Calculation Methodology

ElectroCalc Pro · version 3.5

1. Load current

Single-phase: I = P / (U × cosφ)
Three-phase: I = P / (√3 × U × cosφ)

cosφ accounts for the reactive component. For heaters ≈1, for motors 0.7–0.9.

📘 PUE 1.3.2

2. Cable cross-section selection

The cross-section is chosen so that the continuous allowable current, accounting for ALL factors, is not less than the breaker rating:

I_allow(table) × K_lay × K_temp × K_mat ≥ max(I_calc, I_brk)

Correction factors:

K_lay — installation method. In air 1.0, in conduit 0.82, bundled 0.68, buried 1.15
K_temp — ambient temperature per PUE 1.3.3. At +40°C drops by 18%, at -20°C rises by 18%
K_mat — for aluminium 0.78 relative to copper

Minimum cross-sections (PUE 7.1.34):

Lighting: 1.5 mm² (copper)
Sockets and general circuits: 2.5 mm² (copper)
Aluminium in residential: minimum 4 mm²

3. Voltage drop

1ph: ΔU = 2·I·ρ(t)·L / S
3ph: ΔU_line = √3·I·ρ(t)·L / S
ρ(t) = ρ₂₀·[1 + α·(t−20)]

Copper: ρ₂₀=0.018 Ω·mm²/m, α=0.00393
Aluminium: ρ₂₀=0.029, α=0.00403

📘 GOST 32144 / PUE 1.3.1 — not more than 5% of U_nom

4. Heat losses

Joule's law applied to cable resistance at operating temperature:

1ph: Q = I² × R_loop, R = ρ(t)·2L/S
3ph: Q = 3·I² × R_phase, R = ρ(t)·L/S

5. Short-circuit tripping check

The short-circuit current must be sufficient to reliably trip the magnetic release of the circuit breaker:

I_SC = U_phase × 0.95 / R_loop(70°C)
Condition: I_SC ≥ multiplier × I_brk
Multiplier by characteristic: B=5×, C=10×, D=20×

Resistance is calculated for a "hot" cable (70°C — PVC insulation operating temperature per GOST 31996). Calculation at 20°C underestimates real R and overstates SC current by ~20% — a typical calculator error.

📘 GOST R 50571.5.54 / IEC 60364-4-41
📘 IEC 60909 — factor 0.95 for minimum SC current

6. Illuminance

N = (E × S) / (Φ_lamp × η_util × K_m)
Room index: i = √S / (2·h)
η_util = 0.4 + i·0.08 (clamped 0.4–0.75)

📘 SP 52.13330 — illuminance standards

7. Grounding

R_single = (ρ/2πL) × [ln(2L/d) + ½·ln((4t+L)/(4t−L))]
η_group ≈ 0.92·exp(−0.054·(n−1))
R_group = R_single / (n × η)

📘 PUE 1.7 — allowable grounding resistances

8. Lightning protection

Angular method per GOST R IEC 62305:

R_prot(at height h) = h × tan(α)
Angles: I — 25°, II — 35°, III — 45°, IV — 55°

Authorship & feedback

The calculator is developed in accordance with applicable regulatory documents. All formulas are open — found an error? Write to us: @OmarZhilkibayev

ElectroCalc Pro Team · electrocalcpro.ru · last methodology update: 2026