Reference
Electrical reference
A pocket reference Paul keeps at hand: SI electrical units and multipliers, common circuit symbols, the essential formulas for DC, single- and three-phase AC, and a live unit converter for volts, amps, watts, ohms and energy. All rebuilt from first principles — no third-party content or branding.
Section 1
SI electrical units
| Quantity | Symbol | Unit | Unit symbol | Note |
|---|---|---|---|---|
| Voltage | V, U | volt | V | 1 V = 1 J/C — potential difference |
| Current | I | ampere | A | 1 A = 1 C/s — rate of charge flow |
| Resistance | R | ohm | Ω | 1 Ω = 1 V/A |
| Conductance | G | siemens | S | G = 1/R |
| Reactance | X | ohm | Ω | Opposition due to L or C at frequency ω |
| Impedance | Z | ohm | Ω | Z = R + jX (complex) |
| Power (real) | P | watt | W | 1 W = 1 J/s |
| Power (apparent) | S | volt-ampere | VA | S = V × I (rms) |
| Power (reactive) | Q | volt-ampere reactive | var | Stored in L/C, not consumed |
| Energy | E, W | joule / watt-hour | J / Wh | 1 Wh = 3 600 J |
| Charge | Q | coulomb | C | 1 C = 1 A × 1 s |
| Capacitance | C | farad | F | C = Q / V |
| Inductance | L | henry | H | V = L × dI/dt |
| Frequency | f | hertz | Hz | UK mains 50 Hz |
| Magnetic flux | Φ | weber | Wb | 1 Wb = 1 V·s |
| Flux density | B | tesla | T | 1 T = 1 Wb/m² |
SI prefixes
p · pico
×10-12
n · nano
×10-9
μ · micro
×10-6
m · milli
×10-3
— · (none)
×100
k · kilo
×103
M · mega
×106
G · giga
×109
Section 2
Interactive unit converter
Enter a value in any unit and multiplier — the result updates live in the target unit and multiplier of your choice.
Result
1.5 kW
Section 3
Formula cheat sheet
Ohm's Law & Power
- VoltageV = I × R
- CurrentI = V ÷ R
- ResistanceR = V ÷ I
- Power (DC / resistive AC)P = V × I = I² × R = V² ÷ R
AC single phase
- Apparent powerS = V × I
- Real powerP = V × I × cos φ
- Reactive powerQ = V × I × sin φ
- Power factorpf = cos φ = P ÷ S
AC three phase
- Line/phase voltageV_L = √3 × V_ph
- Apparent powerS = √3 × V_L × I_L
- Real powerP = √3 × V_L × I_L × cos φ
- Current from powerI = P ÷ (√3 × V_L × cos φ)
Resistance & impedance
- Series RR = R₁ + R₂ + …
- Parallel R1/R = 1/R₁ + 1/R₂ + …
- Inductive reactanceX_L = 2π × f × L
- Capacitive reactanceX_C = 1 ÷ (2π × f × C)
- Impedance (series R + X)Z = √(R² + X²)
Energy & efficiency
- EnergyE = P × t
- CostCost = kWh × unit rate
- Efficiencyη = P_out ÷ P_in × 100 %
Section 4
Common circuit symbols
Simplified representations — full BS EN 60617 symbols are used on formal drawings; these are a quick-look guide only.
- ─|├─Cell / battery
Long line = positive, short = negative.
- ─▬▬─Resistor
Fixed resistance.
- ─▬▬↗─Variable resistor
Rheostat / potentiometer.
- ─│├─Capacitor
Two parallel plates; polarised marked with +.
- ─◠◠◠─Inductor / coil
Coiled wire; iron core shown with two lines.
- ─/ ─Switch (SPST)
Single-pole single-throw isolator.
- ─▭─Fuse
Rectangle or wavy line — opens on overcurrent.
- ─◯╱─Circuit breaker
Automatic re-usable overcurrent protection.
- ⏚Earth / ground
Reference to true earth potential.
- ─⊗─Lamp
Filament or LED indicator.
- ─Ⓜ─Motor
Rotating machine; letter denotes type.
- ◠◠║◠◠Transformer
Two coils sharing a magnetic core.
- ─Ⓥ─Voltmeter
Measures voltage in parallel.
- ─Ⓐ─Ammeter
Measures current in series.
Disclaimer — no liability
These calculators are provided for guidance and educational use only. All results must be verified by a competent person against BS 7671:2018+A3:2024/A4:2026, manufacturer data and the specific site conditions before any design, installation or certification decision is made.
All Cabling© Limited accepts no liability whatsoever for any loss, damage, injury, cost or non-compliance arising directly or indirectly from use of, or reliance on, these tools. By using them you accept these terms in full.