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Electrical formulas: a quick-reference tableUseful calculations, in a minute.
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Every calculation from this masterclass, in the order a designer uses them.
| Calculation | Formula | Use it when… | Where |
|---|---|---|---|
| Design current (single-phase) | Ib = P ÷ V | You need the circuit's current from the load. V = the voltage present at the origin of the circuit (usually 230 V). | Power triangle |
| Design current (three-phase) | Ib = P ÷ (√3 × VL) | Three-phase load. VL = 400 V. Answer is per phase. | √3 = 1.73 |
| Tabulated current needed | It ≥ In ÷ (Ca × Cg × Ci …) | Sizing a cable. Only use the factors happening at the same time. | BS 7671 App 4 · Tables 4B1, 4C1, 2.6 |
| Cable size | table value ≥ It | Read down your cable's A table, in the column for your method. | BS 7671 Appendix 4 cable tables |
| Voltage drop | VD = mV/A/m × Ib × L ÷ 1000 | Checking there's enough "push" left. Limits 3% / 5%. | B tables · Table 4Ab |
| R1 + R2 (designer) | B1 × B3 × L ÷ 1000 | Designing for an earth fault (conductors at operating temperature). | GN3 Tables B1, B3 |
| R1 + R2 (ring) | (r1 + r2) ÷ 4 | Ring final circuits. r = single conductor end to end (the "dash" rows). | GN3 Table B1 (+ B3 or B2) |
| R1 + R2 (inspector) | B1 × B2 × L ÷ 1000 | Correcting a measured value to the ambient temperature on the day. Never in a design. | GN3 Table B2 p.175 |
| Earth fault loop | Zs = Ze + (R1 + R2) | Checking ADS. Ze by measurement, enquiry or calculation (or given). | Compare with max Zs (Ch 41) |
| Prospective earth fault current | PEFC = U₀ × Cmin ÷ Zs | How many amps flow in an earth fault. Cmin = 0.95 (218.5 V). | Ohm's law, worst case |
| Ia / disconnection time | read from the curves | How fast the device operates at that PEFC (more amps = faster). | BS 7671 Appendix 3 |
| Adiabatic | S = √(I² × t) ÷ k | Checking the CPC can carry the fault current until the device operates. | k: Chapter 54 (Table 54.3 = 115) |
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- 18th Edition
- Building Regs
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- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing