Unbalanced transformer loads

[LMAO]

If you have two 320A, 208V (line to line), single phase loads fed from one three phase transformer, what should be the power rating of that transformer? 230kva?

thanks

 

[LMAO]

wait, I think it is 200kva not 230.

 

[charlie b]

I would go with a minimum of 225, and probably I would pick the 300 instead. I look at it as if I planned to install a third load of the same size and type, and connect it so as to balance the loads among the three phases. That is a total of 200 kVA. The 225 is the next higher size, and the 300 would give some room for additional loads. Keep in mind that this is a design process, not a code requirement.

 

[LMAO]

I would go with a minimum of 225, and probably I would pick the 300 instead. I look at it as if I planned to install a third load of the same size and type, and connect it so as to balance the loads among the three phases. That is a total of 200 kVA. The 225 is the next higher size, and the 300 would give some room for additional loads. Keep in mind that this is a design process, not a code requirement.

yeah, I just wanted to know if my calculations were right.
One more thing, I am actually thinking of going with 150kva; is there an IEEE guideline for overloading transformers for some time periods and duty cycles? the loads will be at peak for only a few hours a week.

 

[LarryFine]

One more thing, I am actually thinking of going with 150kva; is there an IEEE guideline for overloading transformers for some time periods and duty cycles? the loads will be at peak for only a few hours a week.

I don't know of any such "allowance," but, it would be for the manufacturer to say.

Each load is 66.56kva, so each section (i.e., phase) of the transformer must be able to carry that much.

You'd need either two 1ph units at least that large, or one 3ph unit at least three times that large.

That looks like 200kva (199.68kva to be exact) to me, so you need the 225kva unit.

 

[BJ Conner]

I am missing something. Put three 320 amp loads on your tansformer. A-B 320 amps, B-C 320 amps and C-320 amps.
The KVA is KVA= Vl-l * I l8 1.732
=320*208*1.732= 115.281 KVA
Your only using 2/3 of that but you can't go lower unless you configure the load differently.

 

[david luchini]

I am missing something. Put three 320 amp loads on your tansformer. A-B 320 amps, B-C 320 amps and C-320 amps.
The KVA is KVA= Vl-l * I * 1.732
=320*208*1.732= 115.281 KVA
Your only using 2/3 of that but you can't go lower unless you configure the load differently.

Yes, you are missing something. Putting three 320A single phase, 208V load on the transformer will result in 3 * (320A*208V) = 199.680kVA on the transformer (not 115.281kVA)

In your equation, you are using Il-l, when you should be using Ip. The current on each phase will be 554.256A, with the three 320A loads connected from A-B, B-C, & C-A.

So kVA=Vl-l * I * 1.732 = 208V * 554.256A * 1.732 = 199.680kVA.

 

[scott thompson]

I come up with a much different Apparent Power Value than posted by others.

If you have two 320A, 208V (line to line), single phase loads fed from one three phase transformer, what should be the power rating of that transformer?

Looks like a maximum Load of 133.12 KVA, as the OP defined (2) L-L 208V Heaters, drawing 320 Amps each.

If both Loads are to be driven Coincidentally, Minimum 3 Phase Transformer Capacity would be 150 KVA.
If Loads are Non-Coincidental, the Minimum Capacity would be 75 KVA.

Connect Loads as follows:

Heater #1: Connect between Line "A" and Line "B"

Heater #2: Connect between Line "C" and Line "A"

Line "A" would be common to both Heaters and their corresponding 2 Wire Branch Circuits. The Heaters would be connected in an Open Delta fashion.

Scott

 

[broadgage]

A single phase load of 320 amps at 208 volts is 66.6KVA, by simple multiplication.

THREE such loads, connected in a balanced way to a suitable three phase transformer, would require a transformer of three times this rating.
3 times 66.6 KVA is 199.6 KVA.

Therefore THREE such loads would require a 200KVA transformer, or the next standard size up from this.

In this case it is stated that only TWO single phase loads are to be supplied, but this does not reduce the size of transformer required which is based on a balanced three phase load.

In theory one could use two single phase transformers, each rated at 66.6 KVA, one for each single phase load.
In practice this would be probably be a poor design choice, the two smaller transformers prevent the future use of three phase loads, they would probably cost as much as one larger unit, and installation would be more complex.

A three phase 200 KVA transformer would not allow any extra load on the fully loaded phases, but would allow up to 66.6 KVA of load to be added to the unloaded phase.

In the real world a larger transformer might be advisable to allow for expansion, or a smaller one might be acceptable if the loads are very short term.

 

[Electric-Light]

That creates a huge load unbalance. Depending on magnitude relative to total service size, it maybe acceptable, but if it creates a substantial voltage unbalance upstream, it will adversely affect motors.

If you load a generator with such a load, it will shake itself to death.

To use all the kVA capacity of the transformer or you're going to use a generator, it maybe necessary to use a motor-generator to convert three phases (loaded evenly across) to a single phase alternator

 

[kwired]

If you have two 320A, 208V (line to line), single phase loads fed from one three phase transformer, what should be the power rating of that transformer? 230kva?

thanks

Are you connecting both loads to same phases or are you going to connect to all three phases with one phase having two connections?

 

[LarryFine]

Looks like a maximum Load of 133.12 KVA, as the OP defined (2) L-L 208V Heaters, drawing 320 Amps each.

If both Loads are to be driven Coincidentally, Minimum 3 Phase Transformer Capacity would be 150 KVA.
If Loads are Non-Coincidental, the Minimum Capacity would be 75 KVA.

The problem with your thinking is that each individual load cannot be divided among the three phases. Each load must be supplied individually.

Each heater requires 66.56kva, or a 75kva 1ph transformer. Two such loads would each require a 75kva unit; three such loads, three units.

3 x 75 = 225.

Just because you eliminate one of the loads, does not lessen the required capacity each transformer secondary must be able to supply its L-L load.

A Delta transformer supplying a Delta load is basically three 1ph units supplying three 1ph loads, and they happen to share line conductors.

While the shared lines will see 1.732 times the load current, each transformer secondary will see only the individual load across its two lines.

So, each secondary must be sized for the load across it. The only way to get two 75kva secondaries in one case is a by using a 3-ph 225kva transformer.

 

[LarryFine]

Oops. See below.

 

[LarryFine]

Are you connecting both loads to same phases or are you going to connect to all three phases with one phase having two connections?

The latter, I hope. Otherwise, he'd need a 150kva transformer (and I'd have typed the above needlessly. )

 

[hurk27]

I come up with a much different Apparent Power Value than posted by others.




Looks like a maximum Load of 133.12 KVA, as the OP defined (2) L-L 208V Heaters, drawing 320 Amps each.

If both Loads are to be driven Coincidentally, Minimum 3 Phase Transformer Capacity would be 150 KVA.
If Loads are Non-Coincidental, the Minimum Capacity would be 75 KVA.

Connect Loads as follows:

Heater #1: Connect between Line "A" and Line "B"

Heater #2: Connect between Line "C" and Line "A"

Line "A" would be common to both Heaters and their corresponding 2 Wire Branch Circuits. The Heaters would be connected in an Open Delta fashion.

Scott

Would not Phase A then have twice the current on it?
A 150 kva transformer would only supply 416 amps per phase?

Sorry Larry didn't see the second page:roll:

 

[david luchini]

Would not Phase A then have twice the current on it?
A 150 kva transformer would only supply 416 amps per phase?

Not twice the current, but 1.732 times the current. Scott approach would certainly load phase A beyond its rating.

 

[kwired]

What are the 320 amp loads that apparently can not be balanced across the three phases? Being able to balance across all three phases will allow for a smaller transformer, feeders, switchgear, and so on.

Some have mentioned heaters, but the OP did not mention what the load actually is.

 

[LarryFine]

Being able to balance across all three phases will allow for a smaller transformer, feeders, switchgear, and so on.

For a given total load, yes. But, we're not talking about a bunch of small loads we can move around. This is two individual loads, each of which will be connected between two lines, i.e., across one phase.

The load current for each piece of equipment is 320a (even if intermittently.) Each source's phase must be able to supply that. That fixes the size of each phase's transformer to 75kva. Three of those is 225kva.

Some have mentioned heaters, but the OP did not mention what the load actually is.

It looks like scott said it first in post #8.

Ever play "Telephone?"

 

[LarryFine]

Scott approach would certainly load phase A beyond its rating.

Don't confuse line current and phase current; they're not the same in Delta (which I presumed the OP to intend to use, and which wouldn't change the power anyway.)

 

[kwired]

For a given total load, yes. But, we're not talking about a bunch of small loads we can move around. This is two individual loads, each of which will be connected between two lines, i.e., across one phase.

That is why I asked what the load is, A load that large is somewhat unusual to be limited to a single element or other component and if multiple components often are capable of being reconfigured for single or three phase, multiple voltages, etc.