nateholt
Member
- Location
- Toledo, OH
If anyone has time, please take a look at this and let me know if this is a common industry practice...
Large motor 500 feet away, 3ph motor (1.25 x FLA) = 535A, continuous duty, no ambient temp de-rating required for wiring.
From tables, 1000MCM yields 545A ampacity for 75C insulation. Assuming the three phase conductors in one conduit, no conductor de-rating required so this should work.
But maybe cheaper to go with 3x250MCM rated at 255A non-de-rated ampacity?
That would give 9 current carrying conductors in the conduit? meaning de-rate ampacity by 0.7 to 178.5A each. Times 3 gives just enough to cover the full motor?s (1.25 x FLA) value of 535A.
The additional advantage of the smaller conductors is that the voltage drop would be less. For the 1000MCM at 0.85 PF and 535A for 500 feet, I ran the full calculation using Table 9 reactance, AC resistance, and sine of arccos (messy), came up with 18.3 volts drop. But running the same calculation on the 250 MCM wires gives voltage drop of only 11.3 volts.
So, splitting the load over three smaller conductors saves 25% of the metal and cuts the IR losses in the wiring by 35%. The negative might be that the conduit could be bigger and not sure the cost of 3 -250MCM is that much less than the cost of 1-1000MCM (?).
Is the above logic consistent with industry practices? Any insight appreciated.
Thanks,
Nate Holt.
Large motor 500 feet away, 3ph motor (1.25 x FLA) = 535A, continuous duty, no ambient temp de-rating required for wiring.
From tables, 1000MCM yields 545A ampacity for 75C insulation. Assuming the three phase conductors in one conduit, no conductor de-rating required so this should work.
But maybe cheaper to go with 3x250MCM rated at 255A non-de-rated ampacity?
That would give 9 current carrying conductors in the conduit? meaning de-rate ampacity by 0.7 to 178.5A each. Times 3 gives just enough to cover the full motor?s (1.25 x FLA) value of 535A.
The additional advantage of the smaller conductors is that the voltage drop would be less. For the 1000MCM at 0.85 PF and 535A for 500 feet, I ran the full calculation using Table 9 reactance, AC resistance, and sine of arccos (messy), came up with 18.3 volts drop. But running the same calculation on the 250 MCM wires gives voltage drop of only 11.3 volts.
So, splitting the load over three smaller conductors saves 25% of the metal and cuts the IR losses in the wiring by 35%. The negative might be that the conduit could be bigger and not sure the cost of 3 -250MCM is that much less than the cost of 1-1000MCM (?).
Is the above logic consistent with industry practices? Any insight appreciated.
Thanks,
Nate Holt.