xfmr kva

[Rich Elec.]

A 3 phase dry transformer is installed, the xfmr kva would permit a larger breaker and conductor size than what the panel can supply.
How do you determine the secondary?
Is it by using the percentage of what was used for the primary?

 

[jim dungar]

A 3 phase dry transformer is installed, the xfmr kva would permit a larger breaker and conductor size than what the panel can supply.
How do you determine the secondary?
Is it by using the percentage of what was used for the primary?

The secondary conductors are sized based on the load just like any conductor. They are protected just like any other conductor. The location of their protection is based on the requirements of 240.21(C). There is no requirement that the transformer capacity (KVA) be considered when performing secondary conductor sizing.

Device sizing in article 450.3(B) only applies to the transformer primary only. The primary device is allowed to be "up sized" only if a secondary protective device not larger than 125% has been provided.

 

[augie47]

disregarding the small loss related to the transformer itself; kva is kva...
primary = secondary
If it's 3 phase delta-delta and your secondary loads are three phase, then I would think the % change in voltage idea would work.
Any other load configuration and you would have to calculate the kva based on loads.

 

[engy]

So your primary OCP will be smaller than normal because of the size of the panel serving the xfmr? You may have nuissance tripping of that breaker.

Secondary - See Jim's post

 

[Rich Elec.]

I am thinking that I didn't convey my question well enough.
Let me ask another way.

If you had a 50KVA, 3 phase, 480 to 120/208 xfmr.
50,000 / 480 x 1.73 = 60.2 amps primary.

Now if the panel suppling the xfmr could only supply 40 amps, could you hook the xfmr up with the 40 amp breaker and #8 wire.
If this is acceptable how would you determine the load output on the secondary?
I assume that it could not be loaded at its full secondary kva, since it is not being supplied at its full capacity.

Or am I confused?

Thanks,
Rich

 

[iwire]

KVA in = KVA out (or fairly closely)

40 amp @ 480 V 3 PH = 33 KVA

33 KVA @ 208 V 3 PH = 91 Amps

So if you supply 91 amps of secondary load you will have about 40 amps of load on the primary.

 

[Rich Elec.]

Duplicate post.

 

[Rich Elec.]

I think that I figured out what I was trying to ask.

I was asking if I could protect a xfmr at less than would be required from its kva calculation.

I now see that I would be in violation of 450.3 (B).

450.3 (B) Over current protection shall be provided in accordance with Table 450.3 (B).

Because the primary conductors are for protection of the xfmr.

Does it seem like I am on the right track?

Rich

 

[jim dungar]

Yes, per the NEC you can protect the transformer at less than its rated capacity. However most protective devices sized smaller then 125% of the primary FLA may trip during start up of the transformer due to inrush current. 450.3 provides you with the maximum settings not the minimums.