Underground Service Wire Size

[NEC User]

I?m trying to size the service entrance conductors for a building renovation.
For a building renovation, I calculated a design load of 75 KW (94 KVA)
S = sqrt(3) x I V
94,000 = 1.732 x I 208
I = 261 Amps
Per NEC, we size wires to accommodate 25% spare capacity.
Total Amperes (I) with 25% spare capacity = 261 x 1.25 = 326 Amps
Using NEC Table 310.15 (B) (16) 75 degree column the following cables and conduit sizes are required
4#400 KCMIL in one (1) 4? Conduit

The utility company responded saying that they are going to provide 4#2 cables in the underground service.

How did they get 4#2 from the information I provided? They didn't indicate any transformer prior to the building.

 

[charlie b]

Per NEC, we size wires to accommodate 25% spare capacity.
Total Amperes (I) with 25% spare capacity = 261 x 1.25 = 326 Amps

The NEC has no such requirement. I don't know how you calculated the load at 94KVA. But if you did it right you would have accounted for 125% of any continuous loads and 125% for the largest motor, and would have taken credit for any allowable demand factors that could reduce the calculated total. Having done that, you can size your service at that value. Sizing for an extra 25% is a good design practice, but not an NEC requirement.

Using NEC Table 310.15 (B) (16) 75 degree column the following cables and conduit sizes are required
4#400 KCMIL in one (1) 4? Conduit

For a 326 amp load, I agree with that wire size selection.

The utility company responded saying that they are going to provide 4#2 cables in the underground service. How did they get 4#2 from the information I provided? They didn't indicate any transformer prior to the building.

Utilities typically provide service capacity at or around 40% of the calculated load that you give them. They have years of experience showing that the actual load a building will draw is far less than the calculated load. They do have a transformer that will serve this building. It may (and probably will) also serve other buildings.

 

[Cavie]

Power company's play by a different set of rules.

 

[iwire]

The utility company responded saying that they are going to provide 4#2 cables in the underground service.

How did they get 4#2 from the information I provided? They didn't indicate any transformer prior to the building.

The NEC calculations result in a number much higher than reality.

The power company does not care about those calculations, they use decades of experience to determine what a building will really require and size their equipment from that.

If the power companies had to follow the NEC we would all be paying much higher rates to pay for all that unneeded infrastructure. Larger conductors, larger transformers and the wasted power they use when under loaded etc.

 

[GoldDigger]

And if actual recorded power usage and measured VD justify it, POCO will then upsize the minimum amount necessary, or in the case of transformer size the minimum based on equipment on hand.