# Transformer capacity

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• #10489

I have a oil filled 3 phase transformer 315 kva on a pole pripary voltage is 33 kv secondary is 220/380.

What would be the rated output in amps per phase. Here everyone brings in 3 phase to thier pannel there are hardly any 3 phase loads, maybe a pool pump motor or two. The rest are 1 phase A/C and lighting loads.

Our ampient temperature is 35C

What can I safely draw in current per phase on this transformer?

Thanks

farang

#12088

The KVA per phase would be 315/3=105KVA

The current per phase would be 105,000/220=477.27Amps

#12089

The amps per phase would be 315*1.39=437.85A but its alway good not to exeed 70 % of the maximum..

#12081

power = 1.73 * V* I

V = 380 v

I = ?

power  = 315 kva  =  315 * 1000  = 315000 va

I =  power /  1.37 * v =  315000/1.73 * 380 = 315000/657.4 = 479.16 A

and from my experience  you can  know the Ampere direct by this

if you have   KVA     and output  volt is  380

you can    do that

KVA *  1.5 =  Ampere

if you have  the power be  KW  you can do  that

KW *  2 =  Ampere

you must know her we have  a  power factor not  unity

and you must make a palance on the three phase

Engniee

Ali  younes

#12126

farang said:

I have a oil filled 3 phase transformer 315 kva on a pole pripary voltage is 33 kv secondary is 220/380.

What would be the rated output in amps per phase. Here everyone brings in 3 phase to thier pannel there are hardly any 3 phase loads, maybe a pool pump motor or two. The rest are 1 phase A/C and lighting loads.

Our ampient temperature is 35C

What can I safely draw in current per phase on this transformer?

Thanks

farang

Dear Farang ;

Each transformer hase an ” S : Apparent Power ” in kVA, either for single phase with neutral or for 3 phases with or witout neutral  (depending of the connection).

The formula for Single phase + neutral is : S = V x I        where :

– S : apparent power in kVA

– V : the nominal voltage between phase & neutral without load.

– I : the nominal current for the transformer

and for 3 phase + neutral, the formula is : S = 1.73 x V x I

So. using one of these formula accordingly the the kind of transformer, we can calculat the nominal current for any one, noting that the ” I – nominal current ” for 3 phases with or without neutral means the value of current for each phase, in this case, when we have single phase loads we will distribute them to the 3 phases of the transformer trying to have approximetly an equivalant installed power on each phase.

Normally, we try to have the maximum load on any transformer not more than 80-85% of the apparent power.

Regards.

#12127

Spir Georges GHALI said:

farang said:

I have a oil filled 3 phase transformer 315 kva on a pole pripary voltage is 33 kv secondary is 220/380.

What would be the rated output in amps per phase. Here everyone brings in 3 phase to thier pannel there are hardly any 3 phase loads, maybe a pool pump motor or two. The rest are 1 phase A/C and lighting loads.

Our ampient temperature is 35C

What can I safely draw in current per phase on this transformer?

Thanks

farang

Dear Farang ;

Each transformer hase an ” S : Apparent Power ” in kVA, either for single phase with neutral or for 3 phases with or witout neutral  (depending of the connection).

The formula for Single phase + neutral is : S = V x I        where :

– S : apparent power in kVA

– V : the nominal voltage between phase & neutral without load.

– I : the nominal current for the transformer

and for 3 phase + neutral, the formula is : S = 1.73 x U x I            where :

– U : the nominal voltage between 2 phases without load

So. using one of these formula accordingly the the kind of transformer, we can calculat the nominal current for any one, noting that the ” I – nominal current ” for 3 phases with or without neutral means the value of current for each phase, in this case, when we have single phase loads we will distribute them to the 3 phases of the transformer trying to have approximetly an equivalant installed power on each phase.

Normally, we try to have the maximum load on any transformer not more than 80-85% of the apparent power.

Regards.

#12179

Spir Georges GHALI said:

farang said:

I have a oil filled 3 phase transformer 315 kva on a pole pripary voltage is 33 kv secondary is 220/380.

What would be the rated output in amps per phase. Here everyone brings in 3 phase to thier pannel there are hardly any 3 phase loads, maybe a pool pump motor or two. The rest are 1 phase A/C and lighting loads.

Our ampient temperature is 35C

What can I safely draw in current per phase on this transformer?

Thanks

farang

Dear Farang ;

Each transformer hase an ” S : Apparent Power ” in kVA, either for single phase with neutral or for 3 phases with or witout neutral  (depending of the connection).

The formula for Single phase + neutral is : S = V x I        where :

– S : apparent power in kVA

– V : the nominal voltage between phase & neutral without load.

– I : the nominal current for the transformer

and for 3 phase + neutral, the formula is : S = 1.73 x V x I

So. using one of these formula accordingly the the kind of transformer, we can calculat the nominal current for any one, noting that the ” I – nominal current ” for 3 phases with or without neutral means the value of current for each phase, in this case, when we have single phase loads we will distribute them to the 3 phases of the transformer trying to have approximetly an equivalant installed power on each phase.

Normally, we try to have the maximum load on any transformer not more than 80-85% of the apparent power.

Regards.

Dear Farang ;

I', very sorry for this typing mistake, but in 3 phases we use the formula :

– S = 1.73 x U x I

where ” U ” is the voltage between 23 phases.

Regards.

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