ECN Forum Forums General Topics of the Trade General Discussion Area transformer sizing

I am needing to size a transformer for a new service. My problem is this. Well, one of my problems anyway. I only know that the voltage is 480, but I don't know anymore than that. I am certaintly not ordering one until I know. It is a new commercial building, maybe 2 years old. I am going to put in a 125a 208y/120 panel.

Is my secondary kva amperagex360? If so, would it be a 45kva? This is new to me, I am getting better but I am still a novice at 3ph services. How do I determine my primary amperage? Does not knowing my 480v type affect my transformer size or just the type that I order?

I looked at scotts drawings. Very helpful.

45 kVA is the typical size for 125 A 208Y210 panel boards....

Look at Ugly's...

All of the numbers are laid out there.

Thanks, I forgot about Ugly's.

KVA x1000 = Amps
Example 45 x 1000 divided by 830 = 54 amp primary
45 x 1000 divided by 360 = 125 amp seconday
for three phase

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

________________________________________________________________________________________________________________________

Transformer = 45 KVA
E-pri = 480V; 3ph 3W
E-sec = 208Y/120V; 3ph 4W

Steps to perform:

1. Obtain VA rating from KVA..
   45 KVA x 1,000 = 45,000 VA.
2. 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).
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.

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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

Scott done a wonderfull job to cover all the details for NEC code requriement for transfomer set up.

And there are some transfomer in that size will come in few taps to adjust the voltage to suit the requriement.

And make sure you heed with the SDS rules on the transfomer. { there may have local addmentends to add to it }

Merci,.
Marc

Marc:

Thanks for the reply, and for mentioning the Primary FCAB / FCBN Taps!

-- Scott