Home › Electrical Engineering Forum › General Discussion › Transformer capacity
 This topic has 6 replies, 4 voices, and was last updated 10 years, 1 month ago by Spir Georges GHALI.

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2011/05/22 at 1:53 pm #10489farangParticipant
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
2011/05/23 at 1:57 am #12088erickenchParticipantThe KVA per phase would be 315/3=105KVA
The current per phase would be 105,000/220=477.27Amps
2011/05/23 at 6:42 am #12089adminKeymasterThe amps per phase would be 315*1.39=437.85A but its alway good not to exeed 70 % of the maximum..
2011/05/23 at 11:09 am #12081adminKeymasterpower = 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
about your load and you hvae pool pump motor 3 phase
you must know her we have a power factor not unity
and you must make a palance on the three phase
Engniee
Ali younes
2011/06/02 at 11:18 am #12126Spir Georges GHALIParticipantfarang 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 8085% of the apparent power.
Regards.
2011/06/02 at 11:20 am #12127Spir Georges GHALIParticipantSpir 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 8085% of the apparent power.
Regards.
2011/06/11 at 12:16 pm #12179Spir Georges GHALIParticipantSpir 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 8085% 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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