What this calculator does
A cable’s current rating is a heat-transfer statement dressed up as an ampere figure. The number in the table is the current at which the conductor reaches its insulation’s temperature limit under the specific conditions the table assumed — a nominated ambient, a nominated installation method, one circuit on its own, clear of anything that traps heat. Change any of those conditions and the real capacity changes with it.
This calculator applies the derating chain. You supply the base (tabulated) rating for your cable from the standard you work to, and a correction factor for each condition that differs from the table’s reference. It returns the derated capacity the cable actually has as installed, how much of it your load uses, and — the question people usually mean to ask — the minimum tabulated rating you must select to carry a given design current under those conditions.
It deliberately contains no ampacity tables. Those belong to the standard in force where the installation is built, and reproducing them here would be both a copyright problem and an engineering one, since a table divorced from its installation-method definitions and its reference conditions is worse than useless.
The formula
Each correction factor describes a separate obstruction to getting heat out of the conductor, so they multiply:
I_z = I_t × Ca × Cg × Ci × Cs × Ch
I_z— installed (derated) current-carrying capacity, AI_t— base rating tabulated in your standard for this conductor size, insulation and installation methodCa— ambient temperature correction: air or soil temperature differing from the table’s reference, typically 30 °C in air and 20 °C in groundCg— grouping / bundling correction: other loaded circuits sharing the enclosure, tray, duct bank or trenchCi— thermal insulation correction: cable in or against building thermal insulationCs— burial correction: depth of laying and soil thermal resistivity differing from the reference (commonly 2.5 K·m/W)Ch— harmonic / neutral loading correction: triplen harmonic current loading the neutral of a three-phase circuit
The design condition is then the simplest inequality in cable sizing:
I_b ≤ I_n ≤ I_z
where I_b is the design current and I_n is the protective device rating. Invert the chain and you get the sizing question directly:
I_t,required = I_b / (Ca × Cg × Ci × Cs × Ch)
That is the number to take back to the ampacity table. It is not the cable you install — it is the smallest tabulated rating whose row you may choose from.
This is the method common to IEC 60364-5-52 (Annex B tables, Annexes A and B correction factors), NEC 310.15 (ambient correction and conductor-count adjustment, applied to the 310.16 tables), AS/NZS 3008.1, and BS 7671 Appendix 4. The arithmetic is the same everywhere; the numbers are not. Read every factor from the standard in force for your installation. This tool applies the method; it does not assert compliance with anything.
Reading the result
Total derating factor tells you how much of the cable you paid for you are actually allowed to use. Chains between 0.7 and 0.9 are ordinary. Below 0.5 you are discarding more than half the cable’s capacity, and it is almost always cheaper to fix the installation — space the group out, re-route clear of the insulation, split the bundle across two trays — than to buy the copper that compensates for it.
Derated capacity against the design current is the pass/fail. If I_z < I_b, the cable is undersized as installed, no matter what the table said.
Utilisation is the number worth reporting in a design review. Above 80 % the cable passes but has almost nothing left: one added circuit in the group, one hotter summer than the ambient you assumed, or one load increase and it no longer does. Below about 50 % you were probably driven by voltage drop or fault-loop impedance rather than thermal capacity — normal on long runs, and worth stating so nobody value-engineers the cable back down later.
Required minimum base rating is what you carry back to the table. Note the asymmetry that catches people out: a 0.752 chain does not mean “add 25 %”. It means divide by 0.752, which is a 33 % increase.
Typical values
Ranges you will see in practice, offered as a sanity check on what you type, never as a substitute for looking them up:
- Ca (ambient) — typically 0.87 to 1.0 for air ambients between 30 and 40 °C on 70 °C PVC; higher-temperature insulation tolerates hot ambients better, so 90 °C XLPE derates less for the same ambient. Values above 1.0 are legitimate for ambients below the table’s reference.
- Cg (grouping) — the big one. Two circuits touching in an enclosure is around 0.80; four circuits around 0.65; nine or more can reach 0.40–0.45. Spacing the cables by one diameter or more improves this dramatically, which is why tray layout is worth arguing about.
- Ci (thermal insulation) — 1.0 clear of insulation; roughly 0.88 for 50 mm of contact, 0.78 for 100 mm, and around 0.5 for a cable fully surrounded over more than about 0.5 m.
- Cs (burial / soil) — roughly 0.90 to 1.05 for depth variations and 0.75 to 1.18 for soil thermal resistivities away from the reference. Dry sandy soil is far worse than the assumed value, and this factor regularly ruins a duct-bank design.
- Ch (harmonics) — 1.0 for linear loads; third-harmonic content between 15 % and 33 % typically gives around 0.86. Above 33 % the neutral becomes the governing conductor and the factor is applied to the neutral current instead — check how your standard frames it, because the convention differs.
A factor of exactly 1.0 is the correct way to say “this condition does not apply” — a cable in free air has Cs = 1, a single-phase linear load has Ch = 1. Entering 0 says “this cable has no capacity”, which is why the tool rejects it.
Worked example
A 63 A tabulated cable runs in a tray with one other loaded circuit, in a plant room where the ambient reaches 35 °C. The cable is clear of thermal insulation, run in air rather than buried, and feeds a linear load — so Ci, Cs and Ch are all 1.0. From the tables, the ambient correction is 0.94 and the grouping correction is 0.80. The design current is 40 A.
The chain:
C_total = 0.94 × 0.80 × 1 × 1 × 1 = 0.752
The capacity the cable actually has where it is installed:
I_z = 63 × 0.752 = 47.38 A
That clears the 40 A design current, so the circuit passes — but only just. The spare capacity is 47.376 − 40 = 7.38 A, which is 18.4 % more than the present load, and the utilisation is 40 / 47.376 × 100 = 84.4 % of the derated capacity. The tool flags that: a third circuit added to this tray later would push the grouping factor toward 0.70, drop I_z to about 41.5 A, and leave essentially nothing.
Working the other way, the minimum tabulated rating for this load under these conditions is:
I_t,required = 40 / 0.752 = 53.19 A
So any cable whose table entry is 53.2 A or more will do thermally. The 63 A cable satisfies it — and note that a 40 A load needing a 53.2 A table entry is exactly the 33 % penalty the 0.752 chain imposes.
FAQ
Do I use the design current or the breaker rating?
Both, in sequence. Size the cable so I_b ≤ I_n ≤ I_z: the device must be at least the design current, and the derated capacity must be at least the device rating. Enter the design current here, then check the device you selected also fits under I_z. For a device that is not overload-protective — a motor circuit protected against short circuit only — the rule differs, and your standard will say how.
Should I apply the grouping factor if the other circuits are lightly loaded? Most standards let you relax it when the group is not fully loaded, and several give an explicit formula for that case. Applying the full factor is the defensible default unless you can guarantee the loading of circuits you do not control.
Why is the required base rating so much higher than the load? Because you divide by the chain rather than adding a margin. A 0.75 chain means a 33 % increase; a 0.60 chain a 67 % increase; a 0.50 chain means doubling. This is the reason grouping is worth designing out rather than correcting for.
Does this replace the ampacity tables? No. The base rating must come from the table for your cable construction, conductor material, insulation temperature and installation reference method. This tool does the multiplication and the inversion around it, and keeps every assumption visible as an input you typed.
What about voltage drop? A separate check, and on runs over roughly 30–40 m it usually governs instead of thermal capacity. Size for ampacity here, confirm the drop with the voltage drop calculator, and whichever gives the larger conductor wins.
This tool applies a published derating method to figures you supply. It contains no ampacity tables and makes no claim of compliance. The base rating, every correction factor and the final cable selection must be verified against the wiring rules applicable to your installation.