Cable size for a motor feeder — FLC, the 45°C derate, and the run

Start at the nameplate, not at the chart

Every motor carries its full-load current on its plate, and that figure beats any chart because it already contains the machine's own power factor and efficiency. Read it first. Where the plate is unreadable or the motor has not arrived yet, the current can be estimated: full-load amps = HP × 746 ÷ (√3 × 400 V × power factor × efficiency). Take power factor 0.85 and efficiency 0.90 and the denominator comes to 530, so the estimate collapses to HP × 1.41. A 20 HP motor: 20 × 746 ÷ 530 = 28.2 A.

Motor feeder cable sizing in a Pakistani plant runs in three passes. The conductor must carry the motor's nameplate full-load current after the ambient and grouping correction factors have been applied to its 30°C table rating; the run must keep voltage drop inside the installation's budget, normally 3% of 400 V, which is 12 V; and the protective device must sit between the two. The starting surge is not part of the conductor sum — it is carried by the starter and the overload relay.

Plant drawings in Pakistan usually write this supply as 415 V. The arithmetic here uses 400 V, which returns the higher current for the same output and therefore the safer size; on a measured 415 V supply the real current is about 3.6% lower, since 400 ÷ 415 = 0.964.

Two rows are worth reading twice. At 30 HP the correction changes nothing, because 10 mm² is rated 57 A and 57 × 0.75 = 42.8 A still clears the motor's 42.2 A. At 100 HP it moves the feeder two sizes, because 70 mm² comes to 184 × 0.75 = 138 A against a motor wanting 140.8 A, and three amps short is short. That is the argument for publishing the corrected column instead of a footnote.

Read the chart as a capacity answer only. On a real feeder the run usually pushes the small sizes up anyway, and on a fixed plant feeder 2.5 mm² is the sensible floor whatever the arithmetic says about a 5 HP pump.

Why 30°C is the wrong column in a Pakistani plant

Published ampacity is a laboratory number: one circuit, clipped to a wall in open air, 30°C around it. A motor feeder in a spinning shed, a compressor room or a roof-level tray in June sits nowhere near that, and it rarely sits alone. Two corrections apply, and they multiply — they do not average.

This page uses a combined 0.75 as its worked example and says so wherever it uses it. Take the real ambient and grouping factors for your temperature, your installation method and your number of grouped circuits from BS 7671. A derate is a correction applied to a stated base, so it should always be shown as base × factor = result: anyone who hands you a bare amp figure with no method and no ambient behind it has given you a number you cannot check.

Worked: a 20 HP motor, 120 m away, on a tray at 45°C

A 20 HP (14.9 kW) three-phase motor fed from the plant panel, 120 m of cable on a ladder tray beside other loaded feeders, plant ambient around 45°C. Nameplate first; failing that, the estimate is 20 × 746 ÷ 530 = 28.2 A.

Capacity pass. 4 mm² is rated 32 A for three loaded conductors at 30°C. Apply the worked 0.75 and 32 × 0.75 = 24 A, under the 28.2 A the motor draws, so it fails. 6 mm² is rated 41 A, so 41 × 0.75 = 30.8 A, which clears 28.2 A with 2.6 A to spare. On capacity alone the answer is 6 mm².

Voltage drop pass. Hold the feeder to 3% of 400 V, which is 12 V. On 6 mm² at 7.3 mV/A/m the drop is 7.3 × 28.2 × 120 ÷ 1000 = 24.7 V, twice the budget. On 10 mm² at 4.4 mV/A/m it is 4.4 × 28.2 × 120 ÷ 1000 = 14.9 V, still over. On 16 mm² at 2.8 mV/A/m it is 2.8 × 28.2 × 120 ÷ 1000 = 9.5 V, inside 12 V.

The answer is 16 mm², and it is worth noting what decided it. Heat took the cable from 4 mm² to 6 mm². Distance took it from 6 mm² to 16 mm². A feeder sized off a capacity chart alone would have gone in at 6 mm², started sluggishly, run warm at the terminals and tripped on overload in summer — with nothing visibly wrong with the cable.

One honest note on that sum. The millivolt figures used here are two-conductor values, so applying them to a balanced three-phase run overstates the drop by about 13%, since √3 ÷ 2 = 0.87. Corrected, 10 mm² still comes to 14.9 × 0.87 = 13.0 V and still misses the 12 V budget, so 16 mm² stands either way. Put your own length through the voltage drop calculator before you order.

How far each size reaches before voltage drop takes over

Capacity settles the size at the panel. Length settles it at the machine. This is the same chart read the other way round: for the corrected size in the column above, the one-way run at which the feeder spends its whole 12 V allowance.

Read the 7.5 HP row against the 25 HP row. The small motor on 1.5 mm² spends its allowance at 39 m, while a motor three times its size on 10 mm² reaches 77 m. Feeder runs of 100 to 300 metres are ordinary in a large plant, and past these distances the size is chosen by the run, not by the load: go up a size and work the drop again.

Starting current: what it changes, and what it does not

A cage motor started direct-on-line pulls a heavy surge for a few seconds — as a rule of thumb six to eight times full-load current, though the figure to trust is the motor's own data or the starter's setting. For the 20 HP motor above that is roughly 28.2 × 6 = 169 A up to 28.2 × 8 = 226 A, against a cable chosen for 28.2 A.

That does not mean the cable is undersized. The surge lasts seconds, conductor heating is a matter of minutes, and the copper rides it. What the surge decides is everything around the cable: the protective device has to be motor-rated so it does not trip on inrush, the overload relay is set to the running current rather than the surge, and a star-delta starter, a soft starter or a drive cuts the peak where the supply is weak.

The one place the surge does reach the cable is voltage drop at the instant of starting. A feeder already at the edge of its 12 V budget when running sags much harder on inrush, and the symptom is a motor that hums, struggles and trips instead of spinning up. That is a run-length problem wearing a motor's clothes, and a size up is the fix. A drive-fed motor is a separate specification again: the drive maker states the cable type and a maximum length, and that instruction outranks any chart.

Sizing a plant submain rather than one motor

A submain feeding a shed of machines is not the sum of every nameplate. Total the full-load currents, apply a diversity allowance because the whole plant never runs flat out in the same second, and size against the diversified figure — then check the run, which on a factory submain is usually the constraint that decides the answer. The load total itself is a separate calculation, worked through on commercial load calculation, and the kW-to-mm² chart for a whole three-phase load sits on 3-phase cable size.

• The earth conductor is derived from the phase conductor, not fixed at a favourite size. The derivation is on earth cable size.
• Mechanical protection is a routing decision, not a sizing decision. A run crossing a yard or buried between sheds wants armour; a run on a tray inside the building generally does not. That call is made on armoured vs unarmoured cable.
• Order in mm², and know the strand code you will be quoted. Plant stores here still order in SWG: 7/.052 is the 10 mm² and 7/.064 is the 16 mm², and the two vocabularies do not line up exactly.
• A motor list beats a single line. HP, run lengths and routing quoted together catch the length and routing mistakes that a one-size-at-a-time enquiry hides until the cable is on the tray.

The cable for a motor feeder

Three-phase feeders run as four-core where the earth travels with the phases, or three-core with a separate earth conductor. These are the sizes the chart lands on most often; the worked 20 HP example at 120 m ends on 16mm 4 core cable.

Where the earth runs separately, the three-core versions are 10mm 3 core cable, 16mm 3 core cable and 25mm 3 core cable. For the longest plant distribution runs, where cost per metre matters more than cross-section, compare aluminium cable. Send the sizes and the metres, and we will confirm what we can supply and today's rate.

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