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+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.