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#207973 - 12/14/12 01:45 AM Transformer Feeder Upgrade
bossanova5 Offline
Member

Registered: 10/26/12
Posts: 13
Loc: L.A. CA
Hello all,

Here's my dilema, I'm upgrading an exisiting building to a machine shop.

Currently it has a 400 Amp 3 phase 480 Main Service.It has a 227 KVA Transformer on a(480 volts) 200 Amp breaker feeding a 400 amp 208/120 three phase secondary panel.

I want to put the transformer (227 KVA) on a 400 breaker(Primary),and go to parallel 3/0 feeders for the primary side.

I want to add an additional 225 Amp panel and feed directly off the secondary taps on the transformer.The new 225 Amp panel would have a 225 Amp main breaker.

I'm thinking this is legal.

The 227KVA transfomer at 208 volts according to my calculations can handle 618 Amps.

Thanks for your help in advance

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#207975 - 12/14/12 01:11 PM Re: Transformer Upgrade [Re: bossanova5]
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
I believe you are correct. Your selection of overcurrent devices meets the primary and secondary protection requirements of Table 450.3(B).

BTW, I came up with 630 amps for a 227 kva transformer.

I'm thinking you could actually bump up the size of the new panel to 350 amps.
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#207980 - 12/14/12 08:52 PM Re: Transformer Upgrade [Re: bossanova5]
HTSR77 Offline
New Member

Registered: 12/06/12
Posts: 2
Loc: Washington
Its my opinion that your primary feeder for your 227kva transformer should be calculated as such:

227,000 / ( 1.732 x 480) = 273 amps

273 amps x 125% = 341 amp

341A next size up per 240.6 would be 350A breaker

So parallel sets of (3)#2/0 CU THHN/W @ 75 deg. would be ideal. The 400 amp breaker exceeds the maximum 125% as allowed in 430.3(b) for primary protection only.


Edited by HTSR77 (12/14/12 08:53 PM)

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#207982 - 12/15/12 01:29 AM Re: Transformer Upgrade [Re: bossanova5]
bossanova5 Offline
Member

Registered: 10/26/12
Posts: 13
Loc: L.A. CA
The transformer (Secondary) will be feeding two panels one 400 amp(Which is currently the Main 208/120 panel for the building) and the additional 225 Amp panel I want to add.

As far as I know it's legal to feed multiple panels from one transformer assuming you don't overload it.

Please correct me if I'm wrong Thanks

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#207987 - 12/15/12 06:23 PM Re: Transformer Upgrade [Re: bossanova5]
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
There are two ways of protecting a transformer, primary only and primary and secondary. If you use primary and secondary protection, your primary OCPD can be 250% of the primary current, in this case, 273 x 2,5 = 682 amps, so your 400 amp breaker is fine. The secondary full load current is 630 amp x 125% = 787 amps max ocpd. Your proposed total of ocpds for the secondary side is only 625 amps, well within the limits. you are allowed multiple ocpds, see note 2 to table 450.3(B)

Conductor protection is a separate issue from transformer protection, so parallel 3/0 conductors on a 400 amp breaker is a good pick.


Edited by Tom (12/15/12 06:28 PM)
Edit Reason: added conductor comment
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