What the Electrical Cable Size Calculator does
This calculator sizes a circuit conductor for current-carrying capacity under either IEC or NEC assumptions. It works out the design current, applies the ambient-temperature and grouping factors, shows every factor it used and where that factor comes from, and then picks the smallest size whose tabulated capacity still carries the load after all of them.
The method is the one both bodies of rules use: the tabulated capacity a cable needs is the design current divided by the product of the derating factors. What differs is the tables, the units and how the conductors are counted, and the page keeps that difference visible rather than hiding it behind a single answer.
It is indicative. The reference tables behind it are the widely published headline values for the commonest cases only, every one of them is shown so you can check it, and no result here is a compliance certificate.
How to use it
- Pick IEC or NEC. That choice changes the size series (mm² against AWG), the correction factors and how grouped conductors are counted.
- Enter the load as kW with a voltage and power factor, or enter the design current directly if you already know it. A current takes precedence over a kW figure.
- Set the ambient temperature. It is the temperature around the cable, not the room's comfort temperature - a ceiling void, a plant room or a roof space runs much hotter than the office below it.
- Enter how many circuits or conductors share the route, and how they are arranged. Grouping is usually the harshest factor in the whole calculation.
- Under IEC choose the insulation and the number of loaded conductors; under NEC choose the terminal temperature column your equipment allows.
- Add any factor the tables do not cover - thermal insulation, buried cables, a figure from your own copy of the standard - in the extra factor field, and name it so it appears in the exported schedule.
- Read the recommended size, then check voltage drop and earth-fault loop impedance separately. Both regularly force a bigger cable than capacity alone.
Reading the results
Design current (Ib) is what the circuit actually draws. It is the starting point, not the answer.
The combined derating factor is the product of everything that reduces a cable's capacity. At 0.5 it means the cable can only be worked at half its tabulated rating, so a cable with twice the tabulated capacity is needed.
Required tabulated capacity is the figure to look up in the table: design current divided by the combined factor. It is not a current that ever flows anywhere; it is a table lookup value.
Spare capacity is how much headroom the chosen size has after derating. A few per cent is uncomfortable; 20% or more gives room for the load to grow and for an assumption to be slightly wrong.
Worked example: a 15 kW three-phase load, four circuits bunched in a 40 °C ceiling void
Design current is 15,000 / (root-three x 400 x 0.9) = 24.06 A.
The ambient factor for 70 °C PVC at 40 °C is 0.87, and the grouping factor for four bunched circuits is 0.65. Their product is 0.5655 - the cable can only be worked at 57% of its tabulated rating.
So the tabulated capacity needed is 24.06 / 0.5655 = 42.5 A. In the three-loaded-conductor PVC column, 6 mm² is tabulated at 41 A, which derates to 23.2 A and does not carry the load. 10 mm² is tabulated at 57 A, which derates to 32.2 A - comfortably enough, with 34% spare.
That is a striking result: a load of only 24 A needs 10 mm² rather than the 4 mm² the bare current would suggest, purely because of where the cable runs and what runs with it. Separating those four circuits onto a single-layer tray would change the grouping factor to 0.75, the combined factor to 0.6525, the required capacity to 36.9 A, and the answer to 6 mm².
Both answers still have to survive a voltage-drop check. Over a 60 m run, 10 mm² copper carrying 24 A three-phase drops about 4.3 V, or 1.1% - fine. Over 200 m it would drop 14.3 V, or 3.6%, and the cable would have to grow again.
Formulas and scoring rules
- Design current, three phase
Ib = kW x 1000 / (root-three x V x pf)- Design current, single phase
Ib = kW x 1000 / (V x pf)- Required tabulated capacity
It >= Ib / (Ca x Cg x Ci x ...)Ca ambient, Cg grouping, Ci anything else. The factors multiply.- Derated capacity of a size
Iz = It_tabulated x (product of the factors)Iz is the current that size may actually carry in these conditions.- The selection rule
Ib <= In <= IzDesign current, then the protective device rating, then the cable's derated capacity - in that order. The breaker calculator does the second half.- Aluminium scaling used here
roughly 0.78 x the copper figure of the same cross-sectionA rule of thumb for a first estimate; read the aluminium column of the standard for a design.
Why grouping hurts more than heat
Ambient correction usually costs 10 to 20%. Grouping routinely costs 30 to 50%, because cables bunched together each have to reject heat through their neighbours. Six circuits bunched in conduit sit at a factor of 0.57; nine at 0.50. That halving is applied before you choose a size, so it doubles the tabulated capacity you need.
It is also the factor most often forgotten, because it depends on what else ends up in the same containment after the calculation was done. Where a route is likely to fill up, it is worth sizing for the eventual number of circuits rather than the day-one number, or specifying separate containment.
The three checks a cable has to pass
Current-carrying capacity, which this page calculates, stops the cable overheating in normal service. Voltage drop, which the voltage drop calculator handles, stops the equipment at the far end seeing too little voltage. Earth-fault loop impedance decides whether the protective device will disconnect fast enough when a fault occurs, and it is the check most often skipped by people using an online calculator.
On short runs capacity usually wins; on long runs voltage drop almost always does; on TT systems and long submains the loop impedance check can force a larger protective conductor or a different protective device altogether. A cable that passes only one of the three is not sized.
NEC terminal temperature ratings
NEC 110.14(C) limits the ampacity you may use to the lowest temperature rating of any terminal, device or conductor in the circuit. In practice this means the 60 °C column for equipment rated 100 A or less, and the 75 °C column above that, even when the conductor's insulation is rated 90 °C.
The 90 °C column still earns its keep: it is the column you derate from for ambient and conductor count, and only the derated result has to sit within the terminal rating. That two-stage rule is why the NEC column selector on this page matters more than it looks.
Limitations: what the result does not prove
- The tables here are the headline values for the commonest installation cases only. The standards themselves are copyrighted documents with many more tables and conditions, and they are the authority - not this page.
- It checks current-carrying capacity alone. Voltage drop, earth-fault loop impedance, short-circuit withstand and mechanical protection are separate checks that routinely force a larger cable.
- Buried cables, cables in thermal insulation, cables in ducts and cables in direct sunlight all have their own methods and factors that are not modelled here. Use the extra factor field with a figure from the standard.
- The aluminium figures are scaled from the copper column by a rule of thumb, not read from an aluminium table.
- Nothing here is a compliance certificate. A qualified engineer must verify the design against the applicable standard and the local regulations before anything is installed.
Privacy: where your data goes
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Standards and sources
- IEC 60364-5-52 - Selection and erection of wiring systems - checked 19 Sep 2026
- IEC 60228 - Conductors of insulated cables - checked 19 Sep 2026
- NFPA 70 - National Electrical Code - checked 19 Sep 2026
- IEC 60364-4-43 - Protection against overcurrent
Frequently asked questions
What size cable do I need for a 15 kW three-phase load?
The bare current is 24 A at 400 V and 0.9 power factor, which 4 mm² would carry in ideal conditions. But in a 40 °C void with three other circuits bunched alongside it, the combined derating factor is 0.57 and the answer becomes 10 mm². The conditions matter as much as the load.
What is a derating factor?
A multiplier below one that reduces a cable's tabulated current-carrying capacity because the installation conditions are worse than the conditions the table assumes. The table assumes 30 °C ambient and one circuit on its own; a hotter ambient, more circuits grouped together, thermal insulation or burial all reduce what the cable can safely carry.
Can I convert mm² to AWG and use the other table?
You can convert the area - 10 mm² is close to 7 AWG, so 6 AWG is the next size up - but you cannot mix the tables. The IEC and NEC ampacity values for a given area differ because the assumed installation methods, insulation temperatures and safety approaches differ. Pick one body of rules and stay inside it.
Does the number of conductors include the neutral and the earth?
Under IEC the count is of loaded conductors: three in a balanced three-phase circuit, two in a single-phase one. A neutral carrying only unbalance is not counted, but one carrying significant third-harmonic current is. The protective conductor is never counted. NEC 310.15(E) takes a similar view of the neutral, and the earth is likewise excluded.
Why did the calculator not pick a bigger cable for voltage drop?
Because it does not calculate voltage drop - that is a separate check on a separate page. On runs longer than about 30 to 50 m at low voltage, voltage drop usually decides the size rather than capacity, so always do both.
Should I use PVC or XLPE?
XLPE (90 °C thermosetting) carries roughly 15 to 30% more current in the same cross-section because it tolerates a higher conductor temperature, so it can save a size. The catch is that the equipment terminals and any PVC-insulated tails in the circuit may not be rated for that temperature, and grouping factors bite the same way on both.
What ambient temperature should I assume?
The temperature where the cable actually runs, at the worst time of year. A ceiling void above a suspended ceiling in a warm climate can reach 45 to 55 °C; a plant room next to boilers, more; a shaded external route, less than the rooms it feeds. Assuming 30 °C because that is the table's reference is the commonest optimistic error.
Is this calculation enough to certify an installation?
No. It is a design aid that shows its working. Certification needs the full design - capacity, voltage drop, disconnection times, protective device coordination, earthing, containment and mechanical protection - carried out and signed by a qualified person against the standard that applies where the work is being done.
Last reviewed by the A2Z.Tools team against the sources listed above.
