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Use this calculator to estimate cable voltage drop for sizing conductors in a system when starting and running a motor. A supply transformer and other miscellaneous loads can be modeled.
Methodology and Assumptions
The calculation assumes uncoated copper or aluminum conductors operating at the temperature selected and is based on the ac resistance or impedance from NEC 2005 Table 9 for stranded conductors at 75C operating on an AC 60Hz system. Conductor temperature adjustments are made according to formulas in footnote 2 of NEC 2005 Table 8. The ampacity of each conductor size shown for reference in the dropdown menus is based on NEC 2005 Table 310.16 for 60C insulated conductors rated 0 through 2000 volts with not more than three current carrying conductors in raceway, cable or earth with an ambient of 30C (86F).
Units herein are American Wire Gauge (AWG) and English (feet).
Calculation assumes that the impedance divides proportionally according to the number of parallel conductors.
This load flow assumes 1 p.u. (100%) Voltage at supply transformer primary, which is the same as an infinite source.
Motor parameters are from 2005 NEC
Tables 430.248 and 430.250 for full load currents of ac induction motors running
at usual speeds with normal torque characteristics. The value of the
(efficiency x power factor) can be calculated from the horsepower, current and
voltage given. The Full load efficiency and power factors are estimated as
the square root of this value as follows:
This tends to overestimate the full load current resulting in slightly more
voltage drop.
The "other loads" connected to the bus are treated as a single lumped constant current load. This is equivalent to a combination of resistive loads and motor loads. Resistive loads draw less current with reduced voltage. Motors draw more current with reduced voltage and are modeled as constant kVA. Therefore, the constant current model is a blend of both.
Additional assumptions stated with the referenced NEC Tables also apply, such as cable capacitance being ignored.
Instructions for Use
Do not mix single phase and three phase in same calculation as results will not be accurate.
The parallel runs selection refers to the number of parallel conductors per phase or per hot leg. Examples for parallel runs: 120/240V single phase system with single black-red-white conductors (installed in single conduit) select "single set of conductors", 120/208V 3phase system with 2 conductors per phase and neutral (installed in 2 parallel conduits) select "2 conductors per phase in parallel."
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Actual ampacity and voltage drop for your application may differ from these results but in most cases will be very close to those shown here.
Results are believed to be accurate subject to the inputs and assumptions stated herein. However the user should verify accuracy and suitability before all use for any purpose. See our disclaimer statement here.
Click here for alternate calculator that only includes a single cable and fixed load.
