Voltage Drop Calculator

What is voltage drop?

When current flows through a wire, the conductor’s resistance (DC) or impedance (AC) causes a loss of electrical potential. That loss is the voltage drop. The load receives less voltage than the source provides.

Too much drop can cause:

• Lights that flicker or burn dimly
• Heaters that underperform
• Motors that run hot, stall, or fail early
• Sensitive electronics and low-voltage lighting that brown out (a 1 V loss on 12 V is already ~8%)

A widely cited design guideline is to keep total voltage drop under about 5% at full load; many installers aim for ~3% on the final sub-circuit. Compare with the design recommendation or code requirement applicable to your installation — jurisdiction and circuit type matter. Always follow your local electrical code (NEC, AS/NZS 3000, IEC, etc.).

Supported and not supported

Supported

• DC and resistive single-phase AC with go-and-return path (factor 2)
• Balanced three-phase line-to-line drop (factor √3) as a resistance-only estimate
• Resistance-based estimate from I, one-way length (m), and Ω/km
• Optional system voltage → percent drop and estimated load voltage (3φ: line-to-line)
• Conductor I²R loss Pₗₒₛₛ (always): DC / resistive 1φ I²· 2R; balanced 3φ 3· I²· R
• Optional standard uncertainties on I, length, and Ω/km with coverage k (default 2)
• API result { V_drop, R_one_way, P_loss, R_loop?, percent_drop?, V_load?, uncertainty? } via electrical.voltage_drop
• Shareable query URLs and REST/OpenAPI/MCP via electrical.voltage_drop

Not supported

• Complete IEC 60364-5-52 Annex G impedance-aware voltage-drop model
• AC reactance, skin effect, or power-factor-adjusted impedance drop
• Temperature-corrected resistance / NEC or AS/NZS table lookups
• Unbalanced three-phase or harmonic-rich feeders
• Ampacity, conduit fill, or protective-device sizing
• Code-compliance decisions
• Monte Carlo / correlation / νeff UI on this page (use Uncertainty Propagate directly)

Agent / API notes

Capability id: electrical.voltage_drop · tool id: voltage-drop · pin calculation_version: 1.5.2.

1.5.0: optional V_system adds percent_drop and V_load; result always includes R_one_way and conductor I²R P_loss (and R_loop for dc). Agent input requires mode. Optional uncertainty attaches when any u_* is set.

Stable error codes include MISSING_REQUIRED_INPUT, INVALID_NUMBER, VALUE_MUST_BE_NON_NEGATIVE, VALUE_MUST_BE_POSITIVE, INVALID_MODE, and INVALID_INPUT.

How this calculator works

Enter:

1. Current I in amperes
2. One-way length L in meters (distance to the load, not round-trip)
3. Resistance Rₖₘ in Ω/km

Choose DC / resistive 1φ or balanced 3φ · resistive estimate.

One-way conductor resistance:

R = (L (m) × Rₖₘ (Ω/km))/1000

DC and resistive single-phase AC

Go-and-return path → factor of 2. This AC form assumes PF ≈ 1 and neglects conductor reactance:

V(drop) = I × 2R = I × (2 L Rₖₘ)/1000

Cable heating on the same go-and-return path:

Pₗₒₛₛ = I² × 2R = I × V(drop)

Three-phase AC (balanced, resistive estimate)

Line-to-line drop, same PF ≈ 1 / X ≈ 0 approximation:

V(drop) = √3 I R ≈ 1.732 × I × (L Rₖₘ)/1000

Three-phase conductor loss (three equal line resistances):

Pₗₒₛₛ = 3 I² R = √3 I V(drop)

For inductive loads, motors, and larger AC feeders, use an impedance- and PF-aware model.

Percent drop and load voltage

If you enter system voltage V(sys) (line-to-line for 3φ):

% V(drop) = 100 × (V(drop))/(V(sys)), V(load) ≈ V(sys) − V(drop)

Optional uncertainty

When you supply one or more standard uncertainties u(xᵢ), drop is combined with analytical sensitivity coefficients partial V/partial I, partial V/partial L, partial V/partial Rₖₘ. Expanded uncertainty uses coverage factor k (default 2):

u_c(V) = √(sumᵢ (cᵢ uᵢ)²), U = k u_c

Result UI shows V(drop) ± U when any u_* is present.

Length in feet (same idea)

If you work in imperial units with resistance in Ω/1000 ft:

DC / resistive 1φ:

V(drop) = I × (2 L(ft) R(kft))/1000

Balanced 3φ · resistive estimate:

V(drop) = √3 I × (L(ft) R(kft))/1000

Convert: Rₖₘ ≈ 3.281 × R(kft), and Lₘ = L(ft) × 0.3048.

What drives voltage drop?

Also consider temperature (hotter copper → higher R), AC reactance on large feeders, parallel conductors, and power factor. Professional tools (e.g. NEC-based or AS/NZS 3008 impedance tables) include those effects; this page uses a resistance estimate.

Allowable voltage drop (guidance only)

These are educational summaries — verify against the code that applies to your installation. This page does not treat 3–5% as a single international limit.

How to pick Ω/km

1. Prefer the cable manufacturer or code table for your size, temperature, and construction.
2. Or use approximate copper AWG values from the reference table above.
3. Find size with AWG to mm / SWG to mm, then look up resistance.
4. Aluminum: expect roughly 1.6× the copper resistance for a similar gauge (confirm with data sheets).

Cable selection reminder

Good cable choice usually means:

1. Ampacity — carry the load without overheating under worst-case ambient and bundling.
2. Voltage drop — keep percent drop within limits at full load.
3. Fault / earthing — meet protective-device and safety requirements.

This calculator addresses item 2 only (resistance-based estimate).