LV Cable Sizing Guide: Current Capacity, Derating and Voltage Drop

Cable size is a design result, not a direct conversion from equipment kilowatts. A suitable low-voltage feeder must satisfy current capacity, voltage drop, protection coordination and installation requirements together. This guide explains the checks and the information an EPC or purchasing team should put into its cable schedule.

Start with a complete design basis

Record supply voltage, frequency, phase arrangement, design load, power factor, route length and installation method. For motor loads, use rated input current or account for efficiency when converting shaft power to electrical current. Keep assumptions on diversity, future capacity and operating duty explicit.

For a balanced three-phase load, input current can be estimated as I = P / (√3 × V × power factor), where P is electrical input power in watts and V is line-to-line voltage. Do not use that expression unchanged for single-phase loads or motor shaft output power.

Check current capacity under real site conditions

A published ampacity applies to stated reference conditions. Select the correct installation method and adjust for relevant ambient temperature, grouping and, for buried routes, soil conditions and depth. Apply correction factors consistently with their source; avoid counting the same thermal effect twice.

For a tabulated method: corrected capacity = reference capacity × applicable correction factors. Detailed steady-state calculations can instead use the IEC 60287 framework with the cable construction and site inputs.

Reference: Schneider Electric — General method for cable sizing

Reference: IEC 60287-1-1:2023 — Current rating and losses

Illustrative example: assume a reference rating of 200 A, a temperature factor of 0.91 and a grouping factor of 0.80. The corrected capacity is 145.6 A. It does not support a 160 A design load. These assumed factors are not a recommended rating for any specific BURY CABLE product; use the correct table or calculation for the actual route.

Coordinate the protective device

For the usual overload-protection arrangement, verify Ib ≤ In ≤ Iz, where Ib is design current, In is the protective-device rating or setting, and Iz is the corrected cable capacity. Also verify I2 ≤ 1.45 × Iz using the device’s conventional operating current. Breaking capacity and required disconnection times need their own checks. A cable that passes ampacity alone is not a completed circuit design.

Reference: Schneider Electric — Practical values for a protective scheme

Calculate voltage drop separately

For a balanced three-phase AC feeder, a common approximate expression is ΔV = √3 × I × L × (R cosφ + X sinφ). Use one-way route length L in kilometres, resistance R and reactance X in ohms per kilometre, and current I in amperes. Use resistance appropriate to operating temperature and the actual conductor design.

For a single-phase two-wire circuit, the corresponding multiplier is 2 instead of √3. Calculate percentage drop against the relevant circuit voltage. Check upstream contributions and starting conditions against the project’s permitted voltage-drop budget; one universal percentage does not fit every installation.

Reference: Schneider Electric — Calculation of voltage drop

Worked example: route length changes the result

Assume a balanced 400 V three-phase feeder carrying 160 A at power factor 0.90. For illustration only, take R = 0.34 Ω/km and X = 0.08 Ω/km. These are assumed calculation inputs, not a certified cable datasheet or a conductor-size recommendation.

At 100 m, the feeder drop is about 9.45 V, or 2.36%. If the designer allocates 3% to this feeder, that calculation fits the assumed budget; it does not verify current capacity, upstream drop, motor starting or fault protection. At 150 m, the same assumptions produce about 3.54%, so the design needs to be reconsidered.

Verify short-circuit withstand and other constraints

For an applicable adiabatic thermal check, S ≥ I√t/k, where S is conductor area in mm², I is fault current in amperes, t is clearing time in seconds and k depends on conductor material and the initial/final temperatures. For current-limiting devices, use the manufacturer’s let-through I²t data. Confirm the method’s duration limits and its applicability to the circuit.

Reference: Schneider Electric — Cable short-circuit withstand

Check neutral and protective conductors separately. Nonlinear loads can alter neutral loading. Parallel circuits require a verified sharing arrangement. Terminal temperature limits, cable bending radius, gland sizes and pulling constraints can also change the acceptable selection even after electrical calculations pass.

Turn the calculation into a usable RFQ

If an approved design already specifies the cross-section, send that schedule for quotation. If sizing is still open, include the design inputs and request technical review. Do not present a provisional size as an approved requirement.

Frequently asked questions

Can I choose a cable from motor power alone?

No. The voltage, input current, route, installation and protection arrangement must also be known. Two installations serving the same motor rating may need different cable sizes.

Does a larger conductor always solve the problem?

It may improve resistance-related voltage drop or thermal capacity, but it does not establish the right fire performance, movement duty, approval or termination compatibility.

Does LSZH mean fire resistant?

No. A low-smoke, halogen-free requirement and a circuit-integrity requirement describe different performance objectives. State the required tests and acceptance criteria in the specification.

Discuss your project with BURY CABLE

Send the cable schedule, installation conditions and project specification for construction review and quotation. The final circuit design should be verified by the responsible project engineer using the applicable installation rules and approved product data.

Review our cable range

Compare BS 5467 and BS 6724 cable applications

Request a quotation or technical review

Email: [email protected]