Long Control Cable Runs: Why Contactors Fail to Pull In and How Conductor Size Fixes It

A 24 V contactor coil that closes on the bench and refuses to close at the far end of a 200 m control run is one of the most common commissioning faults in industrial panels. The control cable is rarely damaged. It is simply too thin for the current the coil draws in the opening fraction of a second, and the voltage that reaches the coil falls below the level at which the contactor is built to operate.

Why the inrush, not the holding current, sets the limit

A contactor coil draws far more current while the armature is closing than it does once the contactor is sealed in. The cable must carry that peak without letting the coil voltage sag. Contactor standards (IEC 60947-4-1) require a contactor to close reliably anywhere between 85% and 110% of its rated control supply voltage. On a 24 V circuit, the coil must therefore still see about 20.4 V at the instant of closing.

The voltage lost in the cable is the inrush current multiplied by the loop resistance, and the loop is both conductors, out and back. Doubling the run length doubles the loss. Halving the conductor area doubles it again. A cable chosen by its current rating, which is generous for a coil circuit, says nothing about this.

A worked example

The figures below are illustrative. They assume a 24 V DC coil with a 1 A inrush, a 200 m run and a stable 24 V supply at the panel. Conductor resistances are the standard maximum values for Class 2 copper at 20°C (IS 8130 and IEC 60228): 12.1 ohm/km for 1.5 mm² and 7.41 ohm/km for 2.5 mm².

The 1.5 mm² arrangement fails the 85% threshold even before any supply sag is counted. Two 2.5 mm² cores in parallel on each leg pass with margin. Resistance rises with conductor temperature, so a cable in a hot cable tray loses a little more than the table shows.

Options, in order of preference

There are four practical ways to restore the voltage. They are not equal in cost or reliability.

One option is to use spare cores in parallel. A multicore control cable usually carries spare cores, and paralleling two or three of them on each leg cuts the loop resistance at no extra cable cost. This works only if spare cores were planned in, which is why core-count planning belongs at the design stage. The control cable core-count and spare-core planning guide sets out how many to reserve.

Another is to move the control supply. A higher control voltage tolerates the same absolute drop far better. A 4 V loss on a 24 V coil is serious, while the same 4 V on a 230 V coil is a minor matter. Where the coil voltage can be chosen, the long runs are the place to choose it.

A third is to fit an interposing relay near the load. The long run then carries only a small relay coil current, and the relay contacts switch the contactor coil locally. This adds a component and a failure point, but it solves the problem completely.

A fourth is to step up the conductor area. Where the cable is being specified from scratch, a larger conductor is the simplest answer, subject to the sizes the chosen construction is built in. APAR lists LV XLPE/PVC control cables in 1.5 mm² and 2.5 mm² conductors from 2 up to 61 cores, with armoured and unarmoured options. For loads that need larger conductors on the same route, the LV XLPE power cable range covers a wider size span.

Where the fault turns up

The pattern is most common in plants where the control room or motor control centre sits far from the driven equipment. Conveyor galleries, remote pump houses, cooling tower fans, effluent plants and long process lines all have motor starters at one point and field devices at another. The control cable is often reused from a standard panel schedule drawn up for short runs, and the one outlying run that exceeds the assumption is missed.

The symptom is easy to misread. A contactor that chatters, hums or fails to close is often blamed on a faulty coil or a weak relay. Replacing the coil rarely helps. The contactor may also close on a cold morning and fail in the afternoon, because the copper conductors warm up and their resistance rises. Intermittent starts that correlate with ambient temperature or with other loads switching on at the same time are a strong sign of marginal coil voltage.

The step-by-step check

The check takes only a few minutes per run and can be handled in a spreadsheet.

1. Take the coil inrush current and the rated coil voltage from the contactor datasheet.
2. Calculate the loop resistance: twice the run length in kilometres, multiplied by the conductor resistance in ohm per kilometre, divided by the number of parallel cores per leg.
3. Multiply by the inrush current to get the voltage lost in the cable.
4. Subtract that loss from the lowest expected control supply voltage, not the nominal one.
5. Compare the result with 85% of the rated coil voltage. If it falls short, change the cable arrangement, the control voltage or the switching method and run the check again.

AC coils, DC coils and the holding current

The same arithmetic applies to both coil types, but the margins differ. An AC coil draws a high inrush while the magnetic gap is open and a much lower current once the armature seats, so the closing instant is the critical moment. A DC coil often has an economiser or a series resistor that cuts the holding current after closing, which means the contactor may close and then drop out if the supply sags during the hold. Both conditions should be checked, with the inrush figure for closing and the holding figure for staying in.

Drop-out has a separate limit. A coil must release when the voltage falls to a defined lower threshold, and a long cable with leakage or induced voltage can hold a coil in when it should open. That is a less common fault, but it is another reason to treat a long control run as a design item and not as an afterthought.

What else belongs in the check

Inrush figures differ by coil and by maker, so the value must come from the contactor datasheet, never from an assumption. The check should be run for the longest run in the panel schedule and for the lowest expected supply voltage, since a supply at the bottom of its tolerance leaves no allowance for cable loss.

Signal circuits that share the route need separate attention. Switching a coil on a long run can induce noise into adjacent low-level wiring. The screened instrumentation cables page describes constructions built to resist interference, and the instrumentation cable selection guide explains how pairs, screens and armour are chosen.

APAR recommendation

APAR recommends that every control run longer than about one hundred metres be checked for coil voltage at inrush before the cable is ordered, using the contactor datasheet figure and the 85% operating limit. Where a run fails, parallel spare cores or a higher control voltage are the remedies to try early, with an interposing relay as the fallback. A short calculation at the design stage costs far less than a second site visit to re-pull a cable.