Motor Control Center (MCC) Guide: Sizing, Calculations & Selection

A motor control center (MCC) is a factory-assembled switchgear and controlgear assembly housing multiple motor feeder units — each combining an isolator, a contactor or circuit breaker, overload protection, and control wiring — within a common enclosure fed from a shared busbar, used to start, protect, and control groups of electric motors in industrial and utility plants. This guide walks through the first-principles calculations an engineer needs before specifying an MCC: full-load current (FLC), starting current, short-circuit withstand, and busbar sizing with diversity, followed by selection criteria and the governing IEC standards. The guidance applies equally to projects across the Middle East and Gulf (including Egypt and Saudi Arabia), North Africa, the CIS region, and Sub-Saharan Africa, where ambient temperature, dust ingress, and grid fault levels strongly influence MCC design margins.

Why MCC sizing matters

An undersized MCC busbar or starter will nuisance-trip or, worse, fail to withstand a downstream short circuit; an oversized one wastes capital. The three calculations every specifier must perform are:

1. Motor full-load current (FLC) — sets the minimum rating of the contactor, overload relay, and feeder cable.
2. Starting current — determines inrush withstand, voltage-dip impact, and the choice of starting method (DOL, star-delta, soft starter, or VFD).
3. Prospective short-circuit current at the MCC busbar — sets the required breaking/withstand rating of the incomer and feeder devices, coordinated with the upstream transformer and protection relay settings (see our protection relay guide for relay coordination basics).

The method / formula

Three-phase motor full-load current:

I_FLC = P / (√3 × V × PF × η)

Where P = motor shaft output power (W), V = line voltage (V), PF = power factor, η = efficiency.

Starting current is expressed as a multiple of FLC, depending on the starting method:

I_start = k × I_FLC

Prospective short-circuit current at a busbar fed through a transformer of known impedance:

I_rated(transformer) = S / (√3 × V) I_sc ≈ I_rated / Z%

After-diversity demand current for an MCC with multiple feeders:

I_demand = (Σ I_FLC,i) × Diversity Factor

These four formulas are sufficient to size an MCC’s feeder devices, incomer breaker, busbar, and required short-circuit rating before detailed vendor engineering begins.

Worked example with computed numbers

Step 1 — Motor FLC. For a 160 kW, 400 V three-phase motor with an assumed efficiency of 0.94 and power factor of 0.87:

I_FLC = 160,000 / (1.732 × 400 × 0.87 × 0.94) ≈ 160,000 / 566.6 ≈ 282.4 A

The feeder contactor, overload relay, and cable must each be rated at or above this value, with the overload relay typically set at roughly 1.15 × FLC ≈ 325 A to allow for normal service tolerance while still tripping on sustained overload.

Step 2 — Starting current. For a direct-on-line (DOL) start with an illustrative starting multiplier of 6 × FLC:

I_start = 6 × 282.4 ≈ 1,694 A

This inrush, even if brief, must be checked against upstream voltage-dip limits and against the withstand rating of the contactor and busbar during the starting transient. If the dip is unacceptable, the designer substitutes a star-delta starter (illustrative multiplier ≈ 2–3 × FLC) or a soft starter/VFD (illustrative multiplier below 2 × FLC) to reduce the inrush.

Step 3 — Short-circuit level at the MCC busbar. For a 1000 kVA transformer at 400 V with a nameplate impedance of 5%:

I_rated = 1,000,000 / (1.732 × 400) ≈ 1,443.4 A I_sc ≈ 1,443.4 / 0.05 ≈ 28,868 A ≈ 28.9 kA

The MCC incomer and busbar must therefore be specified with a short-circuit withstand/breaking rating equal to or greater than this computed prospective fault current, coordinated with the upstream transformer protection and the MCC’s own feeder breakers.

Step 4 — Busbar sizing with diversity. Suppose the MCC serves six motor feeders with a combined connected FLC of 1,200 A. Applying an illustrative diversity factor of 0.75 (not all motors run simultaneously at full load):

I_demand = 1,200 × 0.75 = 900 A

The incomer and main busbar would then be selected at the next standard rating above 900 A (e.g., 1,000 A), rather than at the full connected load of 1,200 A — avoiding unnecessary oversizing while retaining margin for future feeders.

Selection / comparison table

Figures above are illustrative engineering ranges for comparative teaching purposes, not ratings of any specific product.

Selection criteria & common pitfalls

• Voltage class. Most MCCs operate at low voltage (≤1 kV) for motors up to a few hundred kW; large motors (several MW, typically above 1 kV) are switched via medium-voltage motor control centers using vacuum contactors or circuit breakers instead of LV contactors.
• Form of internal separation. IEC 61439-2 defines Forms of separation (barriers between busbar, functional units, and terminals) — higher Forms improve personnel safety and limit fault propagation between feeders but increase cost and footprint.
• Short-circuit withstand coordination. The MCC’s rated short-circuit withstand must be checked against the computed prospective fault current at its busbar (see worked example above), not assumed from a catalogue default.
• Starting method vs. supply stiffness. DOL starting is simplest but produces the highest inrush and voltage dip; weak or long-feeder supplies may force a soft starter or VFD even for modest motor sizes.
• Diversity and spare capacity. Sizing the incomer strictly off connected load (sum of FLCs) rather than after-diversity demand leads to oversized, costlier incomers; conversely, ignoring future motor additions leaves no spare feeder space.
• Enclosure and ingress protection. In dusty, high-humidity, or coastal environments common across the Gulf, North Africa, and Sub-Saharan Africa, enclosure IP rating and internal climate control (heaters, filtered ventilation) should be reviewed against IEC 60529 ingress classes.
• Overload and protection relay coordination. Overload relay and feeder breaker settings must coordinate with upstream protection relays and transformer fuses — see our protection relay guide for coordination fundamentals, and our switchgear hub for broader distribution-level coordination.

Standards

Motor control centers and their components are governed primarily by:

• IEC 61439-1 and IEC 61439-2 — general rules and particular requirements for low-voltage switchgear and controlgear assemblies, covering Forms of separation, temperature rise, and verification of assemblies such as MCCs.
• IEC 60947-4-1 — requirements for contactors and motor-starters, including overload relay coordination categories.
• IEC 62271-106 — requirements for high-voltage alternating-current contactors, relevant to medium-voltage motor control centers switching large motors.
• IEC 60529 — degrees of protection (IP codes) for enclosures.

Where a project specifies a different or national standard (e.g., an equivalent NEMA ICS framework), the underlying sizing calculations shown above remain the same; only verification and test procedures differ.

Related guides

See also our guides on medium voltage switchgear, gas-insulated switchgear and ring main units.