Transformer load calculations

[steve66]

I've always sized transformer's using the same load calculations used for conductor sizing. For example, a single motor or a single continuous load would be calculated at 125% FLA.

However, I don't actually see where sizing for transformers is covered in the NEC. At least not in article 450. Can anyone point to where transformer sizing is covered in the NEC?

For example, I have a single 480V, 600 amp chiller that is basically one single motor. So 600*480* sqrt(3) = about 500 KVA.

I have always assumed that I need to take that 500 KVA and multiply by 1.25 which is 625 KVA.

So a 625 KVA transformer would be the smallest transformer allowed to supply this chiller. Is that correct, or am I being overly conservative?

 

[electrofelon]

I've always sized transformer's using the same load calculations used for conductor sizing. For example, a single motor or a single continuous load would be calculated at 125% FLA.

However, I don't actually see where sizing for transformers is covered in the NEC. At least not in article 450. Can anyone point to where transformer sizing is covered in the NEC?

For example, I have a single 480V, 600 amp chiller that is basically one single motor. So 600*480* sqrt(3) = about 500 KVA.

I have always assumed that I need to take that 500 KVA and multiply by 1.25 which is 625 KVA.

So a 625 KVA transformer would be the smallest transformer allowed to supply this chiller. Is that correct, or am I being overly conservative?

NEC doesnt cover sizing transformers. I would say you are being overly conservative. I would skip any extra percentages for motors or continuous loads. Now that said, you 450.3 transformer protection OCPD's need those 125% factors so there is only so much you can "undersize" a transformer. Also it may be prudent to evaluate the voltage sag during motor starting.

 

[tkman85]

When you say 600A chiller, is that 600A FLA or 600A MCA (minimum circuit ampacity)? If the latter, there is already a 125% multiplier on the current rating and your approach would be applying an additional 125% on top of the MCA of 125% * FLA.

 

[steve66]

When you say 600A chiller, is that 600A FLA or 600A MCA (minimum circuit ampacity)? If the latter, there is already a 125% multiplier on the current rating and your approach would be applying an additional 125% on top of the MCA of 125% * FLA.

600 full load amps.

 

[d0nut]

The transformer sizing depends on what your acceptable voltage drop is during motor starting. the IEEE Red book has a really good chart for this. It shows the voltage drop for motors with different sized transformers. It is Figure 3-16 in the version I have.

 

[steve66]

The chiller is on a VFD, so the starting current shouldn't be too much higher than the running current. Or at any rate, it should be less than the across the line starting current.

But it was intended to be more of a general question if we need to include the motor MCA, and/or an 125% of continuous loads for sizing a transformer.

I'm really surprised the NEC doesn't cover transformer sizing. Not sure how I'm never noticed or heard that before.

 

[jim dungar]

I'm really surprised the NEC doesn't cover transformer sizing. Not sure how I'm never noticed or heard that before.

The NEC does not contain any source/supply sizing requirements.
Check with the transformer manufacturer. As others mentioned, one factor to consider is voltage drop during starting, some of which may be desirable.

General purpose transformers are 100% rated.
If you had to apply a 125% factor to the transformer size, why would you then need to also apply a 125% factor to its feeder conductors and its over current protective devices?

 

[steve66]

General purpose transformers are 100% rated.
If you had to apply a 125% factor to the transformer size, why would you then need to also apply a 125% factor to its feeder conductors and its over current protective devices?

Why wouldn't the transformer need the same safety margin that the conductors need?

Looking at a motor load for example, I assume we size conductors at 125% of FLA to give some additional capacity for increased current during motor starting. Or maybe its because the motor load might be a continuous load.

Either way, wouldn't the transformer need the same or some similar additional capacity?

 

[wwhitney]

Why wouldn't the transformer need the same safety margin that the conductors need?

Conductors don't need a 125% for continuous loads, their ampacity is already a continuous rating. It's just regular OCPD that have an issue with continuous loads and require a 125% factor, which then trickles down to the conductor to ensure the conductor is properly protected at shorter time periods. If you use a 100% rated OCPD, then there's no 125% factor for continuous loads. And per Jim transformers are 100% rated, too, so their rating is already a continuous rating.

As to motors and why the FLA gets a 125% multiplier, I've never been clear on that. So no comment on whether that should extend up to transformer sizing.

Cheers, Wayne