Three phase transformer and secondary panel single phase

hhsting

Senior Member
480V 50kVA 3 phase transformer nominal current is 60A.

208V nominal current is 139A.

100A primary OCPD is 166% and is acceptable if secondary OCPD is 125%.

Secondary OCPD would need to be 175A to be kosher.

Jon
Secondary only two legs are loaded third not loaded. Thats single phase not three phase. How can one calculate three phase fla secondary given above situation?


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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
This is not an unusual situation. people use single-phase panels on 208-120 all the time because they are more readily available and cheaper. As long as you do not need 240 volts it will work fine.

250% of the primary current gives you 150.35 amps.

125% of the secondary current gives you 138 amps.

That tells you what rating of overcurrent protection to use. Keep in mind on one side you can go up to the next highest standard rating and the other side you cannot. I will leave it to you to look up which side allows you to use the next higher standard rating.
 

hhsting

Senior Member
This is not an unusual situation. people use single-phase panels on 208-120 all the time because they are more readily available and cheaper.

250% of the primary current gives you 150.35 amps.

125% of the secondary current gives you 138 amps.

That tells you what rating of overcurrent protection to use. Keep in mind on one side you can go up to the next highest standard rating and the other side you cannot. I will leave it to you to look up which side you can go up to the next standard rating.
Secondary current should be 208V single phase. Its three phase transformer only two phase go to downstream single phase panelboard.

How are you calculating it as 138A?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Secondary current should be 208V single phase. Its three phase transformer only two phase go to downstream single phase panelboard.

How are you calculating it as 138A?
The transformer can only deliver so much current through its windings. You have to protect the transformer at its rated capacity which is three phase 50 kVA. It doesn't matter whether you are using all three phases or not.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Secondary current should be 208V single phase. Its three phase transformer only two phase go to downstream single phase panelboard.

How are you calculating it as 138A?
Since the transformer is 208/120 three phase you calculate it as 50000/208/sqrt(3) = 139. (138 or 139 is rounding error)

Then * 1.25 gives 173, next size up gives a 175A breaker.

The fact that you are only using 2 of the 3 phases doesn't change this. You have a 3 phase transformer and must calculate as such. Think of it as a very unbalanced 3 phase load.

Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Note: this is a bit of approximation. If you only load 2 of 3 phases then you would expect less heating and you probably could push the other 2 phases harder.

But absent manufacturer instructions saying something about it, you are stuck doing the 3 phase calc. And since heating goes as the square of the current any difference would be slight.

Jon
 

kwired

Electron manager
Location
NE Nebraska
Since the transformer is 208/120 three phase you calculate it as 50000/208/sqrt(3) = 139. (138 or 139 is rounding error)

Then * 1.25 gives 173, next size up gives a 175A breaker.

The fact that you are only using 2 of the 3 phases doesn't change this. You have a 3 phase transformer and must calculate as such. Think of it as a very unbalanced 3 phase load.

Jon
To add to this, each winding is designed to only carry 139 amps.

You have similar issues if you supplied only 120 volt loads on just one phase to the neutral, 139 amps is all it can handle. It is not able to deliver 50 kVA here it is only able to deliver one third of that because you are only using one third of the total windings of the network. OP's situation is only capable of using two thirds of the available 50 kVA.

Unless it was something already on hand or 208 volts is absolutely needed instead of 240 for some reason it would have been better decision to purchase a single phase transformer. If 50 kVA of load needs to be supplied, portions of this transformer is overloaded if not balancing that load across all three phases.
 

oldsparky52

Senior Member
Okay, trying to understand the currents here. On the secondary side of this transformer (Wye connected), if L1 and L2 have 100amp 120V loads each, then the neutral will also carry 100-amps. What would the currents look like on the primary side conductors (Delta connected on the transformer)?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
As strange as it sounds at first glance, you can have a 150 A breaker on the primary side and a 175 A breaker on the secondary side and still meet code requirements.

You still have to do a feeder load calculation.
 

kwired

Electron manager
Location
NE Nebraska
Okay, trying to understand the currents here. On the secondary side of this transformer (Wye connected), if L1 and L2 have 100amp 120V loads each, then the neutral will also carry 100-amps. What would the currents look like on the primary side conductors (Delta connected on the transformer)?
There would be current in all three lines, but one of them would be minimal current that is just maintaining excitation of primary coils. The other two would be splitting nearly all the real load which would be 24000 VA / 480 = 50 amps
 

oldsparky52

Senior Member
There would be current in all three lines, but one of them would be minimal current that is just maintaining excitation of primary coils. The other two would be splitting nearly all the real load
That doesn't sound quite right to me. I would think one leg on the primary would carry more current than the other two, how much is what I'm trying to understand.

One leg would drive two of the used coils and the other two legs each would pick up the other side of those coils, and the 3rd coil is not loaded.

So I would expect that 2 of the primary conductors would carry about the same current and the one loaded with 2 coils would carry more. Since the phase separation is 120º instead of 180º the one carrying more won't be double the other two.
 

kwired

Electron manager
Location
NE Nebraska
That doesn't sound quite right to me. I would think one leg on the primary would carry more current than the other two, how much is what I'm trying to understand.

One leg would drive two of the used coils and the other two legs each would pick up the other side of those coils, and the 3rd coil is not loaded.

So I would expect that 2 of the primary conductors would carry about the same current and the one loaded with 2 coils would carry more. Since the phase separation is 120º instead of 180º the one carrying more won't be double the other two.
I guess it could. Could also have an open delta primary (intentional or not) and if so would be forced to operate mostly like what I mentioned. Not intentional would be situation of losing one input line, you would still have 208/120 single phase on two output lines and neutral.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Okay, trying to understand the currents here. On the secondary side of this transformer (Wye connected), if L1 and L2 have 100amp 120V loads each, then the neutral will also carry 100-amps. What would the currents look like on the primary side conductors (Delta connected on the transformer)?
In a 480 delta to 208/120 wye transformer, you have 3 coils, each with a 4:1 turns ratio.

So if you have 100A each flowing in the 120V A and B coils, nothing in the C coil, then you will have 25A each in the A and B 480V coils, nothing in C.

The primary coils are in delta, so this means that you will have 25A on two of the phases and 25*1.732A on the third coil.

(The above assuming the same phase angle on the 120V coils, and ignoring the small magnetizing current on the C coil.)

-Jon
 

kwired

Electron manager
Location
NE Nebraska
In a 480 delta to 208/120 wye transformer, you have 3 coils, each with a 4:1 turns ratio.

So if you have 100A each flowing in the 120V A and B coils, nothing in the C coil, then you will have 25A each in the A and B 480V coils, nothing in C.

The primary coils are in delta, so this means that you will have 25A on two of the phases and 25*1.732A on the third coil.

-Jon
I kind of figured if current splits it would be by that 1.732 factor.
 
Secondary current should be 208V single phase. Its three phase transformer only two phase go to downstream single phase panelboard.

How are you calculating it as 138A?
Trust us, you have to use the balanced full load current of the transformer. It doesn't matter if it's serving a non transverse negative 4 phase system feeding a bank of energy convalescing unobtainium flux capacitors.
 
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