Voltage Drop Calculator Widget

Add a voltage drop calculator to your website. Visitors choose the circuit type, conductor material and size in mm2 or AWG, enter the one-way length and load current, and see the drop in volts and percent against common 3% and 5% guidance.

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<iframe src="https://a2z.tools/embed/w/voltage-drop-calculator" title="Voltage Drop Calculator by A2Z Tools" width="100%" height="970" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.

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How it works

The widget uses the voltage drop engine of the A2Z Voltage Drop Calculator. Conductor resistance starts from the standard resistivities at 20 C - annealed copper 1/58 ohm.mm2/m (IEC 60028, the International Annealed Copper Standard also used in NBS Handbook 100) and hard-drawn aluminium 0.028264 ohm.mm2/m (IEC 60889) - and is corrected to the conductor temperature the visitor enters (70 C by default, typical of PVC cable at full load). AWG sizes are converted to their exact cross-sections from the ASTM B258 definition. The drop is then 2 x I x R for DC and single-phase circuits, where current flows out and back, or sqrt(3) x I x R for a balanced three-phase circuit measured against the line-to-line voltage. It also shows the voltage left at the load, the heat lost in the cable and the longest run that stays within 3% and 5%. Only the resistive part is calculated: cable reactance is set to zero and the load is treated as power factor 1.

Calculation method

  • R = rho(T) x L / A, rho(T) = rho20 x (1 + alpha x (T - 20 C)); L = one-way length in m (ft x 0.3048), A in mm2
  • Copper rho20 = 1.724e-8 ohm.m (1/58 ohm.mm2/m), alpha = 0.00393 /K; aluminium rho20 = 2.826e-8 ohm.m, alpha = 0.00403 /K
  • DC and single-phase: Vd = 2 x I x R; balanced three-phase: Vd = sqrt(3) x I x R (reactance and power factor not included)
  • Drop % = Vd / supply voltage x 100; longest run for x% = L x x / drop %; cable loss = 2 I^2 R (single) or 3 I^2 R (three-phase)
  • AWG area from d = 0.127 mm x 92^((36 - n)/39); kcmil sizes from their circular-mil value; volts to 2 decimals, percent to 2 decimals

Worked examples

230 V radial, 2.5 mm2 copper

Inputs: Single-phase, copper, 230 V, 20 A, 30 m, 2.5 mm2, 70 C

Result: 9.9 V drop (4.3%), 220.1 V at the load, 247.53 mohm per conductor, 198 W lost; longest run 20.9 m for 3% and 34.8 m for 5%

rho70 = 1.724e-8 x 1.1965 = 2.0628e-8; R = 2.0628e-8 x 30 / 2.5e-6 = 0.2475 ohm; Vd = 2 x 20 x 0.2475 = 9.90 V.

120 V branch circuit in 12 AWG

Inputs: Single-phase, copper, 120 V, 16 A, 100 ft, 12 AWG, 20 C

Result: 5.08 V drop (4.24%), 158.81 mohm per conductor, longest run 70.8 ft for 3%

12 AWG = 3.3088 mm2, 5.21 ohm/km at 20 C; 30.48 m gives 0.1588 ohm; 2 x 16 x 0.1588 = 5.08 V.

12 V DC run to a caravan fridge

Inputs: DC, copper, 12 V, 10 A, 5 m, 4 mm2, 20 C

Result: 0.43 V drop (3.59%), 11.6 V at the load; longest run 4.2 m for 3%

R = 1.724e-8 x 5 / 4e-6 = 0.02155 ohm; 2 x 10 x 0.02155 = 0.431 V - on low-voltage DC even short runs need thick cable.

Indicative only. This is a calculation aid, not a design or compliance check: verify cable sizes, protection and installation details with a qualified electrician and your local wiring code before anything is installed.

Limitations

  • Resistive drop only: cable reactance and load power factor are ignored, which understates the drop on large conductors (roughly above 25 mm2) feeding inductive loads.
  • Uses nominal cross-sections, not IEC 60228 maximum conductor resistances, and does not model stranding, skin effect or parallel-conductor sharing.
  • Conductor temperature is the visitor's entry; the widget does not derive it from load, installation method or ambient temperature.
  • Checks voltage drop only - not current-carrying capacity, fault-loop impedance, protective-device coordination or the limits of any particular national code.

Where publishers use it

  • Electrical contractors' websites, for customers planning a garden office or EV charger feed
  • Solar and battery installers checking long DC runs between panels, charge controller and batteries
  • Caravan, boat and off-grid blogs sizing 12 V and 24 V cables
  • Training courses on cable selection for apprentices

Questions

What voltage drop is acceptable?

IEC 60364-5-52 Annex G suggests 3% for lighting and 5% for other uses in installations supplied from the public low-voltage network (BS 7671 Appendix 12 follows it), and NEC 210.19(A) Informational Note suggests 3% for a branch circuit and 5% for feeder plus branch. Your local code and the equipment maker have the final say.

Should I enter the one-way or the total length?

The one-way length of the cable route. The widget doubles it for DC and single-phase circuits, because the current flows out and back.

Why does temperature matter?

Copper's resistance rises by about 0.39% per degree C (alpha = 0.00393). A cable carrying its full rated current runs near 70 C, so its resistance is roughly 20% higher than at 20 C, and the drop rises with it.

Why is my result a little lower than a cable table?

The widget uses the nominal cross-section. IEC 60228 conductor tables list maximum resistances, which are higher - 7.41 ohm/km for 2.5 mm2 stranded copper against 6.90 ohm/km from the nominal area - so table-based results come out a few per cent above this one.

Does this size the cable for me?

No. It checks voltage drop only. Current-carrying capacity, installation method, grouping and protection must also be checked against your wiring code.

Sources

  1. IEC 60028:1925 International standard of resistance for copper - International Electrotechnical Commission . Annealed copper 1/58 ohm.mm2/m at 20 C
  2. Copper Wire Tables (NBS Handbook 100) - National Bureau of Standards (now NIST), 1966 . Annealed copper standard, temperature coefficient 0.00393 at 20 C, AWG definition
  3. BS EN 60889 / IEC 60889 Hard-drawn aluminium wire for overhead line conductors - BSI / IEC . Aluminium resistivity 28.264 nohm.m at 20 C
  4. ASTM B258 Standard specification for standard nominal diameters and cross-sectional areas of AWG sizes of solid round wires - ASTM International . AWG sizes: ratio 92^(1/39) between gauges
  5. IEC 60364-5-52:2009 Low-voltage electrical installations - Selection and erection of wiring systems (Annex G, voltage drop) - International Electrotechnical Commission . 3% lighting / 5% other uses (public LV supply)
  6. NFPA 70, National Electrical Code - 210.19(A) Informational Note - National Fire Protection Association . Advisory 3% branch / 5% total

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Voltage Drop Calculator
https://a2z.tools/voltage-drop-calculator

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