Transformer Primary Current

[nkalghatgi]

I have a 25KVA 3phase 480V to a single phase 120/240V transformer which is connected to a single phase panelboard. The panelboard is rated at 100A. Considering the fact that I can load the panelboard upto 80A, how should I calculate current referred to primary side of the transformer to size the feeder conductors coming to the transformer.

 

[david luchini]

I have a 25KVA 3phase 480V to a single phase 120/240V transformer which is connected to a single phase panelboard. The panelboard is rated at 100A. Considering the fact that I can load the panelboard upto 80A, how should I calculate current referred to primary side of the transformer to size the feeder conductors coming to the transformer.

The primary OCPD must be large enough to carry 100% of the non-continuous load plus 125% of the continuous load.

Since your secondary OCPD is less than 125% of the secondary rated current, your primary OCPD can be as large as 250% of the primary rated current, per 450.3(B).

(There is no requirement that the 100A panelboard only be load to 80A. 100% of the non-continuous load plus 125% of the continuous load can equal 100A.)

 

[augie47]

supporting exactly what david stated but adding: select your desired primary overcurrent device per 450.3 and then size the conductor according to the OCP device
(as opposed to the transfer load).

 

[greenspark1]

Unless it's a 100% rated breaker, that 100A main could start tripping at currents of 80A. But otherwise yes, 450.3, you really have your choice of upstream transformer protection.

 

[charlie b]

You lost me here:

I have a 25KVA 3phase 480V to a single phase 120/240V transformer . . . .

I can't imagine such an animal. Do you mean that the primary is single phase 480V, fed from two poles of a three phase 480 volt source?

 

[david luchini]

Unless it's a 100% rated breaker, that 100A main could start tripping at currents of 80A. But otherwise yes, 450.3, you really have your choice of upstream transformer protection.

I don't know of any 100A c/b that would start tripping at currents of 80A, unless it was an adjustable type that could be set lower.

A standard 100A c/b should be able to supply a 100A non-continuous load with no problems.

 

[iceworm]

You lost me here: I can't imagine such an animal. Do you mean that the primary is single phase 480V, fed from two poles of a three phase 480 volt source?

yes, really odd. I've only seen one - don't remember how long ago. I recall it was a 15kva - I don't know what the rated output was - I suspect 10kva

ice

 

[iceworm]

I don't know of any 100A c/b that would start tripping at currents of 80A, unless it was an adjustable type that could be set lower.

A standard 100A c/b should be able to supply a 100A non-continuous load with no problems.

Yes, it should with ease. UL 489 breakers are tested to never trip at 100%.
Okay, they don't test forever, just until the temperature stabilizes.

ice

​

 

[AdrianWint]

The only way to get that configuration would be to connect the transformer line-line on the 480V side.

Since the voltage ratio is 480/240 = 2:1 so the current ratio would be 1:2.

So, a current of 80A on the secondary side would be reflected as 40A on the primary side.

However, this is only 40 x 480 = 19.2 kVA.

25kVA would pull 52A on the primary side, being reflected as 104A secondary current.

 

[zioncools]

Since in the original scenario, you have your choice of primary protection, what would be the minimum (cheapest) primary wiring size and breaker size which would allow for a 100A load on the secondary (non-continuous plus 125% of continuous equals 100A). This would be assuming you just want to save money and do not want to plan for future load growth.

 

[zioncools]

I meant to edit the above post but couldn't for some reason. What I really wanted to ask is this:
When we protect the secondary at below 125% of the transformer's rated current, we have the choice of primary protection up to 250%. But let's say with this same 25kVA transformer (assuming it's single-phase 480-240/120), I want the secondary to have 70A wiring and a 70A bkr (as the main breaker on a panelboard fed by the transformer). If in this case I want to minimize the wiring size on the primary side and add a primary breaker as small as possible, how would I determine the minimum breaker and wiring on the primary side which I could use and still successfully supply the load? I want to feed the load but I don't want to plan for any future additional load.

 

[iceworm]

I think this is a design desion - not a code decision.

Questions:
How often can you (or your customer) stand a spurious trip?
Is it okay for the feeder to trip on power-up after an outage?
If, on normal energizaton, the feeder trips a time or two before finally holding in, is that okay?

These answers have a lot to do with a minimalistic* design.

Example:
Most of my customers would have a freaking fit if it they could still remember the last time it had happened. Or in the case of critical circuit, if it ever happened.

ice
(*is that a word?)

 

[david luchini]

I meant to edit the above post but couldn't for some reason. What I really wanted to ask is this:
When we protect the secondary at below 125% of the transformer's rated current, we have the choice of primary protection up to 250%. But let's say with this same 25kVA transformer (assuming it's single-phase 480-240/120), I want the secondary to have 70A wiring and a 70A bkr (as the main breaker on a panelboard fed by the transformer). If in this case I want to minimize the wiring size on the primary side and add a primary breaker as small as possible, how would I determine the minimum breaker and wiring on the primary side which I could use and still successfully supply the load? I want to feed the load but I don't want to plan for any future additional load.

As noted before, the minimum size breaker on the primary that the code will allow would be sized to carry 100% of the non-continuous load plus 125% of the continuous load.

If you have a 100A c/b on the secondary, you could certainly put a 50A c/b on the primary. However, a small c/b on the primary may trip on inrush current, so there is no guarantee that a 50A primary c/b would be OK.

 

[zioncools]

Thanks for the replies--they make sense.

Here is another "design" type of question:

If you are calculating total load on an electrical system, and you have this same transformer on the system with a secondary load (FLA) of 60A, how would you figure the FLA on the primary side? Would it just be the transformer ratio multiplied by 60A, or would you include some extra for transformer inefficiency?

Thanks!

 

[iceworm]

Thanks for the replies--they make sense.

Here is another "design" type of question:

If you are calculating total load on an electrical system, and you have this same transformer on the system with a secondary load (FLA) of 60A, how would you figure the FLA on the primary side? Would it just be the transformer ratio multiplied by 60A, or would you include some extra for transformer inefficiency?

Thanks!

I can't tell what you are trying to do.

Are you looking to calculate the primary feeder size? If so I use the nameplate primary FLA if given. If not given, calculate the primary fla using the xfm kva and primary voltage. And as david noted, one has to pay attention to the xfm inrush current.

Are you trying to do a facility load calc? If this, I tend to reflect the secondary load kva right to the primary. Calculate the primary amps from the load kva and primary voltage.

ice