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Use this calculator to estimate cable voltage drop for sizing conductors. The calculation assumes uncoated copper or aluminum conductors operating at the temperature selected and is based on the ac/dc resistance or impedance from NEC 2011 Chapter 9, Tables 8 and 9 for stranded conductors operating on a DC or AC 60Hz system. Rather than using the 'k' factor or the 'Effective Z' of Table 9 this method is based on the actual ac resistance and reactance values from the table. The load current input is fixed, as is the base system Voltage. The Voltage drop in the cable is calculated using Ohm's Law where Vdrop = Iload x Rcable. The percent drop is Vdrop / Vsystem x 100. For ac systems the ac impedance is used in place of the dc Rcable. This methodology is similar to the examples given after NEC Table 9.
The ampacity of each conductor size shown for reference in the dropdown menu below is based on NEC 2011 Table 310.15(B)(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).
Note that the 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.
Units herein are American Wire Gauge (AWG) and English (feet).
Note, to run this calculator JavaScripts must be enabled on your browser.
Click here for alternate calculator that also includes a transformer and motor load.
NOTES:
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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", dc system with 3 positive and 3 negative conductors select "3 conductors per phase in parallel".
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Voltage drop for ac systems should total no more than 5% under full load conditions. This is recommended by the NEC 210.19(A)(1) Informational Note No. 4 which states a 3% limit for branch circuits and NEC 215.2(A)(4) Informational Note No. 2 which states a 3% limit for feeders. Both of these set a limit of 5% total for both. Drop may be significantly larger during surge or motor starting conditions -- sometimes in the 15% to 25% range if other devices on the system can withstand this momentary dip. Voltage drop for dc systems should be designed as low as possible or less than 2%.
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For most 120/240V systems using cables of adequate ampacity, voltage drop is not a concern unless cable lengths are well over a hundred feet. A common rule of thumb is to check the voltage drop when the one-way circuit length in feet exceeds the system voltage number. Therefore, using this rule one would check the drop for a 240V system if the circuit length exceeds 240 feet.
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For refining the calculation the conductor operating temperature can be estimated as follows: If the operating current equals the ampacity listed in NEC Tables 310.15 then the temperature can equal the rating of the table column. If the operating current is less than the ampacity listed then the temperature will be less. Since conductor heating is equal to the I2 x R losses, and the heating is proportional to the conductor temperature rise, then the operating temperature will be approximately (Ioperating / Iampacity)2 x (Trating - 30C) + 30C. For example, a 50 Amp load using 75C rated copper conductor requires #8 AWG per Table 310.15(B)(16). If the wire size is increased to #6 AWG for voltage drop considerations then the conductor operating temperature would be (50A / 65A)2 x (75C - 30C) + 30C = 57C. This results in a slight reduction of the voltage drop and may be useful for marginal calculations.
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All references to the NEC refer to National Fire Protection Association, NFPA 70®, National Electrical Code®. or National Electrical Code® Handbook.
More information about voltage drop based on IEC standards is available in the Schneider Electric Electrical Installation Guide.
UPDATE: On 11/4/2009 the 3-phase % calculation was adjusted by a factor of 1.732
UPDATE: On 9/25/2013 added #16 AWG; ac values extrapolated
UPDATE: On 4/27/2018 added 850V, 1000V and 1500V for dc solar systems
UPDATE: On 10/16/2018 added 70V, 80V, 90V for dc systems
UPDATE: On 02/25/2019 updated and added NEC references, expanded methodology description, added NOTE 4 and NOTE 5.
UPDATE: On 4/3/2019 added more Voltage choices between 120 and 208 for dc solar systems
