Machine Shop Service Calculation

[Leonard J Kasper]

Relocating a part of a machine shop to a new location. New service will be240/120 Delta 3 phase.
25 milling machines. name plate 27.5 amps 400 volt 3 phase, buck & boost transformer service each machine, 400 volt/ 230 volt 20 KW 3ph.
27.5x400x1.732= 19,052w
25x 19,052 = 476,300 watts / (240x1.732) = 1,148 amps. This is 3 times the size of the existing service running the eq. now. I know not all eq. is being run at the same time or at max load and that is why there is no problems now. But I need this to work this out on a load calculation. What am I missing? Art. 220.60?

 

[MSU]

Milling machines rarely run at full load, and all machines in any given shop rarely run coincidentally. I suspect the answer will be found in the actual operating load diversity factor.

Question is, will they be operated in the new location at the same utilization or diversity factor as the old location?

NL amps on an induction motor is roughly 30-40% of FLA. Var flow increases somewhat linearly from NL to FL, so total load amps don't increase linearly with load, but rather as indicated by the Pythagorean Theorem. At NL, current is almost all VARS, at FL, current is defined by motor PF. That gives two estimates of VAR flow. Watt flow roughly equals VAR flow @60-80% load. From NL to 50% load load amps may increase from 35% to perhaps 60%. I suspect that few milling machines in any shop coincidentally operate at higher average loads than these. At the point of equal WATT and VAR flow, the WATT flow becomes the predominant portion of total current, and closer dictates total current flow to the motor. below that point, VAR flow dominates.

Hope that helps

 

[Smart $]

.... What am I missing? ...

This...

430.26 Feeder Demand Factor. Where reduced heating of
the conductors results from motors operating on duty-cycle,
intermittently, or from all motors not operating at one time,
the authority having jurisdiction may grant permission for
feeder conductors to have an ampacity less than specified in
430.24, provided the conductors have sufficient ampacity
for the maximum load determined in accordance with the
sizes and number of motors supplied and the character of
their loads and duties.
FPN: Demand factors determined in the design of new facilities
can often be validated against actual historical experience
from similar installations. Refer to ANSI/IEEE
Std. 141, IEEE Recommended Practice for Electric Power
Distribution for Industrial Plants, and ANSI/IEEE Std.
241, Recommended Practice for Electric Power Systems in
Commercial Buildings, for information on the calculation
of loads and demand factor.

 

[Leonard J Kasper]

Thanks. I have reviewed NCE art 430.26 and ANSI 241. I have a meeting set up with the AHJ next week. This will reduce the total service load to a reasonable size.

 

[kwired]

Do you have any history of what the load was like at the first location? Treat anything that is different at new location as if you were going to add that load to the original location, and see what that ends up like, you may want to make sure the AHJ is going be OK with this method.