45 KVA transformer max amperage?

[at123]

What I meant, is I need a panel with sufficient number of circuits; throwing in a 32 circuit panel wouldn't be wise. I understand there is no direct relationship between 375A and the size of the panel, but I need to have sufficient room given the fact I can supply that much current.

I still think that load-balancing with the 3 phase transformer is the key. In 208V dual-pole breaker applications we should be able to draw close to 190A ( 375 / 2 ), but it must be distributed across 3 phases. If I do the other extreme, and put all my dual-pole breakers on the same 2 phases, I'm limited to 125A.

 

[Besoeker]

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 ?

You are.
The circuits are not in parallel and are not single phase.
As I posted earlier, consider the 3-phase arrangement like three 15kVA 120V transformers.

 

[david luchini]

You need to have a panel with sufficient room for the loads you need to supply. That is true whether the loads are all single phase, or the loads are all three phase, or the loads are a combination of the two.

It seems to me that having single phase 120V loads could result in a smaller panel. For instance, (18) 2500VA, 120V loads would need a panel with at least 18 spaces, but (18) 2500VA, 208V, 3ph loads would need a panel with at least 54 spaces.

 

[at123]

You are.
The circuits are not in parallel and are not single phase.
As I posted earlier, consider the 3-phase arrangement like three 15kVA 120V transformers.

three 15kVA 120V transformers rate at 125A each, totaling 375Amps.
Why can't I have a single-pole breaker using 1 phase and neutral ? I still don't get your point.

 

[Besoeker]

three 15kVA 120V transformers rate at 125A each, totaling 375Amps.
Why can't I have a single-pole breaker using 1 phase and neutral ? I still don't get your point.

Because it isn't single phase.
Each transformer is 120-0.
They are connected in a WYE configuration which gives a 120deg electrical displacement between each.
Think of a wheel with three spokes.
The distance from the rim to the hub for any spoke is 120V. The distance between the spokes at the rim is 208V.





Without the phase displacement you couldn't get 208V.
And you can't parallel the outputs - you'd short out the 208V.

 

[kwired]

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.

You can not supply a single 375 amp 120 volt load. You can supply 375 amps of 120 volt load but you have to put a third of it on each phase.

125 amp three phase panel will be able to handle this, you just need one with enough spaces for your branch circuits.

 

[at123]

Besoeker, I think we are just looking at it from different points. You seem to consider only 220V, which is understandable if you are in the UK, but I'm talking about 120V circuits which are connected line-to-neutral.

Here is Mike's example on calculating transformer sizing. Note how three 3kVA 120V loads are distributed among phases and connected to neutral.
http://www.mikeholt.com/mojonewsarchive/EC-HTML/HTML/WyeTransformerCalculations~20030916.htm

[FONT=Arial, Helvetica, sans-serif]Wye transformer sizing example: What size transformer (480 to 208Y/120V, 3? phase) would you need for the following loads?[/FONT]

• [FONT=Arial, Helvetica, sans-serif]208V, 36 kVA, 3? heat strip[/FONT]
• [FONT=Arial, Helvetica, sans-serif]two 208V, 10 kVA, 1? loads[/FONT]
• [FONT=Arial, Helvetica, sans-serif]three 120V, 3 kVA loads[/FONT]

1. [FONT=Arial, Helvetica, sans-serif]three 1?, 25 kVA transformers[/FONT]
2. [FONT=Arial, Helvetica, sans-serif]one 3?, 75 kVA transformer[/FONT]
3. [FONT=Arial, Helvetica, sans-serif]a or b[/FONT]
4. [FONT=Arial, Helvetica, sans-serif]none of these[/FONT][FONT=Arial, Helvetica, sans-serif]Answer: (c) a or b[/FONT]
   [FONT=Arial, Helvetica, sans-serif]Phase A = 23 kVA
   [/FONT][FONT=Arial, Helvetica, sans-serif]Phase B = 22 kVA
   [/FONT][FONT=Arial, Helvetica, sans-serif]Phase C = 20 kVA[/FONT]

	[FONT=Arial, Helvetica, sans-serif]Phase A (L1)[/FONT]	[FONT=Arial, Helvetica, sans-serif]Phase B (L2)[/FONT]	[FONT=Arial, Helvetica, sans-serif]Phase C (L3)[/FONT]	[FONT=Arial, Helvetica, sans-serif]Line Total[/FONT]
[FONT=Arial, Helvetica, sans-serif]36 kVA, 120V, 3?[/FONT]	[FONT=Arial, Helvetica, sans-serif]12 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]12 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]12 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]36 kVA[/FONT]
[FONT=Arial, Helvetica, sans-serif]10 kVA, 208V, 1?[/FONT]	[FONT=Arial, Helvetica, sans-serif]5 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]5 kVA[/FONT]		[FONT=Arial, Helvetica, sans-serif]10 kVA[/FONT]
[FONT=Arial, Helvetica, sans-serif]10 kVA, 208V, 1?[/FONT]		[FONT=Arial, Helvetica, sans-serif]5 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]5 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]10 kVA[/FONT]
[FONT=Arial, Helvetica, sans-serif]3 kVA, 120V[/FONT]	[FONT=Arial, Helvetica, sans-serif]3 kVA*[/FONT]			[FONT=Arial, Helvetica, sans-serif]3 kVA[/FONT]
[FONT=Arial, Helvetica, sans-serif]3 kVA, 120V[/FONT]	[FONT=Arial, Helvetica, sans-serif]3 kVA*[/FONT]			[FONT=Arial, Helvetica, sans-serif]3 kVA[/FONT]
[FONT=Arial, Helvetica, sans-serif]3 kVA, 120V[/FONT]			[FONT=Arial, Helvetica, sans-serif]3 kVA*[/FONT]	[FONT=Arial, Helvetica, sans-serif]3 kVA[/FONT]
	[FONT=Arial, Helvetica, sans-serif]23 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]22 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]20 kVA[/FONT]	[FONT=Arial, Helvetica, sans-serif]65 kVA[/FONT]

[FONT=Arial, Helvetica, sans-serif]* Indicates neutral (120V) loads.[/FONT]
[FONT=Arial, Helvetica, sans-serif]The table sums up the kVA for each phase of each load. Note that the phase totals (23 kVA, 22 kVA and 20 kVA) should add up to the line total (65 kVA). Always use a ?checksum? like this to ensure you have accounted for all items and the math is right.[/FONT]

 

[at123]

You can not supply a single 375 amp 120 volt load. You can supply 375 amps of 120 volt load but you have to put a third of it on each phase.

125 amp three phase panel will be able to handle this, you just need one with enough spaces for your branch circuits.

Sounds good, that's exactly what I thought.

 

[Besoeker]

Besoeker, I think we are just looking at it from different points. You seem to consider only 220V, which is understandable if you are in the UK, but I'm talking about 120V circuits which are connected line-to-neutral.

I already made the point about the 120V being line-neutral. See post #33.
What you don't seem to be grasping is that you cannot combine the three 120V phases to drive one 375A single phase load.
You simply can't add the three line currents arithmetically. Not in UK Not in USA. Not even in Timbuktu ..................

 

[don_resqcapt19]

I already made the point about the 120V being line-neutral. See post #33.
What you don't seem to be grasping is that you cannot combine the three 120V phases to drive one 375A single phase load.
You simply can't add the three line currents arithmetically. Not in UK Not in USA. Not even in Timbuktu ..................

I don't think he has ever said that. I don't see anything that suggests a single 375 amp 120 volt load.

That is my point about him saying "balanced loads"...A total of 375 amps of line to neutral loads, balanced across the 3 phases is 125 amps per phase of line to neutral load.

 

[Besoeker]

I don't think he has ever said that. I don't see anything that suggests a single 375 amp 120 volt load.

I suspect most people would read this:

"Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts" to mean a single load, not three separate loads.
The 375A is a bit meaningless - that magnitude of current doesn't flow anywhere. It isn't being supplied.

 

[don_resqcapt19]

I suspect most people would read this:

"Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts" to mean a single load, not three separate loads.
The 375A is a bit meaningless - that magnitude of current doesn't flow anywhere. It isn't being supplied.

I would never read a "balanced" load as being connected to a single phase. How is it balanced if the load is only connected to one of the three phases?

I also don't see the 375A as meaningless. It is how a lot of people would look at the loading of a panel or a transformer. It conveys the total line to neutral load for multiple loads that the 45kVA transformer can supply, assuming that you connect 125 amps of load to each phase.

 

[iwire]

I suspect most people would read this:

"Does it mean, if properly balanced a 45 kVA three-phase transformer can supply a whopping 375 Amps at 120 volts" to mean a single load, not three separate loads.

I took it to mean any number of 120 volt loads and I expected that number of the loads be say, 15 to 60 separate loads. As one would expect when a transformer supplies a panelboard.

I have never run into a single piece of a equipment that draws 375 amp at 120 volts. Not to say none exist but certainly would be the exception not the rule.

 

[Besoeker]

I took it to mean any number of 120 volt loads and I expected that number of the loads be say, 15 to 60 separate loads. As one would expect when a transformer supplies a panelboard.

I have never run into a single piece of a equipment that draws 375 amp at 120 volts. Not to say none exist but certainly would be the exception not the rule.

From #42

which permits drawing upto 375A

The 375A isn't drawn anywhere.
Three lines at 125A each, fine.
But adding those together arithmetically, which you would have to do to get 375A, makes no sense. There is no 375A. You don't have to rate anything at 375A because it simply doesn't exist.

The currents add vectorially in the neutral and if equal in magnitude and PF, they cancel. That's the main point of 3-phase after all.

I apologise if I have ruffled feathers.

 

[Besoeker]

I would never read a "balanced" load as being connected to a single phase. How is it balanced if the load is only connected to one of the three phases?

I also don't see the 375A as meaningless. It is how a lot of people would look at the loading of a panel or a transformer. It conveys the total line to neutral load for multiple loads that the 45kVA transformer can supply, assuming that you connect 125 amps of load to each phase.

I guess I'm not most people. I would look at the loading for each phase - that's the limitation. Not the 375A sum.

 

[mivey]

I don't think he has ever said that. I don't see anything that suggests a single 375 amp 120 volt load.

That is my point about him saying "balanced loads"...A total of 375 amps of line to neutral loads, balanced across the 3 phases is 125 amps per phase of line to neutral load.

Then you need to read again. He was all over the map. I think he might have picked up on it as the thread went on but was definitely confused to start with.

There is no other way to explain the following:

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 )

Line current for 208 volt case but who knows what for the other. Who ever heard of 3-phase 110/64 volt service or was it 80 amps * 3? Definite confusion.

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.

Seems to me (and David picked up on it as well) that he went from 120 amps to 240 amps to 375 amps. Obviously going down the wrong path.

I still think that load-balancing with the 3 phase transformer is the key. In 208V dual-pole breaker applications we should be able to draw close to 190A ( 375 / 2 ), but it must be distributed across 3 phases. If I do the other extreme, and put all my dual-pole breakers on the same 2 phases, I'm limited to 125A.

And he is still confused.

Agreeing with this amp adding nonsense is not helping him one bit.

 

[mivey]

I would never read a "balanced" load as being connected to a single phase. How is it balanced if the load is only connected to one of the three phases?

I also don't see the 375A as meaningless. It is how a lot of people would look at the loading of a panel or a transformer. It conveys the total line to neutral load for multiple loads that the 45kVA transformer can supply, assuming that you connect 125 amps of load to each phase.

I took it to mean any number of 120 volt loads and I expected that number of the loads be say, 15 to 60 separate loads. As one would expect when a transformer supplies a panelboard.

I have never run into a single piece of a equipment that draws 375 amp at 120 volts. Not to say none exist but certainly would be the exception not the rule.

I know adding amps does not hinder you two one bit because you are clear on what you are doing.

The problem is that it is a condition-specific shorthand substitute for a VA calculation and is going to bite someone in the behind. Better to teach the less experienced to stick with a VA calculation until they get to a point in the calcs where they need to know the amps associated with a particular voltage.

 

[kwired]

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.

You can not supply a single 375 amp 120 volt load. You can supply 375 amps of 120 volt load but you have to put a third of it on each phase.

125 amp three phase panel will be able to handle this, you just need one with enough spaces for your branch circuits.

From #42


The 375A isn't drawn anywhere.
Three lines at 125A each, fine.
But adding those together arithmetically, which you would have to do to get 375A, makes no sense. There is no 375A. You don't have to rate anything at 375A because it simply doesn't exist.

The currents add vectorially in the neutral and if equal in magnitude and PF, they cancel. That's the main point of 3-phase after all.

I apologise if I have ruffled feathers.

I wasn't all that clear myself as to just what was intended, and the reason for post 46. Yes he said a properly balanced supply, then followed it with comments about panel size and transformer size - bottom line was still 45KVA is what can be supplied, and for it to be 120 volts loads it has to be balanced or a portion of the transformer will be overloaded.

 

[mivey]

From #42


The 375A isn't drawn anywhere.
Three lines at 125A each, fine.
But adding those together arithmetically, which you would have to do to get 375A, makes no sense. There is no 375A. You don't have to rate anything at 375A because it simply doesn't exist.

The currents add vectorially in the neutral and if equal in magnitude and PF, they cancel. That's the main point of 3-phase after all.

I apologise if I have ruffled feathers.

I think it was a good point. Amp-adding is only going to get someone in trouble if they aren't careful and it only leads to confusion for the less experienced.

There is no spoon.

 

[mivey]

I wasn't all that clear myself as to just what was intended

Small wonder given that the numbers were all over the place.

Yes he said a properly balanced supply, then followed it with comments about panel size and transformer size

Not to mention some other weird numbers

bottom line was still 45KVA is what can be supplied, and for it to be 120 volts loads it has to be balanced or a portion of the transformer will be overloaded.

I got what you meant as I'm sure BES did. While not a problem for you I'm sure, it would appear BES is pointing out the problems with amp-adding in general.

Better to not do it as it makes no sense in general even if it can be a short-hand for VA under specific conditions.