Feeder Cable Sizing: How Do I Size a Control Panel Supply?

The quick answer first, because that is what gets asked at 2am: no, you do not just add every nameplate current and buy the next cable up. That method is wrong in both directions. It under-sizes when the largest motor's starting and running margin is ignored, and it over-sizes badly when six actuators that never run together are all counted at full load. Get the design current right first, then the cable follows.

Build the Load Schedule Before You Touch a Cable Table

Every sizing argument on site traces back to a missing load schedule. Write one line per load, taken from the nameplate, not from the kW on the P&ID.

• Motors: full-load current from the plate at 400 V. Do not back-calculate from kW and an assumed power factor; the plate already includes efficiency and pf.
• Heaters: rated kW and connection. A 3-phase resistive bank at 400 V draws I = P / (1.732 x 400). Note whether it is star or delta and whether it uses a neutral.
• Single-phase 230 V loads: actuators, controls, sockets, panel heater/thermostat. Record which phase each one lands on.
• Duty: mark each load continuous or non-continuous. Heaters under thermostat control and fans on a process are continuous for sizing purposes.

Check before moving on: the phase columns of your schedule add up, and no 230 V load is unassigned. If all the single-phase load has been dropped on L1 because that is where the fused isolator was convenient, fix that now — it is cheaper than a feeder upsize.

Add the Loads the Motor Way, Not the Lump-Sum Way

The arithmetic that survives scrutiny anywhere in the world is the motor-plus-other-load summation used by the US National Electrical Code in 430.24, and it is a sound design template regardless of which code you certify to:

1. 125% of the full-load current of the highest rated motor
2. 100% of the full-load currents of all the other motors
3. 100% of the non-continuous non-motor load
4. 125% of the continuous non-motor load

Resistive heater banks fall into item 4. Sizing heater circuits at 125% of rated load is standard practice in heat-treat and process heating work, and it covers both the continuous duty and the cold-resistance inrush of an element bank.

In the UK you are designing to BS 7671, which does not hand you this formula — it makes the designer responsible for determining the design current Ib. Use the summation above to derive Ib, then satisfy the BS 7671 coordination requirement Ib <= In <= Iz. Worked example, with every input labelled as an assumption because your nameplates will differ:

Load	Assumed rating	Current per phase	Sizing factor	Contribution
Fan motor 1 (largest)	4 kW, plate FLC 8.5 A	8.5 A	125%	10.6 A
Fan motor 2	2.2 kW, plate FLC 5.0 A	5.0 A	100%	5.0 A
Heater 1 + 2	9 kW each, 3-ph 400 V	13.0 A each	125% (continuous)	32.5 A
230 V actuators, balanced	12 A total	4.0 A / phase	100%	4.0 A
Design current Ib, balanced case				52.1 A
Ib, worst case all 12 A on one phase				60.1 A

Heater current check: 9000 / (1.732 x 400) = 13.0 A. The 8 A spread between the balanced and unbalanced cases is exactly why the phase allocation column matters — it is the difference between a 63 A and an 80 A device.

Check: your Ib is a single number per phase, and you can point at the line in the schedule that drives it.

Apply Diversity Only Where an Interlock Proves It

This is where the quick fixes fail. Two of them recur:

• Wrong fix 1 — sum everything at 100% and move on. It misses the largest-motor uplift and the heater continuous factor, so the feeder is under-rated on the loads that actually run for hours.
• Wrong fix 2 — apply a blanket 0.8 diversity because "they never all run". If nothing in the control logic prevents simultaneous operation, the diversity is a hope, not a design. On a heater panel, coincident demand is the normal state during warm-up.

Diversity is legitimate when it is enforced: a sequencing PLC routine, a mechanical interlock, a stage controller that never energises both heater banks together, or a duty/standby fan changeover. Write the enforcing mechanism next to the factor on the calculation sheet. If someone later deletes that rung, the record shows what the cable assumed.

For the assembly itself, BS EN / IEC 61439-2 handles this through the rated diversity factor (RDF), declared by the assembly manufacturer per circuit or for the whole assembly. Read the declared RDF from the assembly documentation rather than inventing one; if none is declared, the standard's default assumed values apply and they get harsher as circuit count rises. IEC 61439-3 is the one to use if the enclosure is a distribution board intended for operation by ordinary persons.

Check: every diversity factor below 1.0 has a named interlock or a declared RDF behind it.

Pick the Protective Device, Then Prove Iz

Order of operations matters. Choose In at or just above Ib, then find the cable whose installed current-carrying capacity Iz is at or above In.

1. Rating. With Ib = 60.1 A, an 63 A device works only if the unbalanced case is genuinely the design case; otherwise step to 80 A. Do not select 63 A and rely on the load "never really" reaching 60 A.
2. Characteristic. A feeder carrying DOL fan motors sees the largest motor's starting current on top of the heater load. A Type B MCB will trip on that. Use Type C or D, or an MCCB with an adjustable magnetic setting, and confirm the setting clears the highest starting inrush of the largest motor.
3. Upstream discrimination. The panel feeder device must discriminate with the outgoing devices inside the panel and with whatever feeds it. Get the curves side by side; do not assume a rating step is enough.
4. Short-circuit rating. The device breaking capacity and the assembly's rated short-time/conditional short-circuit withstand must exceed the prospective fault current at the panel. This is a declared figure for the assembly under IEC 61439-2 — read it off the assembly nameplate.

Check: you can state Ib, In and Iz as three numbers in ascending order for the selected cable and route.

Derate for the Real Route, Not the Bare Table Value

Tabulated ampacity assumes conditions your cable tray does not meet. Work through the correction factors from the installation-method tables in BS 7671 Appendix 4 in this order:

• Installation method — clipped direct, tray, conduit, buried, in a wall with thermal insulation. This selects the base column and is the single biggest swing.
• Ambient temperature — a feeder running through a boiler house or above a heater bank is not at 30 °C.
• Grouping — count every other loaded circuit sharing the containment.
• Conductor operating temperature — do not use 90 °C thermosetting ratings if the terminals in the panel or the device are only rated for 70 °C. The lower of cable and termination rating governs.

Then check volt drop over the actual run length. On a feeder with motor starting on the end of it, the running volt drop limit is not the whole story — the momentary dip during a DOL start pulls down the 230 V control supply and can drop out contactors. If the run is long, size for volt drop first and the ampacity check becomes trivial.

Neutral: if the 230 V actuator and control load is significant and any of it is switch-mode or electronic, third-harmonic currents add rather than cancel in the neutral. Do not fit a reduced neutral on a feeder that carries a large single-phase electronic load; a full-size neutral, or a check of the neutral current under harmonic loading, is the correct call.

Check: derated Iz is still >= In, and calculated volt drop is inside your project limit (commonly 5% for power circuits — confirm against the current edition of BS 7671).

Verify Before You Energise, and Know When to Stop

1. Earth fault loop impedance. Measure Zs at the panel and confirm the selected device disconnects inside the required time for the circuit type. A correctly sized live conductor with an undersized CPC still fails this.
2. Adiabatic check. Confirm the CPC satisfies for the prospective fault current and the device's let-through.
3. Thermal proof under load. Run all continuous loads together for at least an hour and thermal-image the feeder terminations, the incoming device and the busbar joints. A hot lug at 60 A is a loose lug, not a cable problem.
4. Balance measurement. Clamp all three phases with the heaters and fans running. If the phase currents differ by more than a few amps, the 230 V load allocation drifted from the schedule — rebalance it rather than upsizing the feeder.
5. Documentation. Update the schedule with the measured currents and the declared assembly ratings so the next person sizing an addition starts from reality.

Stop and escalate when the numbers stop being an arithmetic problem. If the prospective fault current at the panel is near or above the assembly's declared short-circuit withstand, if the existing feeder measures hotter than its insulation rating at normal load, or if a modification pushes the total load past what the upstream distribution board was designed for, that is a design review by the assembly manufacturer and the installation designer — not a night-shift substitution. Manufacturer technical support and the assembly's IEC 61439 verification documentation are the correct sources for declared RDF, short-circuit withstand and temperature-rise limits; get those in writing before energising anything you have changed.

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