How to Choose Cable Size for a Distribution Board?

Choosing the correct distribution board cable means matching it to the load and protective device. Size every outgoing circuit first. Then calculate the complete board demand and size the incoming feeder. Each cable must carry its load under the actual installation conditions. Long runs also need a voltage-drop check, while direct-on-line motors require attention to starting current.

There is no single cable size for every 63 A, 100 A, or 200 A board. The result depends on the load, protection, cable, installation conditions, route length, and applicable standard.

The practical rule is simple: calculate the load, select the protective device, and then select a cable that the device can protect safely under the real installation conditions.

The Three-Step Distribution Board Cable Sizing Process

Distribution board cable sizing follows three connected steps. First, size every outgoing circuit. Second, calculate the complete board demand and select its incoming protection and cable. Third, make an additional starting-current check when a motor starts direct-on-line.

Throughout the process, the breaker or fuse must carry the calculated load, while the cable must carry the selected protective-device rating or setting under the actual installation conditions. The protection must also disconnect overloads and short circuits safely.

This load–protection–cable relationship is the foundation of the conductor-sizing method described in the Schneider Electric Electrical Installation Guide, which follows IEC principles.

Step 1: Size Each Outgoing Circuit Cable

Each outgoing circuit should be considered separately because its load type, operating current, cable route, and starting characteristics may be different. A lighting circuit and a motor circuit with similar running currents may still need different protective devices and cable checks.

Calculate the Load Current

Begin with the actual load supplied by the circuit. Check:

• Rated power or equipment nameplate current
• Single-phase or three-phase supply
• System voltage
• Power factor and efficiency where applicable
• Operating duty and expected inrush or starting current

Use equipment nameplate current whenever reliable data are available. Motors require power factor and efficiency, while packaged machines, HVAC equipment, inverters, and electronic power supplies should be assessed from their technical data. The result is the circuit’s expected normal operating current.

Select the Circuit Breaker or Other Protective Device

Next, select the protective device for the circuit. Its rating or adjustable overload setting must be high enough for the design current but not so high that the cable is left without adequate overload protection.

Depending on the load, also check:

• Circuit-breaker trip curve or fuse characteristic
• Motor starting or equipment inrush current
• Breaking capacity at the available fault current
• Coordination with other protective devices

If a breaker trips during motor starting, do not automatically increase its rating. Review the trip characteristic, motor protection, cable capacity, and starting method together.

Select a Preliminary Cable and Check Voltage Drop

After selecting the protection, choose a preliminary conductor from the applicable current-carrying table. The main correction factors are summarized later in this guide.

A cable can meet the ampacity requirement and still be too small for a long route. Excessive voltage drop can cause dim lighting, poor equipment performance, contactor dropout, or difficulty starting a motor. Calculate it from the load, cable length, cable data, supply type, and power factor, then compare it with the applicable limit.

Include both the feeder and outgoing-circuit portions of the route. A long circuit may also need a fault-loop check because its higher impedance can delay automatic disconnection.

Step 2: Calculate the Board Load and Size the Incoming Cable

Once the outgoing circuits are defined, calculate how much power the complete board is expected to supply. Then use that demand to select the upstream protective device and incoming feeder.

Do Not Add All Outgoing Breaker Ratings

Do not size the feeder by adding every outgoing breaker rating. A board with ten 16 A breakers does not automatically have a 160 A demand because the loads may be smaller, intermittent, or non-simultaneous. Equally, excessive diversity can undersize the feeder.

The calculation should distinguish between:

• Connected load: the total rated load connected to the board
• Calculated demand: the load expected to operate simultaneously under the design conditions
• Future load: an agreed allowance for planned expansion

Use demand and diversity factors from the applicable code, client requirement, or documented operating profile—not simply to obtain a smaller feeder.

Check Phase Balance and the Most Heavily Loaded Phase.

For a three-phase board with single-phase circuits, calculate L1, L2, and L3 separately. Size for the most heavily loaded phase, not the three-phase average.

Redistributing single-phase circuits can improve balance. Do not automatically reduce the neutral, because phase imbalance and triplen harmonics from electronic loads can produce significant neutral current.

A practical load schedule should record the information needed to compare phase loading and size each circuit cable:

• Circuit reference
• Load type
• Connected load (kW or kVA)
• Demand factor, where applicable
• Calculated load
• Assigned phase: L1, L2, L3 or three-phase
• Design current (A)
• Protective device rating (A)
• Selected cable size (mm²)

Actual values must be determined from the project load data, installation conditions and applicable electrical standards.

Select the Incoming Protective Device and Cable

The incoming feeder is selected from the calculated demand of the distribution board, not from the total of its outgoing breaker ratings. The same basic sequence used for an outgoing circuit applies again at the board level.

1. Identify the calculated demand and the most heavily loaded phase.
2. Add a documented allowance for planned future load, if required.
3. Select the upstream breaker or fuse rating and protective settings.
4. Choose an incoming cable with enough corrected current-carrying capacity for that protection.
5. Check feeder voltage drop and the total voltage drop to important final loads.
6. Confirm fault protection, breaking capacity, busbar rating, terminals, and assembly rating.

The process may require iteration. A larger cable may change the terminal and entry requirements, while an excessive fault level cannot be solved by changing cable size alone.

Step 3: Check Direct-on-Line Motor Starting

A direct-on-line motor draws high current while accelerating. This can affect its protection, cable, terminal voltage, and nearby equipment.

Running Current and Starting Current Have Different Roles

Full-load current and duty mainly determine continuous cable ampacity. Starting current mainly affects protective-device operation and temporary voltage drop; frequent or long starts can also add thermal stress.

Use the manufacturer’s full-load current, starting-current data, acceleration time, and permitted starts per hour rather than one assumed multiplier for every motor.

Check the Complete DOL Motor Circuit

For a DOL motor circuit, confirm that:

• The cable carries the motor’s normal operating current.
• The protective arrangement permits starting while still protecting the motor and cable.
• The motor-terminal voltage remains sufficient during starting.
• Starting does not disturb other loads or create excessive repeated heating.

If starting voltage drop is excessive, consider a larger cable, shorter route, stronger supply point, soft starter, or variable-frequency drive as part of the complete motor-control design.

What Factors Affect Cable Current-Carrying Capacity?

The conductor size selected from a cable table is only preliminary. Its allowable current, also called ampacity, must be corrected for the cable construction and the conditions in which it will operate.

The cable must still carry the protective-device rating or overload setting after all applicable corrections. If a 63 A breaker is used but a cable can carry only 58 A after derating, select a larger cable or improve the installation arrangement.

IEC 60364-5-52 addresses low-voltage wiring systems. Never use an ampere-to-cable-size chart without checking its cable and installation assumptions.

When Are Additional Cable Checks Needed?

Routine low-voltage designs often use approved tables, standard arrangements, coordination data, or software, so a separate thermal calculation may not appear for every ordinary final circuit. Fault protection must still be satisfied.

An explicit short-circuit thermal-withstand check becomes more important near transformers and main switchboards, on large or parallel feeders, with long clearing times, reduced protective conductors, or generator and inverter supplies. Long routes also require attention to minimum fault current and disconnection time. Dynamic or mechanical stability is more commonly associated with busbars, switchgear, supports, and large single-core cable arrangements.

Will the Selected Cable Fit the Distribution Box?

An electrically suitable cable may still be too large to enter, bend, or terminate inside the selected distribution box.

Cable cross-section and cable diameter are not the same

Conductor cross-section, expressed in mm² or AWG, is used for the electrical calculation. Overall cable diameter is the external size of the complete cable and is used to select cable glands, entry holes, and enclosure space.

Two cables with the same cross-section can have different diameters because of their insulation, core count, armor, sheath, and construction.

Confirm the entries, terminals, and wiring space

Before ordering the distribution box, check:

• Conductor compatibility and terminal range
• Core count, cable diameter, and minimum bend radius
• Top, bottom, or side cable entry
• Gland thread, clamping range, and armor termination
• Required enclosure rating, wiring space, torque, and terminal temperature

Aluminum conductors normally require larger terminals and more bending space than equivalent copper conductors. The UL Solutions Panelboard Application Guide highlights this for North American panelboards.

IEC 62208 applies to empty enclosures before switchgear is installed, while the applicable IEC 61439 parts apply to complete low-voltage assemblies.

A Simple Distribution Board Cable-Sizing Example

Consider a three-phase board with lighting, socket, heating, and motor circuits. These simplified figures explain the method; they do not prescribe conductor sizes.

Enter the actual loads in the board schedule, apply justified demand factors, and distribute the single-phase circuits across L1, L2, and L3.

Assume the maximum phase demand, including planned load, is 54 A. A 63 A upstream device may be selected provisionally. If a candidate feeder is rated at 70 A in the reference table but only 59 A after temperature and grouping corrections, reject it. Select the next suitable size or improve the installation method.

For a long route, check voltage drop. Then verify fault protection, neutral and protective conductors, board rating, terminals, entries, and bending space.

Common Cable-Sizing Mistakes

Avoid these common shortcuts:

• Adding all outgoing breaker ratings to determine the incoming cable
• Choosing a cable from load current without coordinating its protection
• Using a cable table without checking its installation assumptions
• Ignoring ambient-temperature and grouping derating
• Forgetting voltage drop on long routes
• Increasing breaker size only to accommodate motor starting
• Reducing the neutral without checking imbalance and harmonics
• Ignoring cable diameter, terminals, glands, or bending space

A documented load schedule prevents many of these problems.

Frequently Asked Questions

These answers summarize the points most often checked during preliminary cable selection.

What cable size is needed for a 63 A distribution board?

There is no universal answer. Its corrected capacity must remain at least 63 A under the actual installation conditions. Voltage drop, fault protection, neutral and protective conductors, and terminals must also be checked.

Should the breaker or cable be selected first?

Start with the load, choose the protection, and then select a cable that it can protect safely. Voltage drop, motor starting, or fault conditions may require a larger conductor.

Can I add all outgoing breaker ratings to size the incoming cable?

No. Breaker ratings are not operating loads. Calculate demand using justified diversity, check the most heavily loaded phase, and include documented future load where required.

When must voltage drop be checked?

Check voltage drop against the applicable limit, especially for long routes, heavily loaded feeders, and motors with high starting current.

Does DOL motor starting current determine the cable size?

Full-load current and duty mainly determine cable ampacity. Starting current affects protection and temporary voltage drop; frequent or long starts can also add thermal stress.

Can the neutral conductor be smaller than the phase conductors?

Only when the applicable standard and calculation permit it. Phase imbalance and triplen harmonics can produce substantial neutral current.

Is cable cross-section enough to select a distribution box?

No. The supplier also needs its outside diameter, core count, conductor material, bend radius, entry direction, gland, terminal range, and enclosure rating.

Conclusion

Distribution board cable sizing follows three steps: size each outgoing circuit from its load, calculate the total board demand to size the incoming feeder, and check direct-on-line motor starting where applicable. Also verify current-carrying capacity, voltage drop on long runs, and physical compatibility with the enclosure and terminals.

Elmrix can turn these requirements into a practical distribution box specification. We can help confirm enclosure size, cable-entry layout, terminal and busbar arrangement, wiring space, material, mounting, and protection rating. We can also review drawings before quotation or production to reduce installation conflicts. Final cable sizing and protective-device settings should be verified by a qualified electrical designer. Just contact us anytime!

References

1. IEC, IEC 60364-5-52:2009+AMD1:2024 — Selection and erection of wiring systems.
2. IEC, IEC 60364-4-43:2023 — Protection against overcurrent.
3. Schneider Electric, Conductor sizing: methodology and definition.
4. Schneider Electric, General method for cable sizing.
5. Schneider Electric, Calculation of voltage drop in steady load conditions.
6. Schneider Electric, Verification of cable withstand under short-circuit conditions.
7. IEC, IEC 61439-1:2020 — General rules for low-voltage assemblies.
8. IEC, IEC 62208:2023 — Empty enclosures for low-voltage assemblies.
9. UL Solutions, Panelboard Application Guide.