be in the range of a 70 amp breaker, but 450.4 would restrict him to a 45 (If his primary was over 125%).
I will be interested in what others say. but I guess in this case you would keep your primary at 125% to meet 450.4 and since it's delta-delta, the secondary is protected anyway.

 

[mclassen]

Sorry, I've been tied up for a few days, just now getting back. Thanks for your replies. I was thinking that I had primary & secondary OCP. But I guess I really don't because the machine is 80' from the transformer. I also think the machine does have 45amp fuses. I forgot to make a note when I looked at the machine.

 

[mclassen]

Am I right that I can run 2/0 cu from the weatherhead thru the service disconnect (175amp OCP) to the primary lugs. Then install secondary OCP at the transformer sized at 45amp?

 

[mclassen]

Conductors are sized per 409.20. That is also in line with 430.24.


I'm not sure why you are installing a primary disconnect and reducing to # 3 awg. The 175 amp fuses with 2/0 cu. primary is good and the secondary # 6 awg is good, but there is a problem with the secondary O.C.P.D. for the tranformer.

The maximum protection for the 30 kva is 45 amps. You would need an O.C.P.D. rated at 45 amps. This might not be enough to start the 20 hp motor. IMO you would need a 45 kva transformer.

Rick

My question is, why do I need to size the conductors from the disconnect to the transformer by the OCP? Can I not size them 125% of primary load? Isn't this a motor circuit? Does 240.21B3 apply?

 

[Smart $]

Am I right that I can run 2/0 cu from the weatherhead thru the service disconnect (175amp OCP) to the primary lugs. Then install secondary OCP at the transformer sized at 45amp?

Yes... but you are permitted to up to 60A secondary ocp (45 ? 125% = 56, next higher common rating is 60A) — Table 450.3(B).

My question is, why do I need to size the conductors from the disconnect to the transformer by the OCP? Can I not size them 125% of primary load? Isn't this a motor circuit? Does 240.21B3 apply?

The primary conductors must be protected from overcurrent. In most scenarios the ocp is determined by the conductor size. In this case, it is reversed because the ocp is sized to protect the xfmr primary not the conductors feeding it.

Sure you can size the feeders smaller, but your ocpd must have a rating no higher than the next higher standard ocp size.

With primary and secondary protection installed, it can only be a motor circuit as of the secondary ocp. Even if you didn't have secondary protection, the xfmr is a separately derived system and the motor circuit would start at the secondary taps (which you then would have to include the tap rules in your design).

EDIT: What's up with that???!!! (see next post time-wise)

 

[Smart $]

it appears you are sizing your transformer primary conductor by load and not by overcurrent protection. Without secondary protection (assuming you have a delta-delta transformer) the max ocp on the 240 primary would be 90 amps. The #3 would be fine, of course, but your OCP would need to be 90 not 175.

What's up wth that cont'd...

30,000VA/(240V*sqrt(3))=72.17A

Table 450.3(B) permits up to 125% with primary only protection and 250% with both primary and secondary ocp's.

72.17A ? 125% = 90.22A, next standard ocp rating is 100A
72.17A ? 250% = 180.43A, next standard ocp rating is 200A

As said previously, the secondary is permitted up to 60A ocp. That is incorrect because I was using the stated 45A as the rated secondary current. Assuming this is actually a 240/480 xfmr, the max ocpd would be 72.17 ? 2 * 125% = 45.11A... next higher standard size is 50A.

 

[augie47]

I agree with the 100. I just have a "habit" of staying closer to the calculated number when we are talking about .22 amps.
My primary point was to stress that we needed OCP to protect the conductors also, so a #3 at 175 amps would not meet NEC in this situation.