Transformer, OCPD, and Wire Size

[KPR16]

This is rather long and complicated but I would like to make sure I have calculated everything correctly. Sorry for it being so long but I wanted to include all of the info. Please advise.

The scenario.
We will be installing a 112.5 kva 480 to 120/208 3 ph transformer. The transformer will be supplying a 120/208 3 ph mlo panel.
The transformer will be fed from a breaker in a main distribution panel in a separate building. The wire run from the main distribution panel to the transformer is 650 feet in 3 inch schdl 40 pvc 36 inches deep.

1st- The ocpd at the main distribution panel for the primary side - 112,500VA / (480 ? 1.732) = 135.32A,
135.32 X 1.25 = 169.15A - The next standard size up OCPD = 175A

2nd- Sizing the primary side conductors.
175A ocpd = 2/0 AWG at 75 degrees c. according to Table 310.16
Voltage drop for 650 feet - 480x3% = 14.4 - I want my voltage drop to be under 14.4v
14.4v = (1.73 x K x Length x Amps) / kcmil
14.4 = (1.73 x 12.9 x 650 x 170) / Y
Y = 171251 circular mils = 4/0 wire
Now to size the ground I have to increase its size by the same percentage that I increased my current carrying conductors
2/0 wire = 133100 circular mils
4/0 wire = 211600 circular mils
133100/211600 = 63%
According to 250.122 my ground should be a #6 AWG = 26240 circular mils
26240/63% = 40,369 = #4 AWG

Mounted beside the transformer on the primary side there will be a disconnect. Does it need to be fused?

Now to size the secondary of the transformer.
112,500VA / (208 ? 1.732) = 312.27A
312.27 x 1.25 = 390 A
Next size up = 400 amps
After the transformer on the secondary side there will be a 400A fused disconnect.
After the disconnect will be the 120/208 400A rated panel
Wire size will be parallel runs of 4/0
More than 3 current carrying conductors requires reduction in allowable ampacity to 80%
4/0 at 90 degrees c = 260
260 x 0.8 = 216A
Two parallel feeds of 4/0 = 432A

Here is where I get a little mixed up.
The ground on the secondary, does it need to get sized according to the 400A disconnect or the conductors that it will be running with.
I believe it needs to be sized according to the disconnect so each conduit will need a #3 ground with the 4 4/0 conductors. Please correct me if I am wrong or if you find any mistakes in my calculations. Any advice would be appreciated.
Thanks in advance,
Keith

 

[charlie b]

I agree with your basic approach. Here are my observations:

• Agree with 175 amp primary OCPD.
• Agree with 2/0 and #6 EGC as starting point.
• I think you can do the VD calculation using the 100% rated current of 135 amps. That would allow you to use a 3/0, instead of 4/0.
• Agree that using a 4/0 forces the EGC to be #4.
• You have a math error, in that 26240/63% is 41650, not 40369. But it still points to a #4 EGC.
• Yes, you do need a fused disconnect on the secondary of the transformer, if the panel is MLO.
• Agree with 400 amp secondary OCPD.
• You have another math error, in that 260 x 0.8 is 208, not 216. So the ampacity of a set of parallel 4/0s is 416, not 432. But that is still sufficient.
• The ground on the secondary of the transformer is a GEC, not an EGC. It is sized per 250.66, not 250.122. Based on two parallel 4/0, with a total area of 211600 x 2, or just over 400 KCMIL, I would use a 1/0 GEC.

Welcome to the forum.

 

[david luchini]

Mounted beside the transformer on the primary side there will be a disconnect. Does it need to be fused?

No, it does not need to be fused, but it does need to be suitable for use as service equipment (225.36.)

Here is where I get a little mixed up.
The ground on the secondary, does it need to get sized according to the 400A disconnect or the conductors that it will be running with.
I believe it needs to be sized according to the disconnect so each conduit will need a #3 ground with the 4 4/0 conductors. Please correct me if I am wrong or if you find any mistakes in my calculations. Any advice would be appreciated.
Thanks in advance,
Keith

The ground from the transformer to the disconnect needs to be sized per 250.66. The ground from the disconnect to the panel needs to be sized per 250.122.

I'm unclear on whether you have one conduit or two from the transformer to the disconnect.

 

[charlie b]

I read part of the question too quickly. David is right, that the disconnect on the primary side does not need to be fused. I am right, that the disconnect on the secondary side does need to be fused, since you don't have any other overcurrent protection on the secondary side between the transformer and the MLO panel. David's description of the ground wire requirements on the secondary side is more complete than mine.

 

[david luchini]

David is right

Go Irish!

 

[KPR16]

Thanks

Thanks

Thanks David and Charlie
For ease of installation I think there will be two parallel feeds from the transformer to the disconnect on the secondary side. From the disconnect to the panel I think there will be just one feed.
I am glad that I was on the right track with my calculations.
Thanks again for the help and clarifications.

Keith

 

[david luchini]

For ease of installation I think there will be two parallel feeds from the transformer to the disconnect on the secondary side. From the disconnect to the panel I think there will be just one feed.

I would say a #2 in each conduit from the transformer to disconnect, and a #3 in the conduit from the disconnect to panel.

 

[lielec11]

Just a question, but why wouldn't you just run 1 conduit with 500kcmil feeders instead of 2 conduits with 4/0? Are you basing this on pulling 2 sets of 4/0 vs. pulling 1 set of 500's?

 

[Pharon]

I also don't understand why you're de-rating the 4/0 parallel conductors. Aren't you running them in separate raceways?

But I'm with lielec11. I would spec one set of #500kcmil instead of parallel #4/0 anyway.

 

[infinity]

Just a question, but why wouldn't you just run 1 conduit with 500kcmil feeders instead of 2 conduits with 4/0? Are you basing this on pulling 2 sets of 4/0 vs. pulling 1 set of 500's?

500 kcmil is only rated at 380 amps. He needs a 400 amp conductor.

 

[Pharon]

500 kcmil is only rated at 380 amps. He needs a 400 amp conductor.

500kcmil is fine according to 240.4(B).

 

[lielec11]

500 kcmil is only rated at 380 amps. He needs a 400 amp conductor.

That's correct if he's using 75 deg C, however, most contractors I've dealt with usually submit 90 deg C. Regardless, isn't it okay to protect the 500's with a 400A overcurrent protection per 240.4-B? Or are you saying this because the secondary current x 1.25 is 390A?

 

[lielec11]

500kcmil is fine according to 240.4(B).

You beat me to it

 

[infinity]

500kcmil is fine according to 240.4(B).

You beat me to it

Sorry but you're both incorrect.

240.21(C) Transformer Secondary Conductors. A set of con-
ductors feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary, as specified in 240.21(C)(1) through
(C)(6). The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.

 

[Pharon]

That sounds like 240.4(B) doesn't apply only if you choose to not have OCPD on the secondary. But I don't have my Code in front of me to verify the whole context right now.

 

[xformer]

I read part of the question too quickly. David is right, that the disconnect on the primary side does not need to be fused. I am right, that the disconnect on the secondary side does need to be fused, since you don't have any other overcurrent protection on the secondary side between the transformer and the MLO panel. David's description of the ground wire requirements on the secondary side is more complete than mine.

Does the Secondary Side need fusing when the Primary is protected at 125%?

 

[GoldDigger]

It does for a delta-wye transformer or for a center tapped secondary with single phase.
In both of those cases there can be excess current in *part* of the secondary without excess current in any winding on the primary side.

Tapatalk!

 

[don_resqcapt19]

That sounds like 240.4(B) doesn't apply only if you choose to not have OCPD on the secondary. But I don't have my Code in front of me to verify the whole context right now.

240.4(B) does not apply to any conductor protected per the rules found in 240.21. So in this case you can't use 500 kcmil between the transformer secondary and the disconnect, but you could use it from the disconnect to the MLO panel.

 

[lielec11]

That sounds like 240.4(B) doesn't apply only if you choose to not have OCPD on the secondary. But I don't have my Code in front of me to verify the whole context right now.

It appears it doesn't apply from the secondary of the transformer to the first overcurrent device, whether it is a disconnect switch or main breaker or whatever. If you read into 240.21(C)(1) through (C)(6), most of them have a stipulation that states:

"The ampacity of the secondary conductors is... not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the terminations of the secondary conductors."

Using 500's (assuming 380A) and feeding a 400A disconnect would be in violation of this. What if I'm using 90 deg C rated feeders where 500's are good for 430A?

 

[Pharon]

What if I'm using 90 deg C rated feeders where 500's are good for 430A?

Unless the breaker/disconnect terminations are rated for 90 deg C, you need to use the 75 deg C column for ampacity. The 90 deg C column could be used for de-rating (4 or more current carrying conductors in a raceway), however.