Voltage drop calculator

The manual mode uses effective impedance supplied by the user. For two-wire single-phase circuits, the relationship is ΔV = 2 × I × L × Z ÷ 1000. For balanced three-phase circuits, it is ΔV = √3 × I × L × Z ÷ 1000. Current is in amperes, one-way length is in metres, and impedance is in ohms per kilometre. Maximum distance algebraically rearranges the selected relationship using a target percentage supplied by the user.

The percentage-to-volts mode multiplies the supplied reference voltage by the percentage entered. It is a direct arithmetic conversion only; it does not assess a circuit or recommend a target. Converted values may exceed the reference voltage.

The limited DC mode uses listed Southwire RW90 copper DC resistance at 25°C for 14, 12 or 10 AWG solid or seven-strand products, and 8 AWG through 4/0 AWG with the strand construction listed for each size. It applies the two-wire DC relationship using that resistance and does not estimate another temperature.

The limited AC mode uses the same product rows' AC resistance at 75°C and reactance at 60 Hz for a two-wire single-phase run. For conductor resistance R and reactance X in Ω/km, one-way length L in metres, current I, and lagging power-factor angle θ, the source-voltage relation is Vs² = VL² + 2VL·I·(Rloop cos θ + Xloop sin θ) + I²(Rloop² + Xloop²), where Rloop = 2RL/1000 and Xloop = 2XL/1000. The AC distance mode solves the same relation for a user-entered target. The result percentage is drop divided by supplied source voltage.

For the table-backed DC and AC modes, the result also shows one-conductor resistance for the entered length, the two-wire loop resistance, and resistive conductor loss I²Rloop using the entered current. AC loss uses the table's AC resistance; reactance is not counted as resistive loss. These values do not estimate conductor temperature or temperature rise. The manual effective-impedance mode cannot separate resistance from reactance, so it does not show the loss calculation.

This AC estimate assumes the product table's non-ferrous-conduit and one-diameter centre-to-centre spacing basis, a sinusoidal steady-state series model, and lagging power factor. It does not estimate another temperature, frequency or arrangement, and does not support three-phase, leading power factor, capacitance, harmonics, conductor selection or code acceptance. Manual AC mode continues to require user-supplied effective impedance.

The percentage is calculated as voltage drop divided by the supplied reference voltage. Results exclude unbalanced or harmonic-rich systems, split-phase neutral behaviour, conductor selection, breaker sizing, ampacity, protection and code compliance. Values remain in this page and are not saved.

Maximum distance is only the mathematical one-way length at which the chosen model reaches the user-entered target. It does not identify an acceptable target, account for starting conditions or select a conductor.

For a two-wire single-phase circuit at 208 V, 50 A and 30.5 m one-way length, using an explicitly assumed effective impedance of 1.48 Ω/km, the formula gives 4.514 V, or approximately 2.170% of the reference voltage. These values illustrate the calculation; the impedance is not a default for any conductor.

For a reference voltage of 120 V and a user-entered 2.5% drop, direct conversion gives 3 V. This result does not identify an acceptable percentage for a circuit.

For a two-wire DC scenario at 48 V, 10 A and 20 m one-way length using the listed 10 AWG RW90 copper resistance of 3.41 Ω/km at 25°C, the estimate is 1.364 V drop (2.842%) and 13.640 W conductor loss. The two-wire loop resistance is 0.136 Ω. This example uses the table condition and does not estimate another conductor temperature.

For a two-wire single-phase AC case at 120 V, 10 A, 20 m and lagging power factor 0.8, using 12 AWG RW90 copper data, the exact source-voltage relation gives approximately 2.155 V drop and 1.795% of source voltage. One-conductor resistance is 0.131 Ω, two-wire loop resistance is 0.263 Ω, and resistive conductor loss is 26.280 W. This uses the table's 75°C resistance, 60 Hz reactance and stated reactance layout.

Only the display is rounded, to three decimal places; small nonzero results use scientific notation. No minimum or maximum acceptable percentage is applied.

Report a problem or suggest a source. Calculation values are not automatically included.

National Research Council Canada, User's Guide – National Energy Code of Canada for Buildings 2020, 2024 publication, Commentary on Part 7, Example 7-1, printed page 89 (PDF page 98). Source checked November 9, 2026.

Southwire CableTechSupport, RW90 Copper Circuit Sizes (14, 12 & 10 AWG), Canadian specification SPEC22061, table 2. The data mode uses its listed 25°C DC resistance column. AC resistance and reactance are not used by that mode.

Southwire CableTechSupport, SIMpull XHHW-2 / RW90 Copper specification PDF, SPEC10006 revision 1.000.142 (October 21, 2026), tables 1–2, and the current Canadian CableTechSupport detail page for SPEC10006. RW90 sizes 8 AWG through 4/0 AWG use the listed strand counts, 25°C DC resistance, 75°C AC resistance and 60 Hz reactance in this calculator's two-wire modes. The table and product page state the non-ferrous-conduit, one-diameter centre-to-centre reactance basis. Values in Ω/1000 ft are converted to Ω/km using 1,000 ft = 304.8 m. The downloadable spreadsheet export labels its AC resistance column 90°C; it is not used because the versioned PDF and current Canadian product page both specify 75°C. Ampacity columns are not used.

Southwire, RW90 Copper Circuit Sizes (14, 12 & 10 AWG), specification SPEC22061, states the inductive-impedance basis: non-ferrous conduit and one-diameter centre-to-centre spacing. Southwire, Power Cable Installation Guide, AC voltage-drop section, supplies the source-voltage phasor relationship. These are used only for the bounded AC estimate described above.

BCcampus, Basic Motor Control (2020), “Ohm's Law and Watt's Law.” The I²R relationship is used only to report conductor resistive loss; it does not provide a thermal model or conductor rating.

This tool is a physical calculation aid, not a Canadian Electrical Code determination. Verify inputs and the applicable requirements for the job.