1. Junior Member
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Dec 2009
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## Calculating Megawatts

How many Megawatts will a 4/0 Three phase line carry 12470/7200v.According to the chart I am looking at a single 4/0 wire (acsr)will carry 340 amps. So I know on a three phase line you will have three wires of whatever size plus your neutral.How is the correct way to calculate how may megawatts any three phase line will carry according to different wire sizes?

2. Senior Member
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If the 340 Amps for 4/0 ACSR is correct, use the standard calculation for 3-phase power: MW = kV(phase-phase) x Amps x 1.732/1000.

12.47 kV x 340 x 1.732/1000 = 7.34 MW.

BTW, my look up table says 4/0 ACSR rating is 480 A in a 2 ft/sec wind with a 50C rise.

3. Originally Posted by rcwilson
If the 340 Amps for 4/0 ACSR is correct, use the standard calculation for 3-phase power: MW = kV(phase-phase) x Amps x 1.732/1000.

12.47 kV x 340 x 1.732/1000 = 7.34 MW.
MVA, not MW.

4. Junior Member
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Thanks for the info and formula. I had a guy try to tell me that 636 wire would just carry about 8.5 MW but I thought he must have been calculating it wrong. It should carry about 16.61 shouldn't it ?
Thanks

5. Senior Member
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Originally Posted by Besoeker
MVA, not MW.
Correct - I forgot power factor! Thanks for catching my error.

dsmith - 636 ACSR has an typical ampacity rating of 780A in a 2ft/sec wind. That would be about 16.8 MVA at 12.47 kV.
But the actual allowable MW loading depends on the load power factor and the length of the line, the allowable voltage drop, contingency factors etc.

6. Originally Posted by rcwilson
Correct - I forgot power factor! Thanks for catching my error.

dsmith - 636 ACSR has an typical ampacity rating of 780A in a 2ft/sec wind. That would be about 16.8 MVA at 12.47 kV.
But the actual allowable MW loading depends on the load power factor and the length of the line, the allowable voltage drop, contingency factors etc.
Others include ambient temperature, loss limits, allowable sag, elevation, max temp. It would not be unheard of to have a summer limit of 8.5 MW due to heat sensitive things like sag constraints.

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