Transformer/Panel Wire Sizing/Conduit Fill

[Electricalhelp]

I received the base building drawing and was confused if the wire sizes used are correct?

For the 112.5kva transformer, shouldn't the ground on the primary be at least #6 and NOT #3 [Table 250.122]

The wire size for 350 AMP they used 4#400kcmil, #4G, 4"C
Based on my calculation, 380 AMP*.8 = 304 AMP [Table 310.16 and Current Carrying Conductors]
For 350 AMP, shouldn't the ground conductor be at least #3 and NOT #4 [Table 250.122]
The wire size for 350 AMP, I would suggest 4#600kcmil, #3G, 4"C

For the 200AMP, the wire sized they used 4#250kcmil, #6G, 2"C
Based on my calculation, 290 AMP*.8 = 232 AMP [Table 310.16 and Current Carrying Conductors]
For 232 AMP, shouldn't the ground conductor be at least #4 and NOT #6 [Table 250.122, rounded up]

Lastly if they did use 4#250kcmil, #6G, 2"C, I get a conduit fill of 47.46%, which does not meet NEC Code.
The correct wire size should be 4#4/0, #4G, 2 1/2"C or 4#250kcmil, #4G, 3"C


Let me know if any of my calculation are incorrect and if I made any mistakes as well.

See attached riser diagram for your reference

 

[infinity]

There is a lot wrong with this sketch. If building steel the GEC should be #1/0, the SSBJ to the disconnect should also be #1/0, the 400's are too small as the secondary conductors. The EGC's with the taps need to be the same size as the EGC's after the disconnect switch.

 

[augie47]

I agree with infinity that there are deficiencies in the drawing.
Note however, in your calculations you appear to be using a .8 multiplier. Unless you know a major portion of the load in non-linear (very unlikely), you don't need to count the neutral as a ccc so the .8 multiplier isn't necessary.
Do you know the size the transform,er primary OCP ??

 

[infinity]

I agree with Augie the 80% factor should not be used in the calculations.

 

[Electricalhelp]

I agree with infinity that there are deficiencies in the drawing.
Note however, in your calculations you appear to be using a .8 multiplier. Unless you know a major portion of the load in non-linear (very unlikely), you don't need to count the neutral as a ccc so the .8 multiplier isn't necessary.
Do you know the size the transform,er primary OCP ??

The main breaker in the 277/480V/3 Phase Panel is 150 AMP for the transformer.

Thanks for letting me know the .8 multiplier.

There is a lot wrong with this sketch. If building steel the GEC should be #1/0, the SSBJ to the disconnect should also be #1/0, the 400's are too small as the secondary conductors. The EGC's with the taps need to be the same size as the EGC's after the disconnect switch.

So if that's the case, I should use at least 4#500kcmil, #1/0G, 4"C after the disconnect

What about the conduit for the 200AMP Panels?

 

[infinity]

The main breaker in the 277/480V/3 Phase Panel is 150 AMP for the transformer.

Thanks for letting me know the .8 multiplier.


So if that's the case, I should use at least 4#500kcmil, #1/0G, 4"C after the disconnect

What about the conduit for the 200AMP Panels?

Yes 500 kcmil copper (380 amps) would be the minimum size for a single set tomthe 350 amp OCPD. How long are those nipples? 24" or less and the fill jumps to 60%.

 

[Electricalhelp]

Yes 500 kcmil copper (380 amps) would be the minimum size for a single set tomthe 350 amp OCPD. How long are those nipples? 24" or less and the fill jumps to 60%.

It's close to 24", but I thought it was okay to be close to 60%

 

[infinity]

Not sure why they would specify 250 kcmil for a 200 amp panel when the conductors are 10' long. 4-#3/0 and 1-#3 EGC in a 2" conduit is what I would use.

 

[Electricalhelp]

Not sure why they would specify 250 kcmil for a 200 amp panel when the conductors are 10' long. 4-#3/0 and 1-#3 EGC in a 2" conduit is what I would use.

I agree with the 4#3/0 but how did you get #3 EGC.
For 200AMP based on Table 250.122 I get #6

 

[infinity]

For 200AMP based on Table 250.122 I get #6

Post #2:

The EGC's with the taps need to be the same size as the EGC's after the disconnect switch.

So your EGC after the 350 amp fuses is a minimum of #3 based on 250.122. The EGC's run with the tap conductors are required to be at the minimum the same size.

 

[petersonra]

What were you expect from someone that cannot spell transformer correctly on a drawing?

 

[Electricalhelp]

Thank you for the breakdown

Do you know the size the transform,er primary OCP ??

Just waiting on what I need to do with the Primary OCP

What were you expect from someone that cannot spell transformer correctly on a drawing?

LOL, I said the same thing.

Here is the updated riser diagram with the sizes

 

[infinity]

Here is the updated riser diagram with the sizes

The EGC after the 350 amp OCPD does not need to be #1/0. Look at T250.122, for 400 amps the minimum size is #3 AWG. While we're at it what type of electrode is being used for the transformer?

 

[Electricalhelp]

While we're at it what type of electrode is being used for the transformer?

no sure, what should I recommend. This is a multilevel high riser building.
I would assume the metal building frame

 

[infinity]

This is a multilevel high riser building.
I would assume the metal building frame

Sounds likely that building steel will be your electrode so #1/0 would be the GEC size.

 

[Electricalhelp]

Sounds likely that building steel will be your electrode so #1/0 would be the GEC size.

Correct.

But I'm still wondering how they got #3 ground wire for the transformer?

 

[infinity]

But I'm still wondering how they got #3 ground wire for the transformer?

For the same reason that they got 5 other things wrong. Sounds like they used the wrong table. T250.122 would give you a #3 if you used 400 amps. The correct table is 250.66 based on the derived (secondary) conductors.

 

[Electricalhelp]

I just did a quick walk thru and now we have another issue.
Instead of a race way, they installed a 208V/600AMP/3P MCB Panel after the disconnect.
Connected the 2 Subpanels to the Distribution Panel.

I thought a 112.5 KVA transformer can have a maximum size of 400AMP?

 

[augie47]

With a 150 amp breaker on the primary the transformer is considered to be protected and the secondary does not have a limit.
The secondary conductors must be sized for 600 amp minimum
and your grounding electrode conductor must be sized accordingly

 

[Electricalhelp]

With a 150 amp breaker on the primary the transformer is considered to be protected and the secondary does not have a limit.
The secondary conductors must be sized for 600 amp minimum
and your grounding electrode conductor must be sized accordingly

Sorry, can I ask you what code section this is so I can read about it more.

Isn't it typical for most transformers are connected with either a disconnect or a MCB on the primary side before the transformer.

So the Panel can be sized for 600AMP but the total demand load still needs to be around 400AMP only