Temporary Power Cables

In short. Temporary power cables are sized on four checks, not one: the derated current-carrying capacity at site ambient and grouping, the voltage drop over the actual route length, the short-circuit withstand until the protective device clears, and the mechanical duty of a cable that is dragged, coiled and driven over. Flexible copper cable with a tough rubber sheath, laid on a protected route with proper terminations, is what makes a rental generator useful at the load end.

Why cable decides the outcome of a rental job

A generator can hold 415 V at its terminals all day while a pump 150 metres away sees 370 V and will not start. The cable between them has resistance and reactance, and under load that impedance drops voltage and dissipates heat. On rental jobs the cable is chosen under time pressure, the route is longer than planned, and the same drums have been to twenty sites before, so cables deserve the same attention as the generator. The broader method is set out in generator cable sizing; what follows is the temporary-power application of it.

The four checks

Design current and derated capacity

Begin with the real load current, not the generator rating. For a three-phase load, I = kVA × 1000 / (√3 × 415). The published current rating of a cable assumes a reference ambient, typically 30 or 40 degrees Celsius, and a single cable in free air. In a Telangana summer the air on a site can be 45 degrees Celsius, cables lie in the sun or bunched together, and each of those conditions reduces the permitted current. Derating factors of 0.8 to 0.9 for ambient and a further 0.7 to 0.8 for grouping are common, so a cable that carries 300 A in the catalogue may safely carry only 200 A on site.

Voltage drop

Vd = √3 × I × L × (R cos φ + X sin φ)

Here Vd is the line-to-line voltage drop in volts, I the load current in amperes, L the one-way route length in kilometres, R and X the cable resistance and reactance in ohms per kilometre, and cos φ the load power factor. For a 120 mm² copper flexible cable, R is about 0.19 ohm per kilometre and X about 0.08 ohm per kilometre (illustrative, confirm against the manufacturer’s data). A 200 A load at 0.85 power factor over 120 metres gives Vd ≈ 1.732 × 200 × 0.12 × (0.19 × 0.85 + 0.08 × 0.53) ≈ 8.5 V, or about 2 percent of 415 V, which is acceptable. Double the length and halve the conductor and the drop approaches 8 percent, which is not. A 5 percent total limit to the furthest load is a sound working target, with motor starting checked separately because starting current can be six times running current for a few seconds. See voltage drop and motor starting.

Short-circuit withstand

During a fault the cable must carry the fault current without its insulation exceeding its short-circuit temperature until the breaker opens. The adiabatic check I²t ≤ k²S² applies, where S is the conductor area in mm² and k is 115 for PVC-insulated copper and 143 for XLPE or EPR copper. Rental generators produce limited fault current, but a long clearance time, which is common with generator supply because the fault current is low, can still damage a small cable. Protective device and cable are chosen together.

Mechanical and environmental duty

Permanent cables sit still. Rental cables are unrolled, pulled around corners, driven over, left in standing water during the monsoon and exposed to diesel at the generator end. Heavy-duty flexible copper cable with an elastomeric sheath, often referred to as trailing or welding-grade cable, survives this; PVC-sheathed building wire does not. Where vehicles cross the route, use cable ramps or bury the cable in a protected trench. Keep cables off hot exhaust components and away from the generator’s radiator discharge.

Single-core, parallel conductors and current sharing

Above roughly 300 to 400 A per phase, rental practice is to run several single-core cables per phase, because one conductor large enough would be impossible to handle. Parallel cables share current equally only if they are the same size, length and construction, with the same contact resistance at the lugs. Clamp each conductor during commissioning; one carrying 30 percent more than its partners has a loose termination or a longer route and will overheat first. Single-core AC cables must be laid in trefoil or a flat symmetric formation so the magnetic fields cancel, and never individually enclosed in steel.

The table is a starting point for short runs at moderate ambient; length, grouping and derating change the answer.

Neutral and earth conductors

A three-phase four-wire temporary system needs a neutral sized for the actual unbalance and harmonic content. Where the load is mostly single-phase or non-linear, the neutral should be the same size as the phase conductors. The protective earth conductor runs with the power cables from the generator earth terminal to every panel and must be large enough to carry fault current until the protection trips; a separate earth cable of at least half the phase conductor area is typical on temporary systems, with a full-size conductor for the smaller feeders. The generator frame, the panel enclosures and the site electrode are all bonded. Details are in earthing and neutral grounding.

Routing, protection and inspection on site

Plan the route on the site drawing before the drums arrive, avoiding lifting zones, vehicle gates, hot-work areas and planned excavations. Elevate cables on hooks or stands so they stay out of water and away from feet, and use ramps or a covered trench at road crossings. Terminate every cable in a gland or connector. Inspect every shift for cuts, crushing and hot connectors, and run an insulation-resistance test before each new deployment, because damage from the last site is often invisible. Panels and connectors are covered in temporary distribution panels.

Common mistakes

• Sizing the cable from generator kVA divided by voltage with no derating. The cable runs hot in the afternoon sun and the insulation ages in weeks.
• Ignoring route length when the generator position changes on delivery day. Voltage at the far end falls below what motors and drives will tolerate.
• Running parallel conductors of unequal length or mixed sizes. One conductor carries far more than its share and its termination burns.
• Laying single-core cables through separate steel openings or apart from each other. Induced heating and higher reactance appear where nobody expects them.
• Reusing a drum without an insulation test. A crushed section from the previous job becomes an earth fault on the new one.

What to send us

Send the generator rating, the load list with the largest motor, the measured route length from the generator position to each panel, a site sketch showing roads, water and lifting areas the route must cross, and photographs of the connectors on your existing equipment. We will size the feeders, confirm the voltage drop and propose the protection. Send a generator rental enquiry with these details.

Questions engineers ask

Generators by rating

• Under 15 kVA generators
• 15–50 kVA generators
• 50–125 kVA generators
• 125–250 kVA generators
• 250–500 kVA generators
• 500–1000 kVA generators
• 1–2 MVA generators
• 2 MVA and above generators

Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.