Expanding on Yoopersup's post above:
Finding Full-Load Amperes from a given Apparent Power Value - i.e.: "Volt Amps"...Abbreviations Used:a: "VA" = Volt Amps
... VA = Apparent Power
b: "KVA" = Kilo Volt Amps (1,000 Volt Amps)
... KVA = VA × 0.001
c: "FLA" = Full Load Amperes
d: "E-pri" = Primary Voltage
e: "E-sec" = Secondary Voltage
f: E-adj" = Adjusted Voltage
g: "3ph" = 3 Phase
h: "1ph" = Single Phase
i: "2W" = 2 Wire
j: "3W" = 3 Wire
k: "4W" = 4 Wire
l: "L-N" = Line-To-Neutral (Phase Line to Grounded Conductor)
m: "L-L" = Line-To-Line (Phase Line to Phase Line)
n: "Sq. Rt." = Square Root
o: "×" = Multiplication Symbol
p: "÷" = Division Symbol
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Transformer = 45 KVA
E-pri = 480V; 3ph 3W
E-sec = 208Y/120V; 3ph 4W
Steps to perform:
- Obtain VA rating from KVA..
45 KVA x 1,000 = 45,000 VA. - Obtain "Adjusted Voltage Value"..
a: If Transformer is Single Phase, no adjustments to Voltage Value is necessary,
b: If Transformer is 3 Phase, multiply the Rated Voltage by 1.732 (Sq. Rt. of 3). - Divide the Rated VA value by the Adjusted Voltage Value. This result is the Rated FLA Value...
... VA ÷ E-adj = FLA
Example #1:
Find Primary FLA for the 45 KVA 3ph Transformer...
1: 45 KVA × 1,000 = 45,000 VA.
2: E-pri L-L = 480V; 3ph
480 × 1.732 = 831.36 (round off to 831)
3: 45,000 VA ÷ 831 = 54.15 Amps
Primary FLA = 54.1 Amps.
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Example #2:
Find Secondary FLA for the 45 KVA 3ph Transformer...
1: 45 KVA × 1,000 = 45,000 VA.
2: E-sec L-L = 208V; 3ph
208 × 1.732 = 360.26 (round off to 360)
3: 45,000 VA ÷ 360 = 125.00 Amps
Secondary FLA = 125.00 Amps.
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Example #3:
Find Secondary L-N FLA for the 45 KVA 3ph Transformer...
1: 45 KVA × 1,000 = 45,000 VA.
2: 45,000 VA ÷ 3 = 15,000 VA
3: E-sec L-N = 120V; 1ph
120 × 1.00 = 120 (no E-adj necessary for 1ph).
4: 15,000 VA ÷ 120 = 125.00 Amps
Secondary L-N FLA = 125.00 Amps.
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Example #4:
Find Primary & Secondary FLA for a 50 KVA 1ph Transformer...
E-pri = 480V 1ph 2W
E-sec = 120/240V 1ph 3W
1: 50 KVA × 1,000 = 50,000 VA.
2: E-pri L-L = 480V; 1ph
480 × 1.00 = 480
3: 50,000 VA ÷ 480 = 104.167 Amps (round off to 104)
Primary FLA = 104.00 Amps.4: E-sec L-L = 240V; 1ph
240 × 1.00 = 240
5: 50,000 VA ÷ 240 = 208.334 Amps (round off to 208)
Secondary L-L FLA = 208.00 Amps.-------------------------------------------------------
The following NEC Articles shall be referenced for Transformer Installations:
a: Article 240 (Overcurrent Protection),
b: Article 250 (Grounding and Bonding),
c: Article 408 (Switchboards and Panelboards),
d: Article 450 (Transformers and Transformer Vaults).
Hope this information is helpful.
-- Scott
