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Cable Sizing for Generators: 125% Rule vs. PF

Cable Sizing for Generators: 125% Rule vs. PF

Cable Sizing for Generators: 125% Rule vs. PF

This should be a quick one:

For Medium Voltage applications, lets say I have a 2.6MW generator and I need to size the cable for this.

I normally go 2.6MW / 0.8PF to give me 3.25MVA (worst case PF, they typically run at 1PF).
And then go 3.25MVA / root3 / 4.16kV = 451A.
And then on top of that I add the 125% factor to give me 563A for minimum cable ampacity.

My colleagues say, that since you are already including a safety buffer in the 0.8PF, you don't need to add the 125% additional safety factor anymore....

I disagree, b/c the PF during operation can range anywhere between 0.8 to 1.0. So you can't rely on that for cable ampacity calcs.....


RE: Cable Sizing for Generators: 125% Rule vs. PF

0.8 is worst case PF. So 1.0 should give you less cable load - not more.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Cable Sizing for Generators: 125% Rule vs. PF

The rated current of a generator is based on KVA/MVA, not on kW/MW.
The maximum allowable current is at .8 PF
This is not a safety factor. It is the maximum possible load.
Then add 125% safety factor as you do.
You are correct.
In North America the code requires 125% of rated current. That is current at 0.8 PF X 125%.

"Why not the best?"
Jimmy Carter

RE: Cable Sizing for Generators: 125% Rule vs. PF

Taking the amperage rating of the set from the nameplate and multiplying by 1.25 would likely be the simplest and safest way to do it. The calculation you presently do is the next best way.

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