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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsThe correct wire size is the smallest conductor that passes every applicable check—not simply one that carries the load current. Start with design current, then verify ampacity for the actual installation, voltage drop, protective-device coordination, conductor and terminal ratings, and the rules in force where the work is installed. A wire size that is acceptable in one country, wiring method or circuit may be unsafe or non-compliant in another.
This guide explains the calculation process without pretending that one universal ampacity chart exists. UK fixed installations follow BS 7671; US calculations commonly use NEC methods and AWG/KCMIL sizes. Marine, buried and thermally insulated installations introduce additional assumptions.
What determines wire size?
Wire selection is constrained by several inputs at the same time:
- Design current: the load current, including applicable continuous-load or demand assumptions.
- Circuit voltage and configuration: DC, single-phase AC or three-phase AC; source and load voltages; and the conductors included in the circuit.
- Conductor and insulation: copper or aluminium, conductor construction and permitted insulation temperature.
- Run length: one-way distance and the complete current path used for voltage-drop calculations.
- Installation conditions: cable route, ambient temperature, grouping, thermal insulation, conduit or tray, burial depth and soil conditions.
- Protection: the rating and characteristics of the overcurrent device and any separate fault-protection requirements.
- Equipment limits: terminal temperature ratings, connection space, bending radius and the equipment maker’s instructions.
Because these variables interact, “What size wire for 20 amps?” has no defensible answer until the circuit and installation are known.
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Before calculating, identify the governing rules
Establish the jurisdiction, edition and application before using a table or calculator. The IET identifies the current UK On-Site Guide as aligned with BS 7671:2018+A4:2026 (9th Edition) for competent-installer work up to 100 A, three-phase. US-oriented Southwire tools use NEC applications and AWG/KCMIL sizes. Their values and methods should not be mixed into a universal chart.
| Framework or application | Typical size convention | What must be matched |
|---|---|---|
| UK fixed installation | Cross-sectional area in mm² | Current BS 7671 tables, installation method, correction factors, voltage-drop and protective-device rules |
| US building wiring | AWG or KCMIL | NEC ampacity rules, terminal ratings, adjustment factors, raceway/cable conditions and voltage-drop design criteria |
| Marine wiring | Application-specific conductor tables | Marine guidance, vessel voltage, cable routing, heat dissipation and equipment requirements |
| Buried cable | Jurisdiction-dependent | Burial method, depth, soil thermal resistivity, ground temperature and the applicable buried-cable table |
Manufacturer calculators can accelerate a lookup but do not replace the governing code or an installation-specific design. Southwire states that its voltage-drop calculator is designed for NEC applications and does not optimize multiple loads connected at different points along a circuit.
The six-step wire-sizing workflow
1. Establish design current and circuit configuration
Determine the expected load current from the equipment rating or a documented load calculation. Record whether the circuit is DC, single-phase or three-phase, and identify which conductors carry current. Include continuous-load, motor-starting or other demand provisions required by the applicable rules.
2. Record every physical input
Write down source voltage, one-way length, conductor material, insulation rating, installation method, ambient temperature, number of grouped circuits, thermal insulation and any burial details. Missing one of these can invalidate an otherwise careful calculation.
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3. Find the tabulated ampacity
Use the current local code table or the cable manufacturer’s data for the exact installation method. Ampacity is conditional: heat must leave the conductor, and the permitted conductor temperature depends on construction, insulation and surrounding materials.
4. Apply correction and adjustment factors
Apply the factors required for ambient temperature, grouping, thermal insulation, burial and other conditions. Do not apply a factor from a different code or application merely because the number looks convenient.
For example, an IET marine article states that, when a single cable is totally surrounded by thermal insulation for more than 0.5 m and no more precise information is available, its current-carrying capacity may be taken as 0.5 times the ventilated-condition value. That is marine guidance, not a universal shortcut for building wiring.
5. Coordinate the protective device
The protective device must protect the conductors under the governing rules. James Eade’s November 2024 IET Wiring Matters article reproduces this BS 7671 overload-protection condition: “The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions: (ii) the rated current or current setting of the protective device (In) does not exceed the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit.” The article also discusses exceptions and explains why overload checks do not eliminate separate fault-protection requirements.
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6. Check voltage drop and installation constraints
Calculate voltage drop with the circuit-specific resistance and length method required by the applicable rules. If the result exceeds the design limit or the equipment’s operating requirement, increase the conductor size even when ampacity is adequate. Finally verify terminals, lugs, enclosure space, mechanical protection, bending radius and the manufacturer’s instructions.
How to calculate voltage drop
Voltage drop rises with current, conductor resistance and circuit length. A longer run or a higher-current load often needs a larger conductor to keep the equipment voltage within its permitted range.
For the marine example published by the IET, the stated calculation is:
Voltage drop (mV) = resistance per metre (mΩ/m) × design current (A) × run length (m)
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Suppose that marine calculation uses 3 mΩ/m, 10 A and an 8 m run: 3 × 10 × 8 = 240 mV, or 0.24 V. On a 12 V system that is 2% of nominal voltage; on a 24 V system it is 1%. The example demonstrates the arithmetic only. Use the resistance data, circuit path convention and voltage-drop limit specified for the actual marine system or local wiring code. Terminal voltage is determined by source voltage under load minus the total circuit drop.
Special installation conditions that change ampacity
Ambient temperature and grouping
Higher ambient temperatures reduce the heat available for dissipation. Cables installed together also heat one another, so the governing table normally requires adjustment for the number and arrangement of loaded circuits.
Thermal insulation
Insulation around a cable restricts heat flow. Use the correction method for the actual code and installation; do not assume that a cable rated in free air has the same capacity inside insulation.
Buried cables
Buried-cable ratings depend on soil and installation assumptions. An IET comparison cites reference assumptions of 20°C ground temperature, 2.5 K·m/W soil thermal resistivity and 0.7 m depth in BS 7671, versus 15°C, 1.2 K·m/W and 0.5 m in ERA 69-30. These are the compared reference assumptions in that article, not universal field conditions. Where site data are unavailable, use the appropriate current BS 7671 tabulated values and correction factors rather than treating either set as a measurement of the site.
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Marine and cyclic loads
Marine cable routes, enclosed spaces and cyclic loading can require application-specific data. The IET identifies IEC 60287 for steady-state cable-rating calculations and IEC 60853 for cyclic cable ratings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why ampacity alone is not enough
An IET cable-selection explainer, based on BS 7671:2008, lists electric-shock protection, thermal effects, overcurrent protection, voltage drop and terminal temperature among the selection concerns. Its historical tables should not be used as current ratings, but the selection principle remains useful: a conductor can be large enough for current and still fail a voltage-drop, termination or protection check.
- Undersized for voltage drop: lights may dim, motors may struggle to start and electronic equipment may fall outside its operating range.
- Overheated installation: grouping or insulation can reduce ampacity below the value printed for an open installation.
- Protection mismatch: an oversized breaker can leave a smaller conductor inadequately protected.
- Terminal incompatibility: the conductor may require a different termination temperature rating or may not fit the equipment connection.
- Fault conditions: overload protection does not by itself prove that fault current will be cleared safely.
A practical worksheet
- Jurisdiction and code edition: record the country, adopted code and application.
- Load: record equipment rating, design current and any continuous or starting-load assumptions.
- Circuit: record voltage, phase, DC/AC type and number of loaded conductors.
- Route: record one-way length, conductor path, installation method, ambient temperature, grouping, insulation and burial data.
- Conductor: choose material, insulation and candidate sizes from the applicable table.
- Ampacity: apply every required correction or adjustment factor.
- Protection: verify breaker or fuse rating, characteristics and fault-protection requirements.
- Voltage drop: calculate with the method and limit required for the application; increase size if necessary.
- Hardware: check terminals, lugs, enclosure fill, mechanical protection and manufacturer instructions.
- Verification: have a qualified person review the design and installation where local rules require it.
Common wire-sizing mistakes
- Using an online chart without confirming its country, code, conductor material or installation assumptions.
- Confusing one-way cable length with the complete circuit path used by the voltage-drop method.
- Applying a buried-cable, marine or insulation factor to an unrelated installation.
- Choosing a breaker first and then selecting a conductor to match it without checking ampacity and fault protection.
- Treating nominal source voltage as the voltage available at the equipment under load.
- Relying on a historic BS 7671 table or an unqualified internet calculator as current compliance evidence.
When to stop calculating and obtain design verification
Use qualified electrical design or inspection support for service equipment, high fault-current systems, large motors, generators, parallel conductors, buried routes with unknown soil conditions, hazardous locations, complex multi-load feeders and any work controlled by licensing or inspection rules. The final design must use the code adopted at the project location, not a mixed set of tables from different jurisdictions.
Frequently Asked Questions
Can I choose wire size from amps alone?
No. Current is only the starting point. Voltage, phase, length, conductor and insulation, installation conditions, voltage-drop limit and protective-device coordination are also required.
Are AWG and mm² interchangeable?
They are different sizing conventions. Convert only with an accepted conversion and then verify the resulting conductor against the local code table; do not substitute a nominally similar size without checking ampacity and termination rules.
Should I use a manufacturer voltage-drop calculator?
It can be a useful supporting check when its stated scope matches the project. Southwire’s calculator is for NEC applications and AWG/KCMIL sizes, and Southwire notes that it does not optimize multiple loads at different points on a circuit.
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