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Joined: May 2007
Posts: 55
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Up2code Offline OP
Member
My calculations for this transformer's primary conductors & primary overcurrent protection, do not match what was submitted by the engineer. Any advice would be appreciated.

Transformer is as follows:
300kva, 3 phase, 3 wire, 480V primary with
3 phase, 4 wire, 208/120V secondary.
Transformer is being fed off of existing 800 amp switchgear.

Submitted plans show primary overcurrent device to be
400 amp, 3 pole breaker & (3) 750kcm AL primary conductors.

I'm coming up with 361 amps of current, for this 300 kva transformer. According to 215.2(A)(1), the current gets multiplied by 125%, which now brings my 361 amps to 451 amps, which under table 310.16, requires...

1250kcm AL or 700kcm CU (using 75C column for terminal ratings.)
And table 240.6, requires a 450 amp overcurrent device.
Am I missing something?

Joined: Jul 2004
Posts: 9,663
Likes: 4
G
Member
What is the computed load and the secondary O/C protection? Maybe the transformer is just bigger than it needs to be for the installation.


Greg Fretwell
Joined: May 2007
Posts: 55
U
Up2code Offline OP
Member
Not sure what total connected load is. Will check in AM. Secondary conductors ( 2 sets of 4-750kcm AL) will terminate into an 800A ocpd.

Is it common practice to size the transformer primary ocpd & conductors off of calculated load & not it's kva rating? Perhaps that's why my calcs are diffent from engineer's.

Joined: Apr 2002
Posts: 7,279
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400 amp primary (480) OCP for a 300 KVA xfr is 'normal, as 800 amp on the secondary (208).

750KCmil Al on the primary is 'OK' (385 A at 75 degree), pushed to 400 OCP, allowed by code.

2x750 on the secondary side (385x2 = 770 amp) pushed to 800 amp OCP, allowed by code.

Although you have AL conductors, this method in Cu is a common occurance on plan review for me. (500Kcmil Cu)



John
Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
It seems to hinge on 215.2

I do not see, from the OP's description, why he is assuming a maximum continuous load. He can probably skip the 125% calculation, and size his wire to 362 amps. After all, 215 also tells us to base the wire size on the nameplate, as a minimum.

I agree with the 75 degree column.

I would not size the wire to the 400-amp breaker; in this instance, you need to oversize the breaker to allow for current inrush at start-up. Similar to the principles used for motor OCPD sizing.

So, I'm looking at smaller wire .... say, 500mcm for copper and 700mcm for aluminum. Did I err?


Last edited by renosteinke; 01/18/12 12:03 PM.
Joined: May 2007
Posts: 55
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Up2code Offline OP
Member
Reno, why would installation not hinge on 215.2? Load would be considered continuous load, correct? So my primary current of 361 amps should be multiplied by 125%, requiring primary conductors to have ampacity of at least 451 amps. Why would 125% calculation be skipped? Perhaps I'm possibly missing something.

Greg, engineer's calculated demand on the transformer is 409 amps. Secondary conductors terminate into an 800amp ocpd.

Perhaps as the electical inspector, I am just making something more of an issue than it really is. As Hotline1 stated, it's normal for engineer's to size transformers and conductors this way.

Joined: Jul 2004
Posts: 9,663
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G
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The thing you have to remember is 215 applies to the load, not the size of the transformer. It would not be unreasonable to spec a transformer that is good for 125% of the maximum computed load but that doesn't mean you would have to upsize the feeders to 156%


Greg Fretwell
Joined: Jan 2005
Posts: 5,316
Cat Servant
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The transformer may be "on" all the time, but it is not fully loaded all the time.

If the actual connected loads were 300 amps, and the transformer rated at 361 amps ..... your load on the transformer is only 300 amps.

Now, if the connected loads were a continuous 300 amps, I would want the wires to be able to handle 300 x 1.25, or 375 amps.

It's the actual loads that matter - except that the transformer feeders cannot be smaller than the 362 amps called for by the transformers nameplate.

Joined: May 2007
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Up2code Offline OP
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Thanks everyone for the help! I just need more practice sizing transformer primary & secondary conductors.

Joined: Jul 2004
Posts: 9,663
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G
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At a certain point you just have to trust the engineer to know what he is doing but there is nothing wrong with checking his work.


Greg Fretwell
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