Sizing the Neutral Conductor?

[Mattula]

Transformer:
75KVA
Primary-480v 3Ph 3w - (3#1 & 1#6G)
Secondary- 120/208V 3Ph 4w - ( 4#4/0 & 1#2G)

Neutral: Art.250.66 Table
Largest Ungrounded Conductor - #4/0 Cu Conductor
Neutral Size: #2 Cu Conductor
(This seems too small to me.)

Show me your math please. Thanks!

 

[skeshesh]

How can we? we don't have information about the load...

 

[Mattula]

Well if the max load was the transformers rated output it would be 208 Amps. Does that help?:-?

 

[augie47]

In the other related post the secondary breaker was 225 amps.
With no calculations, you neutral would have to be sized accordingly (with 220.61 allowing a small reduction--70% of over 200).

With calculations, per220.61, you can reduce your neutral accordingly.

(Note I could not come up with an answer as to a minumum size so I addressed that seperately rather than muddy your post. You may want to also read:
http://forums.mikeholt.com/showthread.php?p=1140693#post1140693 )

 

[SG-1]

First, Table 250.66 is the bare miminum size allowed.

Do you have any neutral loads to serve ?
Is this unknown ?

Sometimes we are asked to provide a transformer for customer use. The loads are unknown to us, only the transformer size. We provide a 100% neutral for the customer connections in this case.

If you do not have any neutral loads, you do not need the neutral, just an Equipment Bonding Jumper between the transformer and the disconnect. This could be part of the wiring method.