Sizing primary on transformer if load is known

[deetz]

So when sizing the primary wires for a transformer that's powering a machine, can you use the machine loads for the sizing of the wires for the primary? That is after you do your calculations. Say you have a 70kva load machine that is supplied by a 75kva transformer. Can you take 70kva times the 831.36 to get your amps for wire sizing? The machine is 70kva at 220v, so the secondary wire will be bigger of course. The reason for this is I have a buss plug that is 100 amps already from a different machine installed and was wondering if I could use it. The finished calculation would be 105 amps on 480v, and was hoping I could use it, if the powers that be would be okay with it.

 

[xnuke]

You can always put in a smaller OCPD than the maximum code allows, but you may experience nuisance trips.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[deetz]

Okay, so let me get this straight. Machine load. 70kva at 220v three phase. 70,000 divided by 831.36 times 1.25 is 105 amps. Run #2 wire for the primary to a 100 amp Buss disconnect, using 100amp RK5 fuses. 70,000 divided by 381.04 times 1.25 is 230 amps. Run 4/0 wire for the secondary to machine. That should be okay with code?

 

[deetz]

Also do you need to multiply 125% to the transformer load capabilities? Meaning the machine load is 70kva, would a 75kva transformer suffice, or do I need to go to a 112.5kva one?

 

[dpc]

The overcurrent devices should be sized based on the transformer kVA and the conductors must be sized based on the resulting overcurrent device rating. The transformer inrush seen by the primary overcurrent device is basically independent of the actual load. If you size the primary device too small, it will trip (sometimes) on transformer inrush.

 

[waross]

Canadian Electrical Code:
26-256 Overcurrent protection for dry-type transformer circuits rated 750 V or less (see
Appendix B)
(1) Except as permitted in Subrule (2), each ungrounded conductor of the transformer feeder or branch circuit
supplying the transformer shall be provided with overcurrent protection rated or set at not more than
125% of the rated primary current of the transformer, and this primary overcurrent device shall be
considered as protecting secondary conductors rated at 125% or more of the rated secondary current.
OR
(2) Notwithstanding Subrule (1), a transformer having an overcurrent device on the secondary side set at not
more than 125% of the rated secondary current of the transformer shall not be required to have an
individual overcurrent device on the primary side, provided that the primary feeder overcurrent device is
set at not more than 300% of the rated primary current of the transformer.
(3) Where a value not exceeding 125% of the rated primary current of the transformer as specified in
Subrule (1) does not correspond to the standard rating of the overcurrent device, the next higher standard
rating shall be permitted.

26-258 Conductor size for transformers
(1) The conductors supplying transformers shall have an ampacity rating
(a) not less than 125% of the rated primary current of the transformer for a single transformer;


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Ohm's law
Not just a good idea;
It's the LAW!