45 KVA transformer max amperage?

[Besoeker]

Load should always be determined in KVA. Amps means nothing until you size the conductors and select the breaker size.

If you put all the load on a single phase of a three phase panel, add up the total KVA of that phase, multiply by 3, you will get the minimum size of your 3-phase transformer.

Which contradicts this from post #3:

What matters is only the total KVA of the loads.

That's the point I was getting at.

 

[Smart $]

Which contradicts this from post #3:

What matters is only the total KVA of the loads.

That's the point I was getting at.

You must be using a different thread display. That's not in Post#3 on my display of this thread. I'm using the linear display.

 

[Besoeker]

You must be using a different thread display. That's not in Post#3 on my display of this thread. I'm using the linear display.

Regardless of where posted, the point remains.

 

[Smart $]

Regardless of where posted, the point remains.

So does the post quoted... wherever it may be :lol:

 

[lemming]

For a wye:
V_Line = ?3 V_Phase = ?3 * 120V = 207.84V rounded to 208V, which produces the insignificant error in debate.

So there is no error. Rounding to 3 significant figures means you are within 0.1% of answer.

For line voltages:
kVA = ?3 V_Line I_Line

For phase voltages:
kVA = 3 V_Phase I_Phase

So ?3 * ?3 = 3. Hard to argue with basic math. :happyyes:
Personally I'd go with line quantities, since they are more general than phase quantities. You don't need to know how it is connected inside.

To answer question:
kVA = ?3 V_Line I_Line
I_{Line =} kVA / ?3 V_Line = 45 kVA / ?3 208V = 125A
I_{Line =} kVA / ?3 V_Line = 75 kVA / ?3 208V = 208A

So 45 kVA is a no go.

 

[GoldDigger]

The "?" I see is supposed to be a sqrt symbol?

 

[Besoeker]

The "?" I see is supposed to be a sqrt symbol?

I suppose you are correct..................
Mathematical operators don't work well on forums like this.

 

[at123]

load current calculation

load current calculation

Is it correct to say that for a three-phase 45 kva transformer, the load on each one of 3 lines going to the electrical panel should not exceed 80 Amps ?

If the load is properly balanced, this will produce maximum 120 Amps at 208 V ( 43 kVA ) or 240 Amps at 110 V ( 45 kVA )

 

[kwired]

Is it correct to say that for a three-phase 45 kva transformer, the load on each one of 3 lines going to the electrical panel should not exceed 80 Amps ?

If the load is properly balanced, this will produce maximum 120 Amps at 208 V ( 43 kVA ) or 240 Amps at 110 V ( 45 kVA )

45000/208/1.732=124.91A That means each line is rated for a net of 125 amps, whether it be a 125 amp three phase load, 125 amp 208 volt load, a 125 amp 120 volt load, or a combination of any of those that results in up to 125 on any one leg maximum. If you go higher then 125 the transformer will likey still supply the load but will see increased heating that may end up leading to a failure at some point.

 

[at123]

45000/208/1.732=124.91A That means each line is rated for a net of 125 amps, whether it be a 125 amp three phase load, 125 amp 208 volt load, a 125 amp 120 volt load, or a combination of any of those that results in up to 125 on any one leg maximum. If you go higher then 125 the transformer will likey still supply the load but will see increased heating that may end up leading to a failure at some point.

Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts ?
This will affect the panel size I choose.
This is a lot of current for a relatively small transformer.

 

[don_resqcapt19]

Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts ?
This will affect the panel size I choose.
This is a lot of current for a relatively small transformer.

375 x 120 = 45,000

 

[Smart $]

Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts ?
This will affect the panel size I choose.
This is a lot of current for a relatively small transformer.

It's 125A maximum per Line. Panel rating is per Line.

 

[Besoeker]

Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts ?

It doesn't. Read again what kwired posted:

45000/208/1.732=124.91A That means each line is rated for a net of 125 amps, whether it be a 125 amp three phase load, 125 amp 208 volt load, a 125 amp 120 volt load, or a combination of any of those that results in up to 125 on any one leg maximum. If you go higher then 125 the transformer will likey still supply the load but will see increased heating that may end up leading to a failure at some point.

Line current is 125A maximum. The 120V is derived from line to neutral. You simply cannot do what you appear to have done (375*120=45000).
I don't know if this helps or hinders....
Think of it as three 120V 15kVA transformers connected in a WYE (Star) configuration. Each transformer can deliver 15000/120 = 125A.

 

[Tony S]

I like an easy life.

 

[Besoeker]

I like an easy life.
Beware of Robin Hood lest he at your FLC.....

 

[don_resqcapt19]

Line current is 125A maximum. The 120V is derived from line to neutral. You simply cannot do what you appear to have done (375*120=45000).
...

I posted the 375*120=45000.

He asked that if he used balanced loads, could he supply 375 amps of 120 volt loads? I assumed by balanced, he meant equal loads on each phase.

 

[Besoeker]

I posted the 375*120=45000.

He asked that if he used balanced loads, could he supply 375 amps of 120 volt loads? I assumed by balanced, he meant equal loads on each phase.

Yes, I saw both posts.
The answer is still that you can't.
Maximum line current is 125A regardless of load configuration. Balanced or otherwise.

 

[don_resqcapt19]

Yes, I saw both posts.
The answer is still that you can't.
Maximum line current is 125A regardless of load configuration. Balanced or otherwise.

He can put 125 amps line to neutral load on each of the 3 phases. That is what I understand balanced means.

 

[at123]

Yes, I saw both posts.
The answer is still that you can't.
Maximum line current is 125A regardless of load configuration. Balanced or otherwise.

Besoeker, I don't understand your argument. You agree that each line can supply up to 125A. Hypothetically, if all circuits in a panel are single-phase, all three lines are connected line-to-neutral, which permits drawing upto 375A. Am I missing something ?

 

[david luchini]

Besoeker, I don't understand your argument. You agree that each line can supply up to 125A. Hypothetically, if all circuits in a panel are single-phase, all three lines are connected line-to-neutral, which permits drawing upto 375A. Am I missing something ?

Post #30 suggests that you might be missing something.

Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts ?
This will affect the panel size I choose.
This is a lot of current for a relatively small transformer.

Why do you think its a lot of current for a relatively small transformer? That's what the transformer is rated for (45kVA.)

And why would it affect the panel size you chose? The panel size could be the same if you were supply a balanced 45kVA of all single phase 120v loads or if you were supplying a single balanced 45kVA 208V three phase load.