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Voltage drop calculator: cable drop and minimum wire size
Drop in volts and percent for a copper or aluminium cable, and the minimum cross-section in mm² to stay within your limit (NF C 15-100 method). Free.
- Instant result
- Formula and standards shown
- On desktop and phone
Result
8.84VVoltage drop ΔU
- Relative voltage drop
- 3.84%
- Caution : Cable ampacity Iz and protection are not checked here.
- Caution : Beyond 100 m, NF C 15-100 allows 0.005 % more per metre, up to 0.5 %.
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Does not replace the installation design or its check by a qualified person.
What is voltage drop?
Voltage drop is the loss of voltage along a cable, between the origin of the circuit and the load, caused by the resistance and reactance of the conductors carrying the current. It grows with length and current, falls as the cross-section increases, and is given in volts or as a percentage of the nominal voltage. IEC 60364-5-52 (Annex G) and its national versions, BS 7671 in the UK or NF C 15-100 in France, set limits for low-voltage installations, while the US National Electrical Code recommends values in informational notes.
A maintenance team usually meets excessive voltage drop as a symptom rather than on a drawing: a motor that struggles to start or runs hot, contactors that chatter, a drive that trips on undervoltage. The risk is highest when a machine moves to the far end of the workshop or a load is added to an existing feeder, because the cable was sized for the situation before.
IEC limits, public supply
Table G.52.1 of IEC 60364-5-52: 3% for lighting and 5% for other uses, from the origin of the installation to the load. BS 7671 Appendix 4 uses the same values.
IEC limits, private supply
When the site has its own LV transformer, the same table allows 6% for lighting and 8% for other uses, since the cables from the transformer belong to the installation.
NEC recommendation (US)
Notes to sections 210.19 and 215.2 suggest 3% on a branch circuit and 5% for feeder and branch circuit combined. Under section 90.5(C), informational notes are not enforceable requirements.
Steady state, not motor starting
These limits apply to normal operation. At start-up a motor draws several times its rated current, so the drop at its terminals is far larger for a few seconds.
Reference documents : IEC 60364-5-52, Annex G (Table G.52.1) ; BS 7671, Appendix 4 ; NFPA 70 (NEC), sections 90.5(C), 210.19 and 215.2
Voltage drop results filed with the wiring change
Here, the calculator is free and the result is gone when you close the page. In Reglova, it is attached to the work order on the right machine: you find it again at the next breakdown, and on audit day.
The CMMS software comparison, our gaps included- Attached to the work order with its inputs: the server recalculates it, it stays verifiable.
- On the technician's phone, even without a network.
- In the machine's history, for the next breakdown.
- The asset's nameplate data pre-fills the calculation.
When to calculate cable voltage drop
01
Check the supply of a machine added at the far end of the workshop.
02
Explain a motor that struggles to start at the end of a long run.
03
Prepare a wiring change with the design office or the electrician.
Good to know
The current-carrying capacity Iz and the cable protection are not checked here, and the permitted drop depends on your country's wiring rules. The calculation replaces neither the installation design nor its inspection by a qualified person.
Voltage drop formula and minimum cross-section
The calculator applies the formula of the French wiring standard NF C 15-100 and its guide UTE C 15-105, ΔU = b·(ρ1·L/S·cos φ + λ·L·sin φ)·IB. It gives the drop in volts and as a percentage for the cross-section entered and, if you set a limit, the calculated minimum cross-section and the standard size to choose.
Formula · Reference documents
ΔU = b·(ρ1·L/S·cos φ + λ·L·sin φ)·IB, b = 1 three-phase, 2 single-phase ; ΔU % = 100·ΔU/U0 ; Smin = b·ρ1·L·cos φ·IB / (ΔUmax − b·λ·L·sin φ·IB)
NF C 15-100, UTE C 15-105 ; ρ1 copper 0.023, aluminium 0.037 Ω·mm²/m (Eduscol)
Inputs and accepted ranges
| Input | Unit | Accepted range | Example |
|---|---|---|---|
| Supply | 2 options | Three-phase | |
| Cable length | m | 0.1 to 10,000 | 110 m |
| Design current IB | A | 0.01 to 10,000 | 140 A |
| Power factor cos φ | 0.01 to 1 | 0.8 | |
| Conductor | 2 options | Copper | |
| Resistivity ρ1 (otherwise from conductor) | Ω·mm²/m | 0.01 to 0.1 | Optional |
| Reactance per length λ | mΩ/m | 0 to 1 | 0.08 mΩ/m |
| Phase-to-neutral voltage U0 | V | 1 to 1,000 | 230 V |
| Cross-section | mm² | 0.5 to 1,000 | 35 mm² |
| Permitted drop | % | 0.1 to 20 | Optional |
Calculated results
- Voltage drop ΔUV
- Relative voltage drop%
- Standard cross-sectionmm²
- Minimum cross-sectionmm²
- Drop with that cross-section%
What the calculator flags
- Cable ampacity Iz and protection are not checked here.
- The drop exceeds the permitted limit.
- Beyond 100 m, NF C 15-100 allows 0.005 % more per metre, up to 0.5 %.
Electrical changes traced from request to inspection
- Breakdowns reported by QR code
- Work orders on the phone, even without a network
- Mandatory inspections reminded by email
- Audit file as a PDF, in one click
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