On a 230 V single-phase lead, every metre of copper costs voltage, and the whole installation from the origin to the tool has a budget of about 3 % for lighting and 5 % for other loads under BS 7671. With a 16 A load, a 2.5 mm² lead uses that 5 % on its own at about 40 m. With 10 A on 1.5 mm², about 38 m. Use a shorter or thicker lead, never two reels in a chain, and unwind every reel before it carries load.
STAT
3 % and 5 % are the voltage drop limits for lighting and other uses in an installation fed from the public network, measured from the origin to the equipment terminals, under BS 7671 Regulation 525.202 as NICEIC set it out in May 2019. Source: Professional Electrician, Calculating Voltage Drop with NICEIC
Key takeaways
| Topic | In brief |
|---|---|
| The budget | 3 % lighting, 5 % other uses, from the supply origin to the tool. |
| What uses it | Length, current and cross-section. Double the length, double the drop. |
| 230 V vs 400 V | A balanced three-phase lead can run about twice as far. |
| Reels | Unwind fully before loading. A coiled reel overheats within its rating. |
| Chains | Two reels in series double the drop and add a joint. |
What is voltage drop on an extension lead?
Current flowing through copper loses some voltage in the conductor’s resistance. The longer the lead and the higher the current, the less voltage reaches the socket at the far end.
TSR’s own product pages put the symptom plainly: an undersized cable heats up, and "the machine runs slowly at the end of a long cord." The fix they give is "a thicker cable, not a bigger fuse." A larger fuse lets more current through the same thin copper. It does nothing for the volts lost on the way.
Motors and heaters show it first: the motor slows and the heater gives less heat.
How much voltage drop is allowed?
In the UK, BS 7671 sets the budget for an installation supplied from the public network. As NICEIC summarised it in May 2019, Regulation 525.202 points to section 6.4 of Appendix 4, which allows 3 % for lighting and 5 % for other uses, measured from the origin of the installation to the equipment terminals.
At 230 V that is 6.9 V for lighting and 11.5 V for everything else. Regulation 525.102 accepts a larger drop while a motor is starting.
The budget covers the whole route. The cable from the supply to the board and the board to the socket have already used part of it before an extension lead is plugged in. The lead gets whatever is left.
How is the drop on a lead calculated?
The method in HD 60364-52 Annex G, as Trilux sets it out, multiplies resistance by current. For a single-phase lead with a resistive load:
ΔU = 2 × ρ × L ÷ S × I
ρ is 0.0225 Ω·mm²/m for copper at operating temperature, L the lead length in metres, S the cross-section in mm² and I the current in amperes. The factor 2 counts the conductor out and back. For a balanced three-phase lead the factor is 1, which is why Trilux notes that a three-phase line with evenly loaded phases may be twice as long.
A worked case. A 3 kW site heater on single-phase 230 V draws about 13 A. On a 40 m reel of 3 × 2.5 mm², the lead resistance is 2 × 0.0225 × 40 ÷ 2.5 = 0.72 Ω, and the drop is 0.72 × 13 = 9.4 V. That is 4.1 % of 230 V, from the lead alone.
How long can a lead be for each cross-section?
The table gives the length at which a single-phase copper lead uses the whole budget by itself, with a resistive load and nothing upstream. These lengths come from the formula above, not from any product rating.
| Lead and load | 3 % (6.9 V) | 5 % (11.5 V) |
|---|---|---|
| 1.5 mm², 10 A | 23 m | 38 m |
| 2.5 mm², 10 A | 38 m | 64 m |
| 2.5 mm², 16 A | 24 m | 40 m |
| 4 mm², 16 A | 38 m | 64 m |
Read it as an upper bound. Upstream cable has already used part of the budget, so a real site lead should be shorter. Trilux, working to a 4 % budget, gets 30 m for 10 A on 1.5 mm² and 32 m for 16 A on 2.5 mm², the same arithmetic.
TSR’s 20 m reel of 3 × 1.5 mm² has 230 V outlets rated 10 A. At its full 10 A it drops about 6.0 V, or 2.6 %. That is fine for a work light or a hand tool. A 3 kW heater is over that outlet rating and belongs on a 2.5 mm² lead.
Why must a cable reel be fully unwound?
Heat. Ireland’s Health and Safety Authority warned in April 2014 that a reel left coiled in use cannot shed the heat its cable generates. The insulation can melt until earth and neutral cores become live, a serious electrocution risk to anyone touching the appliance or nearby metal.
Brennenstuhl, whose reels TSR stocks, says the same: wound cable cannot dissipate heat, so unwind it completely. Each reel’s label gives a lower maximum load wound than unwound. TSR’s reel pages add that a cable left on the drum "heats up in the coils even when the current stays within its rating."
Reels built to EN 61242 or EN 61316 carry a thermal cut-out, per the HSA. A cut-out that trips is telling you the reel was coiled or overloaded. Unwind it and check the load before resetting.
Can you plug one reel into another?
You can, and it is the most common way a site runs out of voltage. Two 40 m reels of 2.5 mm² in series make an 80 m lead. With the same 13 A heater the drop is 18.7 V, or 8.1 % of 230 V, well past the 5 % budget before anything upstream is counted.
The chain also adds a plug and socket joint in the middle of a walkway. TSR’s product pages call a joint "always a weak point" and advise measuring the cable along the actual route, not as the crow flies. Where the work is far from the board, move the power closer. A sub-distribution board near the work point with short leads beats any chain of reels. Our guide to specifying temporary power distribution covers where boards go.
Lead check before a long run
- Measure the route. Around structures and up stairs, not the straight line.
- Find the load current. Nameplate watts ÷ 230 for single-phase.
- Pick the cross-section. Use the table, and leave room for the upstream drop.
- One lead, no chains. If one lead is not long enough, move a board.
- Unwind every reel before it carries a heater, pump or saw.
- Check sheath and plugs. A cut sheath means a replaced cable, not tape.
- Match the rating outdoors. Read the IP marking with the lids closed; our post on IP ratings for site equipment explains the digits.
- RCD at the source. The HSA wants every reel and everything fed through it RCD-protected at source. On generator supplies see how a generator-fed supply is earthed.
Board-level monitoring, such as TSR’s Spine system, shows load and voltage at the board. It does not see the volts lost along a lead after the socket, so the lead check stays a site task.
Summary
Voltage drop is the budget of 3 % for lighting and 5 % for other loads from the supply origin to the tool. Long, thin leads and chained reels spend it. Size the lead from the current and the length, keep it to one run, and unwind every reel before it carries load. TSR’s extension cords and cable reels come in 1.5 and 2.5 mm² single-phase and 16 A three-phase versions.
Frequently asked questions (FAQ)
How long can a 230 V extension lead be?
With 16 A on 2.5 mm², about 40 m uses the whole 5 % budget by itself. With 10 A on 1.5 mm², about 38 m. Upstream cable shares the budget, so keep site leads shorter.
Is 2.5 mm² better than 1.5 mm² for a long lead?
Yes. The drop falls in proportion to cross-section, so 2.5 mm² loses 40 % less voltage than 1.5 mm² at the same length and current, and it carries a 16 A load.
Why does a cable reel need to be fully unwound?
A coiled reel cannot shed heat. The HSA warned in 2014 that insulation can melt and make earth and neutral cores live, and reel makers rate a wound reel for less load.
Can I plug one cable reel into another?
It doubles the length and the voltage drop and adds a joint. Two 40 m reels of 2.5 mm² at 13 A drop about 8 % of 230 V. Move a board closer instead.
Does a bigger fuse fix voltage drop?
No. TSR’s own guidance is a thicker cable, not a bigger fuse. A larger fuse only lets more current through the same thin copper, which raises the heat and the drop.
Is a three-phase lead better over distance?
With evenly loaded phases, yes. The calculation counts one conductor instead of two, so Trilux notes a balanced three-phase line may be about twice as long as a single-phase one.
Sources
- Professional Electrician, Calculating Voltage Drop with NICEIC (2019)
- Trilux, Line voltage drop
- Health and Safety Authority, Electric cable reel alert (2014)
- Brennenstuhl, Why you should always unwind a cable reel completely
- TSR-Elsite, Cable reel 20m 3×1.5mm² 230V
- TSR-Elsite, Cable reel 16A (400V) 20m
- TSR-Elsite, Electric heater 3kW
TSR-Elsite. Published 30.10.2026.