Motor Circuit Protection: Fuses and Conductor

Short answer

A motor circuit is protected with separate elements that serve different functions: disconnect, short-circuit and ground-fault device, controller, overload protection and conductor. The conductor is sized above the motor full-load current, and the short-circuit device is chosen higher than that current to allow the start, but low enough to protect the cable and the starter. The exact margins depend on the applicable code and its edition in force.

What is being protected

A motor is a load with three particular features: it starts by drawing several times its rated current, it can overheat from a slow overload, and it can give rise to a short circuit in the cable or the winding. Those three situations call for different protections, which together make up the motor circuit:

Element	Function	What it prevents
Disconnect switch	Isolate the motor and its control from the supply for maintenance	Working live
Short-circuit and ground-fault device	Clear a fault in milliseconds	Damage to the cable, starter and panel
Controller (contactor or starter)	Switch the motor in normal service	None; it is the operating device
Overload protection	Detect heating and open the contactor	Thermal damage to the winding
Conductor	Carry the starting and running current	Cable heating

That they are separate elements does not prevent some devices from combining them: a motor circuit breaker integrates isolation, short circuit and overload. See motor circuit breaker (MPCB): how to select one.

Two schools of sizing

The North American code (NEC, Article 430) and IEC practice solve the motor circuit with different criteria, and it is common to find both in the same plant.

North American scheme. It uses the full-load current given by the code itself in tables, not the nameplate current, to size the conductor and the short-circuit device. The branch-circuit conductor of a continuous-duty motor is sized with a margin above that table current, which public sources quote as 125 %. The short-circuit device may be larger than that current, with different maximum limits for time-delay fuse, non-time-delay fuse, inverse-time breaker and instantaneous-trip breaker. The limits are of the order of 175 % for the dual-element fuse and 250 % for the inverse-time breaker. They can be raised if the motor does not start. Those factors must be confirmed against the code text in the edition adopted in your jurisdiction; this guide does not fix them. Overload is set with the nameplate current.

IEC scheme. The conductor is sized with the motor operational current, applying the grouping, temperature and installation-method factors of the installation standard. The short-circuit device and the contactor are chosen by coordination (see type 1 and type 2 coordination), and overload is protected with a relay of suitable class (see thermal relay and trip classes).

Fuses in motor circuits

Fuses are widely used in motor circuits for their ability to limit short-circuit current, which reduces the energy that reaches the starter. Three types appear often:

Time-delay or dual-element. It tolerates the starting peak for a few seconds and blows on a short circuit or a strong sustained overload. It is the usual one for motors.
Non-time-delay (fast-acting). It responds quickly to any overcurrent and requires larger ratings to let the start through. It is used when high current limitation is needed.
Semiconductor-protection class. Used in soft starters and drives, not for the motor.

The fuse curve must cross the overload relay curve: at moderate currents the relay should act, and at high currents the fuse. If that crossover is wrong, the fuse blows on slight overloads or the relay is damaged in a short circuit. In addition, the fuse interrupting rating must exceed the available short-circuit current, and the disconnect must be able to switch at the motor current. In a motor control center these conditions are verified bucket by bucket.

The conductor

A motor conductor is sized with four criteria, and the most demanding is chosen:

Ampacity in continuous service, with the code margin and the installation factors.
Voltage drop in service and, above all, at start; a long cable can keep the motor from starting even if its size is correct for current. See starting large motors and voltage drop.
Short-circuit withstand, that is, the cable must withstand the energy the protection lets through before opening.
Environmental conditions, such as ambient temperature, grouping and raceway.

Illustrative example

Invented figures to show the reasoning.

An 18.5 kW, 400 V motor, nameplate current 35 A (illustrative), direct-on-line start, 80 m cable in a tray.

The cable is sized for current with the margin of the chosen standard or code, and then voltage drop is checked in service and at start. With a starting current of about six to seven times rated (illustrative), a cable sized tightly for ampacity can give a voltage drop at start that keeps the motor from accelerating. The size is raised until both drops are acceptable.
The time-delay fuse is chosen above the starting current with the manufacturer’s table, and its curve is checked to cross the overload relay curve in the intermediate zone.
It is verified that the fuse interrupting rating exceeds the panel short-circuit current and that the cable withstands the let-through energy.

The order matters: the cable is governed by voltage drop more often than you might think.

Ground fault, multi-speed motors and drives

Three cases depart from the simple direct-on-line circuit.

Ground fault. The short-circuit device usually also serves the ground-fault function in the branch circuit. In systems with high-impedance grounded or isolated neutral, the ground-fault current is small and a fuse does not see it. Specific ground-fault protection with a core-balance transformer is needed. On medium and high power motors, this protection is usual and is combined with differential protection or the motor relay, as seen in motor protection relay.

Multi-speed motors or staged starting. A two-speed motor has two different rated currents, and the circuit must be protected for the larger, with separate overload protection for each winding. In wye-delta and autotransformer starts, there are sections of the circuit that carry only a fraction of the line current. Overload protection is placed according to the position of the relay in the diagram.

Circuits with a drive. The drive input draws a non-sinusoidal current with harmonics. The output to the motor carries a high-frequency switched voltage, which in many cases requires shielded cable and limits length. Input short-circuit protection is set by the drive manufacturer, often with a specific semiconductor fuse class, and the table in its manual applies. On the output, no switching devices connected to the motor are installed while the drive is running, unless the manufacturer says so. Opening with the drive energized can damage its output stage.

In any of these cases, the common criterion is the same: each section of the circuit must be protected by a device capable of clearing the most severe fault possible in that section, with selectivity toward upstream and without the normal start tripping it.

Common mistakes

Sizing the conductor by current only and finding later that the motor does not start because of voltage drop.
Using a fast-acting fuse with a tight rating that blows on every start.
Using a fuse so large that the cable or relay does not survive the short circuit.
Confusing nameplate current and code table current when setting protections.
Applying the percentages of a code without checking which edition governs in the project’s country.
Not checking the curve crossover between relay and fuse.
Forgetting the local disconnect when the motor is far from the panel.

Checklist

Applicable installation code or standard and its edition.
Nameplate current, table current (if applicable) and motor starting current.
Conductor verified for ampacity, voltage drop in service and at start, and short circuit.
Short-circuit device chosen with the manufacturer’s table and the code.
Interrupting rating higher than the short-circuit current at the point.
Fuse curve crossover with the overload relay reviewed.
Disconnect sized and located per the standard.
Short-circuit study updated if the source changed.

When to call us

The motor circuit looks simple until the source changes, a cable gets longer or a drive is added. Our team can carry out the short-circuit study and the sizing of your panel and cables remotely, worldwide, and test and commission the protections in Colombia, or elsewhere on request. Write to us through contact with the single-line diagram, the list of motors and the cable lengths.

References

NFPA 70 (NEC), Article 430, Motors, Motor Circuits and Controllers.
IEC 60364, Low-voltage electrical installations (series).
IEC 60947-4-1, Contactors and motor-starters – Electromechanical contactors and motor-starters.
Fuse manufacturers’ technical documentation on motor circuit protection.

Frequently asked questions

Why is a motor fuse larger than the motor current?

Because the motor draws several times its rated current at start and the fuse must not blow on that peak. The fuse protects against short circuit, not overload; the relay handles overload. That is why short-circuit protection is accepted to be noticeably larger than the full-load current.

What is a dual-element or time-delay fuse?

It is a fuse that tolerates short-duration overcurrents, such as those of starting, and blows only if they persist. It allows a rating closer to the motor than a fast-acting fuse, which is why it is widely used in motor circuits.

Is the conductor sized with the nameplate current or the table current?

It depends on the code. In the North American scheme, the conductor and the branch-circuit device are sized with the full-load current from the code tables, and overload with the nameplate current. In IEC schemes the starting point is the motor operational current. Check the installation standard that applies to your project.

Does a drive change the conductor sizing?

Yes. Between drive and motor, non-sinusoidal currents, cable length and peak voltage at the motor are considered, and between supply and drive the manufacturer's instructions apply. The input and output cables are sized with different criteria.