AC wire size calculator (installation chain)
Checks not one cable but the whole chain: service entrance → meter → subpanel → load. In Germany three limits interlock — 0.5%, 3% and 4% — and the calculator shows which link breaks. With real cable reactance from catalogue data instead of a DC-only calculation.
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.
The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Operating current | P/(sqrt(3)*U*cos(phi)) | 15.877 A | exact |
| Resistivity at operating temperature | rho_20*(1+alpha*(T_iso-20)) | 0.020629 Ohm*mm^2/m | exact |
| Voltage drop of the whole chain | Summe DU_i / U_nenn | 0.91122 % | exact |
| Power loss of the chain | sqrt(3)*DU*I*cos(phi) | 100.23 W | exact |
- Formula
ΔU = k·I·L·(R′·cos φ + X′·sin φ), k = 2 (single-phase) or √3 (three-phase) · R′ = ρ(T_ins)/A · X′ = 2πf·L′ from the cable catalogue- Valid for
- Three-link chain with the German partial limits (0.5 % TAB to meter, 3 % meter-to-load per DIN 18015-1, 4 % overall per DIN VDE 0100-520) or IEC profiles; ΔU = k·I·L·(R′·cos φ + X′·sin φ) with catalogue reactances (NYY-Cu, 2026-08-03; missing cross-sections use a named 0.08 Ω/km fallback). Reference: 11 kW three-phase over 16/10/4 mm² → 0.91 % total, all links within limits. The reactance share is reported once notable.
- Not covered
- Ampacity and installation method (calculator 18), selectivity and protective devices (calculator 17), harmonics, phase imbalance, voltage rise from feed-in as its own limiting case (same maths, different limit — explained in the FAQ).
Frequently asked questions
Why does the calculator check a chain instead of a single cable?
Because in Germany three limits interlock: at most 0.5% from the service box to the meter (TAB/DIN 18015-1), at most 3% from the meter to the load (DIN 18015-1) and at most 4% for the whole run (DIN VDE 0100-520). One cable can meet its own section and still break the sum — so the calculator checks each link individually plus the chain as a whole, and marks where it breaks.
What distinguishes the AC calculation from the DC one?
Two things: power factor and reactance. The voltage drop follows ΔU = k·I·L·(R′·cos φ + X′·sin φ) — at cos φ = 1 the reactance term vanishes, at motor loads with cos φ 0.8 it contributes noticeably. The reactance per km is manufacturer-specific and comes from real catalogue data here (NYY, 0.23–0.34 mH/km depending on cross-section) instead of a blanket value. Three-phase also computes with √3 instead of 2 against 400 V instead of 230 V — which is why three-phase drops less.
Where does the reactance come from — and what about exotic cross-sections?
From the versioned cable-catalogue dataset (manufacturer datasheet, NYY per VDE 0276-603, with source and retrieval date): resistance and inductance per standard cross-section from 1.5 to 300 mm², from which X′ = 2πf·L′. If an entered cross-section is not in the catalogue, the calculator continues with the typical 0.08 Ω/km and says so explicitly via a warning — standard tables are deliberately not reproduced.
Why does the calculator use 70 °C conductor temperature?
Because the voltage-drop limit must hold in the worst permissible operating state: at the maximum operating temperature of PVC insulation (70 °C per IEC 60364-5-52) copper has about 20% higher resistivity than at 20 °C. Calculating with room temperature flatters the chain. With XLPE insulation (90 °C) the surcharge grows further — switchable in the “insulation” field.
Is 4 mm² enough for an 11 kW wallbox?
In the example yes, with margin: 15.9 A per phase, 20 m on 4 mm² yield 0.71% in the last link; the DIN meter→load section sits at 0.82% of the allowed 3%, the whole chain at 0.91% of 4%. But voltage drop is only one of two checks: ampacity by installation method (calculator 18) can demand a larger cross-section under grouping or thermal insulation — the stricter of the two governs. And longer runs tip it quickly: 60 m instead of 20 m triple the last link.
Does the calculator apply to circuits with intermediate branches?
Only with limits: the chain here carries the full load current to the end — the design case for a single final circuit such as a wallbox or an instantaneous water heater. If the meter or subpanel feeds further circuits, the upper chain carries their sum current and the real drop there is higher than calculated here. Standards-compliant whole-building planning remains the electrician’s job.
Why three limits instead of one?
Because in Germany three rules interlock: 0.5 % from the service connection box to the meter (TAB/DIN 18015-1), 3 % from meter to load (DIN 18015-1), and 4 % overall (DIN VDE 0100-520). A chain can pass every individual run and still fail in total — or vice versa. The calculator therefore checks each link separately and names the one that breaks the chain.
Where does the cable reactance come from?
From a versioned manufacturer catalogue dataset (NYY copper, sourced 2026-08-03) — not copied from the standard, which names no per-length values. If a cross-section is missing from the catalogue, the page computes with a named fallback of 0.08 Ω/km and says so as a warning. At cos φ = 1 reactance plays no role; with motor loads its share grows, and the page reports it once it becomes notable.
What does the power factor change in practice?
Little in the 11 kW reference chain — 0.92 instead of 0.91 % total drop at cos φ 0.9 — because the cross-sections are generous and resistance dominates. But the formula ΔU = k·I·L·(R′·cos φ + X′·sin φ) shows when it tips: long runs, large cross-sections (small R′) and inductive loads. Exactly then a pure resistance calculation understates the drop.
Does the chain also apply to PV feed-in?
Yes, in reverse — and there it often matters more strictly: the voltage drop becomes a voltage RISE at the inverter, and inverters trip on overvoltage (253 V). Same cables, same maths, but the symptom is a curtailing inverter instead of a dim lamp. The 3 % recommendation for the feed-in line has its reason here.
Why are 230 and 400 volts adjustable?
For grids outside the DACH standard — 120/208 V, for instance. The limit profiles remain selectable: the German chain profile or the IEC recommendations. In the normal German case you leave both voltages untouched; they sit deliberately in the advanced assumptions.