Conductor sizing 15kva transformer

[JJackson2491]

I am installing a new machines in a friends shop and could use a hand with conductor sizing.

120/208 3 phase panel

curing lamp machine
requires:
480v 25a 3 phase circuit continuous load

I found a step up transformer and my local supply house but its been a while since ive done this type of calculation and any help would be greatly appreciated.

 

[augie47]

480 X 1.732 X 25 X 1.25 = 25.980 kw

A 30 kva would be your next standard size.

 

[texie]

As Augie indicates, a 30 KVA will do it assuming this is not a high inrush load. The 208 input current would be 30,000 /208/1.732=83 amps. Breaker would be 83 X 1.25=103 amp (next size up=110 amp.)
The 480 load side would be 30,000/480/1.723=36 amp. Secondary breaker = 36X 1.25=45 amp.
But, you have other things to consider. Is the load balanced? Do you need a neutral to the machine? Do you have a 208 delta to 480 Y transfarmer? It you have a 208Y X 480 delta that you are using in reverse (I would not recommend), that presents other issues that need consideration.

 

[JJackson2491]

thanks for the refresher guys it is greatly appreciated! I think i might wait and order this transformer since the price is better than my supply house,

http://www.swgr.com/store/Dry-Type-Low-Transformer/product/11-4387.aspx

 

[conmgt]

Since this transformer is strictly for a dedicated load and that load is 26kVA, could the primary current be considered to be 26,000/208/1.732=72A?

 

[Smart $]

Since this transformer is strictly for a dedicated load and that load is 26kVA, could the primary current be considered to be 26,000/208/1.732=72A?

The load is actually 480V?25A?sqrt(3)= 21kW. Gus put in the 125% for continuous... I imagine because the thread title asks for conductor sizing.

 

[conmgt]

The 208 input current would be 30,000 /208/1.732=83 amps. Breaker would be 83 X 1.25=103 amp (next size up=110 amp.)
The 480 load side would be 30,000/480/1.723=36 amp. Secondary breaker = 36X 1.25=45 amp.

I'm not agreeing with a 110A breaker. It seems overkill...to me.

That 103A 208 current is based on the capacity of the transformer and not the actual load.
Does the transformer need to have conductors and an OCP rated for the transformer...or just the load?
The load already had 25% added to the 21 kW load get to 26kW and then that was bumped 15% to 30kVA and then the load of that 30kVA transformer had another 25% added to get to 103A.

In this example, the actual load is 480V * 25A * 1.732 = 20,784. The primary side would see 20,784/208/1.732 = 58A plus 25% (for continuous)...58 * 1.25 = 72A. Add 15% for transformer losses for a total of 80A. 90A breaker or you can apply 450.3B and go with a 100A breaker.
Primary conductors: #4 Cu or #2 AL. 1" EMT for AL.
With a 103A current, conductor size: #2 Cu or 1/0 AL 1.25" EMT for AL.

Not much of a difference is the run is short and the bends are few but if we're talking a 100'+ run, that could add up. Or make a difference in other situations when maybe you are dealing with an existing transformer or getting a good deal on a used transformer.

Basically, I just seeing if everything can be based on that actual load of the machine.

 

[Smart $]

I'm not agreeing with a 110A breaker. It seems overkill...to me.

That 103A 208 current is based on the capacity of the transformer and not the actual load.
Does the transformer need to have conductors and an OCP rated for the transformer...or just the load?
The load already had 25% added to the 21 kW load get to 26kW and then that was bumped 15% to 30kVA and then the load of that 30kVA transformer had another 25% added to get to 103A.

In this example, the actual load is 480V * 25A * 1.732 = 20,784. The primary side would see 20,784/208/1.732 = 58A plus 25% (for continuous)...58 * 1.25 = 72A. Add 15% for transformer losses for a total of 80A. 90A breaker or you can apply 450.3B and go with a 100A breaker.
Primary conductors: #4 Cu or #2 AL. 1" EMT for AL.
With a 103A current, conductor size: #2 Cu or 1/0 AL 1.25" EMT for AL.

Not much of a difference is the run is short and the bends are few but if we're talking a 100'+ run, that could add up. Or make a difference in other situations when maybe you are dealing with an existing transformer or getting a good deal on a used transformer.

Basically, I just seeing if everything can be based on that actual load of the machine.

I think a 25kVA transformer is sufficient. Are not transformers rated for contiuous load? As such, the primary rated current would be 25kVA/360=69.4A. I've not heard of being able to add in transformer losses as part of the rated current, so I won't. If we go with primary only protection (other conditions apply), max is 125% of rated current, i.e. 86.8A... limiting primary ocp to next standard size of 90A. But as stated, the load current (58A) is less than the rated current (69.4A). Sizing to the load, the ocpd must be sized at least 125% of 58A... or 72A as mentioned. Next standard size is 80A, also as mentioned...

So unless you go with secondary protection, 90A primary breaker is the max, 80A the minimum. Conductor sizing follows this choice.

 

[jim dungar]

Basically, I just seeing if everything can be based on that actual load of the machine.

The protection and sizing for equipment on the output of the transformer (its secondary) can be based solely on the load.

The input to the transformer (its primary) needs to be based on its inrush (usually the deciding factor in OCPD sizing) and losses (usually relatively minimal) as well as its output load. It is usually easiest to consider the transformer as being 100% loaded.

 

[conmgt]

I'm not sure about a continuous rating for transformers...as in, I don't know one way or the other.

If the actual load is 21kW and add 25% for continuous loads to get 26kW, what allows a 25kVA transformer? Or is it splitting hairs? What about inrush? If the nameplate on the equipment says 25A circuit, is that like when a microwave says "20A" on it so you don't run a 15A circuit to it and it really draws less.

I recently looked at a spec sheet on a 30 kVA transformer and it showed a 1400W consumption for itself while connected to a load. The 15% addition for transformer loss was rough guess and possibly on the high side.

 

[conmgt]

The protection and sizing for equipment on the output of the transformer (its secondary) can be based solely on the load.

The input to the transformer (its primary) needs to be based on its inrush (usually the deciding factor in OCPD sizing) and losses (usually relatively minimal) as well as its output load. It is usually easiest to consider the transformer as being 100% loaded.

Ahhh, so the transformer's inrush is the same independent of the load attached to it and therefore the OCP needs to be sized for that. And the conductors can still be sized for the load the transformer will see? With motors, conductors are sized the the FLA.

 

[jim dungar]

If the actual load is 21kW and add 25% for continuous loads to get 26kW, what allows a 25kVA transformer?

The transformer is not a load, it is a supply. Maybe up to 99% of the time, where you have a 21KW load you need a 21KVA transformer (assuming unity PF load).

The difference between continuous and non-continuous (the ubiquitous 1.25 multiplier) is involved with sizing conductors and their protection.

 

[Smart $]

...

If the actual load is 21kW and add 25% for continuous loads to get 26kW, what allows a 25kVA transformer? Or is it splitting hairs? What about inrush?

Where are you getting that you have to add 25% to a transformer's rating for continuous loading?

Inrush will affect sizing of the OCPD, not the rating of the transformer itself.

If the nameplate on the equipment says 25A circuit, is that like when a microwave says "20A" on it so you don't run a 15A circuit to it and it really draws less.

If it says that and actually draws less then it is not considered a continuous load. The circuit rating is a result of considering the maximum current, not the average or typical current.

I recently looked at a spec sheet on a 30 kVA transformer and it showed a 1400W consumption for itself while connected to a load. The 15% addition for transformer loss was rough guess and possibly on the high side.

Losses are a consideration but AFAIK not part of the rated current or kVA rating.

 

[jim dungar]

Ahhh, so the transformer's inrush is the same independent of the load attached to it and therefore the OCP needs to be sized for that. And the conductors can still be sized for the load the transformer will see? With motors, conductors are sized the the FLA.

Yes, knid of.
In fact, sometimes a loaded transformer has less inrush than an un-loaded one, but for all intents inrush is not load dependent.

The primary conductors need to be sized based on the primary protective device.

The secondary conductors and protection are load dependent.

 

[Smart $]

...

The secondary conductors and protection are load dependent.

If secondary protected.

If primary-only protected, must calculate proportional current... right?

 

[conmgt]

The transformer is not a load, it is a supply. Maybe up to 99% of the time, where you have a 21KW load you need a 21KVA transformer.

The difference between continuous and non-continuous (the ubiquitous 1.25 multiplier) is involved with sizing conductors and their protection.

Duly noted.

Where are you getting that you have to add 25% to a transformer's rating for continuous loading?

I was thinking that the 21kW load was 25% higher because of the 3hr continuous rule.

If it says that and actually draws less then it is not considered a continuous load. The circuit rating is a result of considering the maximum current, not the average or typical current.
Losses are a consideration but AFAIK not part of the rated current or kVA rating.

So maybe the 25A on the nameplate has all of the factoring calculated...like HVAC minimum circuity ampacity?

The primary conductors need to be sized based on the primary protective device.

Can you point to a code reference for that?

 

[jim dungar]

If secondary protected.

If primary-only protected, must calculate proportional current... right?

Kind of. You must check that the primary protection and the turns ratio are not oversized based on your choice of secondary conductors.
While the end result is probably the same, the basic steps I always try to use are: 1) secondary load, 2) secondary conductors, 3) secondary conductor protection (which might be located on primary), 4) transformer, 5) primary protection, 7) primary conductor.
Work from the load towards the source, not the source.

 

[Dennis Alwon]

I believe transformers can be loaded at 100%

 

[jim dungar]

The primary conductors need to be sized based on the primary protective device.

Can you point to a code reference for that?

450.3 and 240.4

The informational note to 450.3 refers you to article 240 for conductor protection
240.4(G) that allows 'oversizing' for motor circuts does not apply to transformer primary conductors.

 

[conmgt]

450.3 and 240.4

The informational note to 450.3 refers you to article 240 for conductor protection
240.4(G) that allows 'oversizing' for motor circuts does not apply to transformer primary conductors.

Hmmm...so any exceptions to a conductor's amperage rating needing to match an OCP's amperage rating would be listed on this table and the Articles/Sections referenced.