Electrical Installation Guide: conductor voltage drop and short-circuit current

Overview

The voltage-drop formula for a balanced three-phase circuit is sqrt(3) x current x cable length x (R cos(phi) + X sin(phi)); a two-conductor circuit uses a multiplier of 2 instead of sqrt(3). Because both resistance and reactance appear, power factor cos(phi) and cable cross-section together decide the result: at low power factor the reactive term can outweigh the resistive term, especially with large cables.

The three-phase fault-current estimate divides the phase-to-phase voltage by sqrt(3) times the upstream per-phase impedance. Series resistances and reactances from the source to the fault point are first summed separately and only then is the magnitude of the combined impedance found; adding the impedances arithmetically would give a wrong result. The method allows a quick estimate of the expected fault level at switchboards and sub-boards.

The page supports preliminary electrical calculations with stated circuit assumptions, but it does not replace a coordinated protection study covering device ratings, selectivity and thermal withstand. The topic relates to IEC 60364; circuit type, phase balance, power factor, conductor temperature and the upstream fault assumption must be recorded explicitly, and device ratings and conductor impedance taken from the relevant manufacturer data.

Key points

1. Steady-state voltage drop in a balanced three-phase circuit: sqrt(3) x I x L x (R cos(phi) + X sin(phi)).
2. A two-conductor circuit uses the same formula with a multiplier of 2 instead of sqrt(3).
3. Three-phase fault current = phase-to-phase voltage / (sqrt(3) x upstream per-phase impedance).
4. Series resistances and reactances are summed separately before the impedance magnitude is found.
5. The method is for preliminary sizing; it does not replace a coordinated protection study.
6. Related standard: IEC 60364.

Using it in design

• Make circuit type (3-phase or two-conductor), cos(phi) and conductor temperature explicit inputs to the cable voltage-drop calculation.
• In the short-circuit calculation sum R and X separately and take the magnitude afterwards; do not add impedances arithmetically.
• Show the upstream fault assumption in the result report.
• Take device ratings and conductor impedance from manufacturer data and label results as 'preliminary'.

Who should read it

Design engineers doing preliminary electrical sizing who need quick estimates of voltage drop and fault level.

Related standards

• IEC 60364

Publication details

Authors

Schneider Electric

Publisher

Schneider Electric

Year

Year not stated

Document number

Electrical Installation Guide, Chapter G

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation