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TP EIMSA study materials
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TP EIMSA: Electrical Theory and Fundamentals

TP EIMSA · Work through your exam, one topic at a time.

All guide sections

Electrical Theory and Fundamentals

Theory is used as a tool in this exam: cable sizing, device selection, motor loading and instrument interpretation all rest on a small set of relationships. Distractors are usually built from the wrong formula rather than from arithmetic slips, so the marks go to candidates who know which relationship applies.

The relationships you will actually use

  • Ohm's law V = IR, with P = VI = I squared R = V squared / R.
  • Three-phase: P = root-3 x V(line) x I(line) x cos phi; apparent power S = root-3 x V x I in kVA; reactive power Q = root-3 x V x I x sin phi.
  • AC impedance: Z = square root of (R squared + (X_L - X_C) squared), with X_L = 2 pi f L and X_C = 1 / (2 pi f C).
  • RMS and peak: 230 V RMS is about 325 V peak, so peak = RMS x root-2.
  • Conductor resistance rises with temperature: R2 = R1 [1 + alpha (T2 - T1)], with alpha near 0.00393 per kelvin for copper.

Voltage drop

Appendix 4 limits voltage drop from the origin to 3 per cent for lighting and 5 per cent for other uses on a public low-voltage supply. Tabulated mV/A/m values already contain the factor of two for single-phase circuits and the root-3 for three-phase, so one formula serves both: Vd = (mV/A/m x Ib x L) / 1000, with L in metres.

Worked example

A 4.0 mm2 two-core thermoplastic cable (11 mV/A/m) supplies 20 A over 18 m. Vd = 11 x 20 x 18 / 1000 = 3.96 V, which is 1.72 per cent of 230 V and inside the 5 per cent limit. The same cable on a three-phase circuit has a tabulated value near 9.5 mV/A/m, giving 9.5 x 20 x 18 / 1000 = 3.42 V, or 0.86 per cent of 400 V. The current is unchanged; only the coefficient and the reference voltage differ.

Traps that cost marks

  • Sizing cable and switchgear from kW when the load must be expressed in kVA; a 0.85 power factor turns 8.8 kW into about 10.4 kVA.
  • Applying root-3 to a single-phase calculation, or omitting it on a balanced three-phase load.
  • Comparing a cold measured loop impedance with a tabulated maximum without the temperature correction.
  • Using DC resistance tables for AC circuits that carry significant reactance.