In what order do I apply derating factors?

Start with the capacity at the conductor's own temperature rating, multiply by the ambient correction, then by the grouping factor. That gives the adjusted capacity. Then apply the termination limit, which caps the result instead of scaling it. The continuous-load uplift goes on the load side, not the conductor side, so it never multiplies into this chain.

Why does my conductor fail when the terminations are rated lower?

You may derate from a hotter insulation column, but you can't use more current than the terminations allow. A conductor derated from the 90 degree column can show a good margin and still be limited by 75 degree terminations at each end. The termination limit wins, and it's the step most often missed.

Does this page know the ampacity of 10 AWG?

No. Current-carrying capacity tables are in national wiring rules and they're copyrighted. You read the one figure from your own copy, and everything after it is calculated here. In the app, a code pack fills that figure in for you.

What happens if I leave a field blank?

That check is reported as not made, and the result is marked optimistic. Nothing is assumed for you. A blank grouping factor isn't treated as 1.0 without a word: you get a line saying no grouping adjustment was applied, and that the answer is too high if the conductors are bundled.

Can a conductor pass every thermal check and still be wrong?

Yes, often on long runs. Voltage drop depends on resistance and length, not heat, so a cable with plenty of capacity can still deliver too low a voltage at the far end. That's why the drop check is part of the same chain.

Wire size check

Sizing a wire looks like one table lookup, but it’s eight steps, and only one of them needs a number from your wiring rules. Here they are in order, worked through on one circuit. The app runs the same steps on yours.

Worked example: one circuit, start to finish

System: single phase, 230 V
Conductor: 10 mm² copper, 70 °C
Load: 32 A, power factor 0.9
Run: 35 m one way
Capacity from the book: 50 A
Ambient: 40 °C, quoted at 30 °C
Grouping: 0.80
Terminations: 45 A
Device: 40 A
Drop limit: 3 %

Every figure here is made up for the example, including the 50 A. Real capacities come from your own copy of the wiring rules. This site doesn’t reproduce them.

Usable capacity

34.6 A

Design current

32.0 A

Voltage drop

4.24 V, 1.84 %

The cable carries the load with 2.6 A to spare, but a 40 A breaker is too big for it, and one check couldn’t be made. A table lookup alone shows neither problem.

ok
Adjustment for grouped conductors
Applied a factor of 0.800 as supplied.
ok
Termination temperature limit
The conductor is the binding limit at 34.6 A; the terminations allow 45.0 A.
not checked
Continuous load uplift
The load was treated as non-continuous, because no uplift factor was supplied. A load that runs for three hours or more is generally treated as continuous and carries an uplift from your rules.
ok
Conductor carries the design current
34.6 A usable against 32.0 A required, a margin of +2.6 A.
no
Protective device
A 40 A device does not protect a conductor good for only 34.6 A.
ok
Voltage drop
4.24 V, 1.84 % against a limit of 3.00 %.

The calculation sheet, as the app writes it

FISHTAPE · WIRE SIZE CHECK worked example, every figure invented 1. Capacity from your rules formula: read from your own copy, not from this engine size = 10 mm2 conductor temperature rating = 70 degC quoted at ambient = 30 degC result: 50.00 A 2. Ambient correction formula: k = sqrt((Tc - Ta_actual) / (Tc - Ta_quoted)) conductor rating Tc = 70.0 degC ambient of the quoted rating = 30.0 degC ambient at the installation = 40.0 degC result: 0.8660 3. Adjusted capacity formula: I_adjusted = I_base * k_ambient * k_grouping base = 50.00 A ambient factor = 0.8660 grouping factor = 0.8000 result: 34.64 A 4. Termination limit formula: I_usable = min(I_adjusted, I_termination) adjusted = 34.64 A at termination rating = 45.00 A result: 34.64 A 5. Design current formula: I_design = I_load * k_continuous load current = 32.00 A continuous factor = 1.0000 result: 32.00 A 6. Resistivity at the conductor temperature formula: rho(T) = rho20 * (1 + alpha * (T - 20)) material = copper rho20 = 1.724100e-8 ohm.m alpha = 0.00393 /K T = 70.0000 degC result: 2.062886e-8 ohm.m 7. Resistance per metre formula: R' = rho(T) / A * k_stranding size = 10 mm2 A = 10.0000 mm2 k_stranding = 1.020 result: 0.002104 ohm/m 8. Effective impedance per metre formula: Z' = R' * cos(phi) + X' * sin(phi) R' = 0.002104 ohm/m X' = 0.000000 ohm/m cos(phi) = 0.9000 sin(phi) = 0.4359 result: 0.001894 ohm/m 9. Voltage drop formula: Vdrop = k * I * L * Z' system = single phase two-wire k = 2.00000 (k = 2 (out and return)) I = 32.0000 A L = 35.0000 m Z' = 0.001894 ohm/m result: 4.2420 V 10. Voltage drop as a share of nominal formula: drop% = Vdrop / Vnominal * 100 Vdrop = 4.2420 V Vnominal = 230.0000 V result: 1.844 %

The lookup is the easy bit

Everyone finds the right row in the table. The mistakes come after it: derates applied in the wrong order, forgetting that lower-rated terminations cap the result, missing the uplift on a continuous load, or picking a breaker that protects the cable but trips on the load. Those are all rules, not table values, and rules can be calculated.

The order matters

Capacity at the conductor’s own temperature rating, from your code book.
Ambient correction, worked out here as the square root of the ratio of the temperature headrooms. The correction tables in the codes are this same formula in five-degree steps, rounded, so using your actual ambient is more accurate.
Grouping, if more current-carrying conductors share the raceway than the table assumes. The factors come in steps and can’t be derived, so you enter them.
The termination limit. You can derate from a hotter column, but you can’t then draw more than the terminations are rated for. This is the step people skip most, and skipping it gives a cable that only passes on paper.
The continuous-load uplift, applied to the load, not to the cable.
The comparison: what the cable can carry against what the load needs.
The breaker window: at least the design current, at most what the cable can carry.
Voltage drop, which doesn’t depend on any of the steps above and often decides the size on a long run.

A skipped check isn’t a pass

In the example above no continuous-load factor was given, and the result says so. It doesn’t just treat the load as non-continuous and carry on. The same goes for grouping, the terminations, the breaker and the drop limit. Calculators that fill in a default for something you didn’t enter are how wrong answers end up in front of an inspector.

Where the one number comes from

This site won’t tell you the capacity of a conductor. That figure is in your national wiring rules, it’s copyrighted, and it isn’t reproduced anywhere here. In the app, with a code pack, you pick your code and edition and the number comes from the pack, with the clause printed on the sheet. Without a pack, the app tells you which table and column to look in, and you type the figure from your own book. The other seven steps are the same either way.

Run it on your own circuits

In the app all of this is one screen, in your code, edition and units, offline, with every circuit saved to its job. The free calculators here each do one piece of it: voltage drop, the smallest size for a drop limit, conduit fill and Ohm’s law. See the app.

More free calculators

Voltage drop calculator →

Single phase, three phase and DC, in mm², AWG or kcmil. Shows every step and gives you a sheet to print.

Wire size by voltage drop →

The smallest size that keeps the drop under your limit, plus a list of what it doesn’t check.

Ohm’s law and power →

Volts, amps or ohms from the other two, plus real power for single and three phase.

Wire gauge chart →

AWG and kcmil sizes with diameter, mm², circular mils and resistance, worked out from the gauge formula.

Conduit fill →

How full your conduit is, for any conduit and any cable, in mm or inches, against the limit from your rules.

The app

Fishtape does the rest

Current capacity with derating, for your code and edition. Short circuit, protective conductor, transformer fault current and conduit fill. Sheets saved by job, and all of it works offline. Leave your email and we’ll send the link.

€1.99 a month, or €29.99 once, for everything. Free for 7 days.

Coming toiPhone and iPad Coming toAndroid

Cable size screen: one size that passes every check, with the binding constraint named

Get it on launch day

We’ll send one email when it’s out. Tell us which code you work to and that pack moves up the list.