sparky 134
Senior Member
- Location
- Joliet, IL
I installed a 480v 3phase to 120/240v 3phase 75KVA transformer that has a center tap (X4) between X1 and X2. X3 is the high leg.
I called the manufacturer today to verify that X3 was the high leg and that X4 was the neutral. The tech verified that X3 is the high leg and that X4 is the neutral between X1 and X2. She also said, "The X4 terminal is limited to 5% of the KVA of the transformer."
This came as a surprise. This transformer is to supply power to a 120/240v panelboard which will serve both single and three phase loads. Am I correct in calculating the maximum amount of current the X4 terminal can handle is 9 amps ? The nameplate states the maximum primary amps are 90 and the secondary is 180. 5% of 180 equals 9.
If this is correct, this transformer will not work. I contacted the salesman who sold us the transformer since she is the person who specified the transformer. I told her the transformer would be feeding a 120/240v panelboard which she also supplied. Not once did she mention the current limitation of the neutral. She did send me an email with the following information which she acquired from the SquareD website:
"240 Delta with 120 center taps have historically been limited to 5% capacity on the center tap. The new units from Schneider Electric offer greater limits on 120 V center tap. Limits are determined by the total transformer loading and the following formula used to size new 120 V center tap units:
(240 V balanced loads) + 2.5 x (120 V loads)= kVA required"
I do not understand this paragraph. If I add four 120volt loads to this panel using a multiwire branch circuit and each 120volt load is equal, would the current returning to the transformer be zero ?
I called the manufacturer today to verify that X3 was the high leg and that X4 was the neutral. The tech verified that X3 is the high leg and that X4 is the neutral between X1 and X2. She also said, "The X4 terminal is limited to 5% of the KVA of the transformer."
This came as a surprise. This transformer is to supply power to a 120/240v panelboard which will serve both single and three phase loads. Am I correct in calculating the maximum amount of current the X4 terminal can handle is 9 amps ? The nameplate states the maximum primary amps are 90 and the secondary is 180. 5% of 180 equals 9.
If this is correct, this transformer will not work. I contacted the salesman who sold us the transformer since she is the person who specified the transformer. I told her the transformer would be feeding a 120/240v panelboard which she also supplied. Not once did she mention the current limitation of the neutral. She did send me an email with the following information which she acquired from the SquareD website:
"240 Delta with 120 center taps have historically been limited to 5% capacity on the center tap. The new units from Schneider Electric offer greater limits on 120 V center tap. Limits are determined by the total transformer loading and the following formula used to size new 120 V center tap units:
(240 V balanced loads) + 2.5 x (120 V loads)= kVA required"
I do not understand this paragraph. If I add four 120volt loads to this panel using a multiwire branch circuit and each 120volt load is equal, would the current returning to the transformer be zero ?