Calculate ampacity with derating
Shows the full derating waterfall instead of a single figure: from the table value through temperature and grouping correction down to the permissible continuous load. The factor between them is often close to two — and the most common cause of overheated installations.
- Thermally not permissible: 22 A required, 16.43 A available. Use a larger conductor, a different installation method, or unbundle the circuits.
AMPACITY_INSUFFICIENT
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
Safety-relevant calculation. This is an estimate based on the stated assumptions. The final design must be checked by a qualified electrician against the rules that apply where you are.
Data as of: 2026-07-15 · The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Base ampacity | I_table(code, method, insulation, conductors) | 32 A | measured |
| Permissible continuous load | I_table * f_temp * f_group * f_soil | 16.432 A | exact |
| Required base ampacity | I_B / (f_temp * f_group * f_soil) | 42.843 A | exact |
- Formula
I_z = I_table · f_temp · f_group · f_soil · check I_z ≥ I_B- Valid for
- Derating waterfall I_z,real = I_z,table × f_temp × f_grouping × f_soil; all table values are USER INPUTS from the named code (deliberately no embedded table — copyright concept), the installation-method code labels the output. Reference: 32 A (B2) × 0.79 × 0.65 → 16.4 A against 22 A operating current (134 %). The three remedies are computed rather than asserted: unbundling 25.3 A (sufficient), next larger size 21.6 A (not sufficient), other method named. NO SUBSTITUTE FOR PROFESSIONAL DESIGN.
- Not covered
- The code tables themselves (user input — one value per configuration), special installation methods and grouping geometries beyond the standard factors, cyclic loading, harmonic allowances, short-circuit withstand.
- Data sources
- Official publications of the standards bodies and state authorities (NFPA, IEC, DKE/VDE, CEN) · retrieved 2026-07-15
Frequently asked questions
How many amps can my cable really carry continuously?
The permissible continuous load is I_z = I_table · f_temp · f_grouping · f_soil, checked against I_z ≥ I_B. The calculator shows the full derating waterfall: in the specification example, a table value of 32 A drops to 16.4 A after temperature correction (× 0.79) and grouping (× 0.65) — impermissible against a 22 A operating current. The factor between table value and real limit is often close to two, and it is the most common cause of overheated installations.
Why do I have to look up the base ampacity in the code myself?
Deliberately no ampacity table is stored: the calculator accepts exactly one configuration and returns exactly one value — it is not a reference work. Base ampacity and correction factors are user inputs from the applicable code, and every result names the code and installation method so that you can — and must — verify the value yourself.
What are my options when the ampacity check fails?
The calculator works through three remedies with their effect: a larger conductor, a different installation method, and unbundling the circuits — each with the resulting permissible continuous load. Mind the limits: short-circuit withstand, harmonics, voltage drop and time-varying loads are not covered, and the calculator does not replace professional design.
Why can I not simply use the value printed on the cable jacket?
Because the print describes an ideal case: a single cable, free in air, at 30 °C. In reality it lies bundled in a duct in a hot attic — and every deviation derates capacity multiplicatively. The waterfall shows each derating step individually: at 45 °C with four bundled cables, a 40 A table value quickly leaves only 26 A.
What do I do when the operating current exceeds the derated capacity?
Three ways out, checked in this order: unbundle (the grouping factor disappears), choose a better installation method (free in air instead of ducted raises the table value), or increase the cross-section. The “required table value” tile answers the reverse question: which base capacity you need to buy given your factors.
How do ampacity and fuse relate?
The fuse must sit BELOW the derated capacity — not below the table value. Fusing to the 40 A table value while the cable carries only 26 A derated gives you a cable that overheats permanently at 35 A without the fuse ever tripping. The fuse calculators (16/17) therefore take the derated capacity as the upper limit.
Why is no ampacity table embedded here?
Copyright discipline — and method. The code tables are protected works; this calculator neither prints nor queries them. You fetch the one table value for your configuration from the code, and the calculator delivers what no table reprint can: the combination logic of the correction factors. Every result names code and installation method so you can — and must — verify the base value yourself.
How large is the gap between table value and real limit?
A factor of two in the reference case: 32 A per table becomes 16.4 A of permissible continuous load after temperature factor 0.79 and grouping factor 0.65 — 134 % utilisation at 22 A operating current. Practically all DIY builders stop at the table value; exactly this factor is the most common cause of overheated installations. The example fails on purpose so the warning side is visible.
How do the three remedies compare?
The calculator computes them instead of asserting them — and the ranking often surprises: in the reference case the next larger cross-section is NOT enough (42 A table → 21.6 A after derating), while unbundling is (25.3 A), because it attacks the harshest factor directly. The thicker conductor keeps the bad factors; the better installation removes them. Compute first, buy second.
Where do I get the I_z of the next larger size?
From the same code table as the base value — one row down. The switch in the advanced assumptions accepts it; without it the “larger conductor” remedy stays uncomputed rather than running on an invented table value. Same discipline as the base value: table values come from the user, never from the calculator.
Does the soil factor apply for installation in air?
No — it describes the soil's thermal conductivity for buried runs (D1/D2) and therefore stays at 1.0 as long as you install in air or conduit. For buried cable it depends on soil type and moisture and sits in the same table set. Three multiplicative factors — temperature, grouping, soil — the code knows no more for the standard case.