We are working on this mansion with a 600 amp panel and our boss wants to pull 250 kcmil aluminum wire (we don't even think his load "calculations" are correct for the house, probably only need 400 amp service). We have a guest house and a detached garage we are pulling to from the house. We measured the garage piping and it will be 450 feet and a 200 amp panel. The two journeymen on the job aren't getting the same numbers in their calculations for the of size wire we need, but we all agree that the boss is wrong because with the voltage drop that aluminum wire isn't going to cut it. What equations should we be using to calculate this? And what size wire should we actually be using?

This is a commonly used voltage drop calculation formula:


(2xK) x A x L
_________________
CMils

SINGLE PHASE (1Ø) FORMULA

The "K" for Al is 21.2
The "A" is your load in amps (200)
The "L" is the length of the run, in feet, one way
The "CMils" is the conductor size (250,000)

You should get an answer of 15.26 Vd, which is excessive for a feeder.

Understand that the "200 amp" load is a major factor. Do you know what the calculated load is for this panel??

Elaborating on John's Post (AKA "HotLine1"):



Using my Custom Made Volt-Loss Spreadsheet, the following values were obtained across several scenarios:

FEEDER SCENARIO #1: 250 MCM ALUMINUM FEEDERS
Feeder Length: 450 Feet,
Duct Material: Non-Metallic,
System: 120/240V 1 Phase 3 Wire,
E L-L at point of Delivery: 240V,
Loads: =/> 0.884PF; connected L-L...
"L-L" = Line-to-Line
"(AVG)" = Averaged
"(WCS)" = Worst-Case Scenario

1A.: 30 Amp Load:
Volt Loss Percentile: 0.893% (AVG), 0.954% (WCS)
Averaged Working Voltage: 237.86 VAC

1B.: 50 Amp Load:
Volt Loss Percentile: 1.489% (AVG), 1.590% (WCS)
Averaged Working Voltage: 236.43 VAC

1C.: 80 Amp Load:
Volt Loss Percentile: 2.382% (AVG), 2.544% (WCS)
Averaged Working Voltage: 234.28 VAC

1D.: 125 Amp Load:
Volt Loss Percentile: 3.722% (AVG), 3.975% (WCS)
Averaged Working Voltage: 231.07 VAC

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FEEDER SCENARIO #2: 250 MCM COPPER FEEDERS
Feeder Length: 450 Feet,
Duct Material: Non-Metallic,
System: 120/240V 1 Phase 3 Wire,
E L-L at point of Delivery: 240V,
Loads: =/> 0.884PF; connected L-L...
"L-L" = Line-to-Line
"(AVG)" = Averaged
"(WCS)" = Worst-Case Scenario

2A.: 30 Amp Load:
Volt Loss Percentile: 0.707% (AVG), 0.8325% (WCS)
Averaged Working Voltage: 238.30 VAC

2B.: 50 Amp Load:
Volt Loss Percentile: 1.178% (AVG), 1.3875% (WCS)
Averaged Working Voltage: 237.17 VAC

2C.: 80 Amp Load:
Volt Loss Percentile: 1.884% (AVG), 2.220% (WCS)
Averaged Working Voltage: 235.48 VAC

2D.: 125 Amp Load:
Volt Loss Percentile: 2.949% (AVG), 3.46875% (WCS)
Averaged Working Voltage: 232.94 VAC

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The above figures plot High Power Factor Loads connected across the Secondary (Output) of a Single Phase Transformer, with a Center-Tapped & Grounded Secondary Winding - either Physical Tap, or Tap between X2-X3 Terminals.

Balanced L-N Loads may be figured as L-L
(i.e.: Two separate 50A L-N Loads connected A-N and B-N)
For a completely L-N Load, double the Volt-Loss Percentiles listed above.
Example: Feeder = CU, 30 Amp L-N Load:
Volt Loss Percentile: 1.413% (AVG), 1.665% (WCS)
Averaged Working Voltage: 118.30 VAC

Volt Loss results are from Two (2) Calculation Types:
a. Conductor Resistance-Only
and
b. Conductor Impedance (Resistance & Reactance).

Averaged Volt Loss Calculations are the mean of the Two, while Worst-Case Scenario is the higher of the Two.

Lastly;
If the System is "120/208V 1 Phase 3 Wire Open Wye", derived from a 208Y/120V 3 Phase 4 Wire Transformer's Output, values will be much different than the results from a 120/240V 1PH. 3 Wire Transformer.

--Scott (EE)