Every cable fights back against the current running through it, and that fight costs you voltage. On a long run, there might not be enough left at the other end for the gear to work properly.
Voltage drop is the calculation that separates a proper job from one that scrapes through inspection and bites you later. Nail it at the design stage, and you pick the right cable on the first go. Miss it, and you're pulling new cable through a conduit that's already sheeted over.
This guide covers what voltage drop is, what causes it, the DC and AC formulas, how to test it onsite, and the limits under the main wiring standards — with a worked example for each circuit type.
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Voltage drop is simply the loss of electrical pressure as current travels through a wire or component. Think of it like water pressure in a hose — the longer the hose and the narrower the pipe, the less pressure arrives at the other end.
In electrical terms: when current flows through a conductor, the wire's resistance converts some of that electrical energy into heat. The result is that the voltage at the far end of the circuit is always lower than at the source. That difference is your voltage drop.
Every circuit has some voltage drop — that's normal. The problem starts when it becomes excessive. Excessive voltage drop causes lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out.
Voltage drop is energy going missing on the way down the cable. Whatever the supply's putting out — 230 V single-phase in the UK, say — isn't what's landing at the load. Some get left behind.
That's because every cable's got resistance, and pushing current through resistance kicks off heat, which comes at the expense of voltage. So the far end always reads lower than the supply end. The longer the run, the higher the current, the more energy you lose.
AC circuits have a bit of extra pushback too, but for most jobs it's small enough to ignore.
Four things drive voltage drop in any installation:
Beyond the cable itself, dodgy connections make things worse. A corroded terminal or a loose lug adds resistance at a point in the circuit, creating a localised voltage drop that a cable calculation won't pick up. That's why a voltage drop test on an existing installation can turn up connection problems that a visual inspection misses.
The acceptable limit depends on which standard applies in your region and what type of circuit you're wiring.
BS 7671 sets two limits, measured from the origin of the installation to the load:
Lighting's held to the tighter number because even a slight drop shows up as flicker or dimming. For installations fed from a private LV source like a generator, BS 7671 allows more headroom: up to 6% for lighting and 8% for other circuits.
These are mandatory limits, not guidelines. A circuit that exceeds them is a compliance failure.
The Wiring Rules set a single limit: the voltage drop from the point of supply to any outlet must not exceed 5% of the nominal supply voltage. That’s 11.5 V on a 230 V supply.
If the point of supply is the LV terminals of a dedicated on-site substation, you get a bit more room, up to 7%. For stand-alone systems, a total drop of up to 11% from nominal may be permitted under specific conditions set out in the standard.
AS/NZS 3000 also splits the allowance between consumer mains (typically 2% from the point of supply to the main switchboard) and the final sub-circuit (the remaining allowance to the outlet). In practice, a lot of designers target 3% on the final sub-circuit to keep some headroom within the 5% total.
Always check against the current edition of the standard for your region. Standards get updated. E.g. AS/NZS 3000:2018 has been amended since it was first published, and New Zealand formally cited the latest version into law via the Electricity (Safety) Amendment Regulations 2025.
DC circuits are the simplest case. No reactance, just resistance, so it's a straight application of Ohm's Law.
The formula accounts for the fact that current travels from the source to the load and back again, so the total conductor length carrying current is twice the one-way cable length:
VD = 2 × I × R × L
Where:
A 24 V DC control circuit feeds a panel 15 m from the supply. The design current is 8 A. The cable is 2.5 mm² copper with a resistance of 0.00741 Ω/m.
VD = 2 × 8 × 0.00741 × 15
VD = 1.78 V
VD% = (1.78 ÷ 24) × 100 = 7.4%
That's over the 5% target. Step up to 4 mm² copper (resistance ≈ 0.00463 Ω/m):
VD = 2 × 8 × 0.00463 × 15
VD = 1.11 V
VD% = (1.11 ÷ 24) × 100 = 4.6% ✓
For single-phase AC circuits, the formula follows the same structure as DC, but resistance values are taken at operating temperature. For longer runs or inductive loads, use impedance (resistance and reactance combined) rather than resistance alone.
BS 7671 Appendix 4 gives mV/A/m values for standard cable types. This is the method most UK sparkies use because the resistance and reactance are already combined into a single table value for the cable type and installation method.
VD (V) = (mV/A/m × Ib × L) ÷ 1000
Where:
The mV/A/m value varies by cable CSA, conductor material, insulation type, and installation method. Bigger cables have lower values because they have less resistance per metre. Always use the table that matches your cable type. For thermoplastic (PVC) twin and earth clipped direct, that's Table 4D5 in BS 7671.
A 230 V lighting circuit, 25 m long, design current 12 A. Cable: 2.5 mm² twin and earth copper, clipped direct. From BS 7671 Table 4D5, the mV/A/m value is 18 mV/A/m.
VD = (18 × 12 × 25) ÷ 1000
VD = 5.4 V
VD% = (5.4 ÷ 230) × 100 = 2.35%
The limit for a lighting circuit under BS 7671 is 3% (6.9 V). That's a pass.
Where mV/A/m tables aren't available, or for circuits where power factor matters, use the impedance formula:
VD = 2 × I × Z × L
Where Z is the cable impedance in Ω/m, calculated as:
Z = √(R² + X²)
Where R is resistance per metre and X is inductive reactance per metre. For most domestic and light commercial cables, X is small relative to R and the resistance-only formula gets you close enough for design purposes. For bigger cables (above 16 mm²) or long runs, use the full impedance value.
When you've got loads sitting in parallel, each branch only carries its own slice of the total current. You work out the drop on the supply cable (the run feeding all the branches) using the full current, then work out the drop on each branch cable using just what that branch is pulling.
The bit to remember: all the parallel branches are getting fed off the same voltage at the junction point. So if the supply cable's already dropped a fair bit of voltage, every branch is starting from a lower number before you even factor in its own branch drop. You have to check both ends — supply cable and branch cable — not just one.
A 230 V supply feeds a junction box 20 m away via a 6 mm² cable. From the junction box, two branch circuits run 10 m each: Branch A draws 10 A, Branch B draws 8 A. Total current in the supply cable: 18 A.
Using a resistance of 0.00308 Ω/m for 6 mm² copper:
Supply cable VD = 2 × 18 × 0.00308 × 20 = 2.22 V (0.97%)
Branch A (2.5 mm² copper, R = 0.00741 Ω/m):
Branch A VD = 2 × 10 × 0.00741 × 10 = 1.48 V (0.64%)
Total drop to Branch A load: 2.22 + 1.48 = 3.70 V (1.61%). Well within the 5% limit.
Calculating voltage drop at the design stage tells you what to expect. Testing it on an existing installation tells you what's actually going on. The two don't always match.
A voltage drop test is easy:
If the reading is significantly higher than your calculated value, you've usually got a connection problem somewhere: a loose terminal, a corroded joint, or an undersized connector adding resistance at a point in the circuit. Isolate the circuit, work through the connections from the supply end, and retest.
Use a calibrated multimeter or a proper installation tester. Some of the multifunction ones have a voltage drop mode built in that sends a known test current through and gives you the drop straight up.
9. What happens when voltage drop is too high?Gear is designed to work within a voltage tolerance band. When the voltage at the load falls below the bottom of that band, things start going wrong:
If your calculation shows excessive voltage drop, there are a few ways to bring it within limits. The right fix depends on the job:
The most important thing: sort voltage drop at the design stage. Upsizing a cable before it goes in costs a fraction of what it costs to replace it after the walls are closed up.
A voltage drop calculator does the maths for you. Key in the circuit parameters (cable size, length, current, circuit type, and supply voltage) and it spits out the voltage drop in volts and as a percentage, with a pass/fail check against the applicable standard.
Tradify's free voltage drop calculator handles DC, single-phase AC, and three-phase circuits. It's handy for a quick design check on site or in the van before you commit to a cable size.
For trickier jobs (long runs, motor loads, or circuits where power factor matters) a dedicated cable sizing tool that applies the full AS/NZS 3008 or BS 7671 Appendix 4 method will give you more precise results.
12. Keep your electrical business running as well as your installationsVoltage drop calculations are one part of running a tight electrical business. The other part is making sure your quotes, jobs, and invoices don't fall through the cracks while you're on the tools.
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The regulatory limits and standard references in this article are correct as at 7 September 2026. Electrical standards are updated periodically — always verify against the current edition of BS 7671, AS/NZS 3000, or the standard applicable in your region before designing or certifying an installation. This article is for general information only and does not constitute engineering or compliance advice.