Technical Hub

Engineering calculators

Screening tools with the formula, assumptions and limitations shown for every result. Results must be verified by a qualified engineer before use.

Screening tool

Transformer full-load current

Rated current on each winding of a three-phase transformer.

I = S / (√3 × V)

Safety-critical · verify

Transformer secondary fault current

Three-phase symmetrical fault current at transformer secondary terminals, with optional source impedance.

Isc = V / (√3 × (Zt + Zs)), Zt = Z% × V² / S, Zs = V² / Ssource

Safety-critical · verify

Cable current & voltage drop

Three-phase voltage drop using cable resistance and reactance.

ΔV = √3 × I × L × (R cos φ + X sin φ)

Safety-critical · verify

Earthing conductor sizing (adiabatic)

Minimum conductor cross-section to withstand an earth fault for a given duration.

S = I × √t / k

Safety-critical · verify

CT knee-point check

Required knee-point voltage of a protection CT to avoid saturation for maximum through-fault.

Vk ≥ K × (If / N) × (Rct + 2Rlead + Rrelay)

Screening tool

Power factor

Apparent power, power factor and current from active and reactive power.

S = √(P² + Q²), PF = P / S

Screening tool

Capacitor bank sizing

Reactive compensation needed to correct to a target power factor.

Qc = P × (tan φ1 − tan φ2)

Screening tool

Harmonic distortion (THD) check

Voltage THD from individual harmonic levels, compared with a planning limit.

THD = √(Σ Vh²) / V1 × 100

Screening tool

Battery energy & runtime

Usable energy and runtime of a battery system at a constant load.

Runtime = Ah × V × DoD × η / Pload

Screening tool

BESS C-rate & duration

Charge/discharge rate and duration of a battery energy storage system.

C-rate = P / E, duration = E / P

Screening tool

Transformer sizing

Indicative transformer rating from maximum demand, growth and loading margin.

S = P / PF × (1 + growth) / loading

Screening tool

MV cable charging current

Capacitive charging current of a screened MV/HV cable.

Ic = 2π × f × C × (V / √3) × L

Safety-critical · verify

Arc-flash screening (Lee method)

Conservative open-air incident energy estimate for initial screening only.

E (J/cm²) = 2.142 × 10⁶ × V × Ibf × t / D² (V in kV, Ibf in kA, t in s, D in mm)

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