MCC sizing: drawers, incoming cubicle and busbar

What MCC sizing is

Sizing a Motor Control Center (MCC) is not just picking breakers: it is turning a load list — power, rated current, voltage and starter type for each motor — into a real physical layout of drawers, columns and busbar. The result is the switchgear assembly that will feed and protect the whole plant from a single incoming point, and it has to close on three fronts at once: protection (each motor with the right breaker and contactor), distribution (a busbar and main breaker that can carry the demand current) and space (each drawer fitting the usable column height, with a spare).

The method here follows the market reference — WEG CCM06 and Schneider Blokset — and the IEC 61439-1/2 standard governing low-voltage switchgear and controlgear assemblies. Every decision is deliberate and auditable: the drawer size comes from a power × starter rule, the components come from commercial series, and the column assembly follows a height-based packing.

The drawer: module by starter and power

The basic unit of the MCC is the drawer, a standardized (usually withdrawable) compartment that houses one motor’s starter. Its module — 1X, 2X, 3X or 4X — sets the height it occupies (200, 300, 400 and 600 mm respectively) and the room for components.

The size grows along two axes: the load power and the starter complexity.

• Feeder (no motor, distribution only): the criterion is current. In ≤ 63 A → 1X; ≤ 160 A → 2X; ≤ 400 A → 3X; above → 4X.
• DOL (direct-on-line): power sets the base. kW ≤ 7.5 → 1X; ≤ 30 → 2X; ≤ 75 → 3X; above → 4X.
• DOL reversing: adds one module to the DOL base (two contactors + interlock take more room), capped at 4X.
• Soft starter and VFD: have a 2X floor, because the electronic component does not fit in 1X; they rise to 3X/4X with power.

This rule reflects the physical reality of the catalog: a 30 kW reversing starter does not fit in the same bay as a 30 kW DOL, and a drive always demands more volume and more depth.

Components of each drawer

With the size set, the tool selects the switching and protection components per load, always rounding up to the first value of a commercial series:

• Motor circuit breaker (WEG MPW / Schneider GV): selected at 1.15 · In, to absorb the starting current and give the thermal trip range to set to the service factor. The series runs from 1.6 to 160 A.
• AC-3 contactor (WEG CWB / Schneider LC1): selected from the rated current In in utilization category AC-3 (squirrel-cage motors), from 9 to 630 A.
• Relay: thermal or intelligent protection relay (IR).

The composition changes with the starter: DOL takes motor breaker + contactor + relay; the reversing starter takes two contactors + interlock; the soft starter takes the electronic module + bypass contactor; and the VFD takes the drive in place of the switching contactor. The feeder takes only a distribution breaker sized by current.

The demand current and the incoming cubicle

No MCC is sized from the raw sum of rated currents — that would assume every load at full load at the same time. What sizes the assembly is the demand current:

I_dem = (Σ In_i · fd_i) · fs

where each load enters with its demand factor fd (how much it actually draws) and the whole assembly gets a diversity factor fs. It is this current that selects:

• the main breaker, in 3 poles, rounded up in the moulded-case series (16 to 1000 A);
• the main busbar cross-section, S = I_dem / J, with J between 1.5 and 2.5 A/mm² for copper in a ventilated assembly.

The incoming cubicle (the first column) concentrates the main breaker, the control transformer (mandatory), and optionally the power transformer (step-down), the heating transformer (if any motor has an anti-condensation heater), the 24 Vdc supply and the terminal blocks. The heights of these items must fit the usable column height; when they overflow, the method raises an “incoming column full” warning.

The short-circuit rating

Selecting rated currents is not enough — the MCC must survive a short circuit. The prospective fault current at the installation point, computed per IEC 60909 from the transformer and network impedance, sets the breaking capacity (Icu) of the breakers and the withstand (Icw) of the busbar.

The tool rounds the prospective Icc up in the commercial series (3 / 4.5 / 6 / 10 / 15 / 25 / 36 / 50 / 65 kA). This is where undersizing is most dangerous: a busbar with a kA below the prospective value may not contain a dead fault, and that is a safety failure, not just a performance one.

The column assembly

With drawers and incoming cubicle defined, the method packs everything into columns by a greedy height criterion:

1. Column 0 is the incoming cubicle, with its own items.
2. The load drawers (and the network drawer, if there are managed switches and remote I/O) are stacked into drawer columns: while it fits the usable height, the drawer goes in the current column; when it overflows, a new column is opened.
3. The free space of the last column is filled with spare drawers (1X), and the method guarantees at least one required spare — opening a new column if needed. A spare is an expansion requirement of good practice and IEC 61439.

In the end, the total width is the sum of the column widths, the assembly height is the usable height plus base/roof (~300 mm), and the depth is 600 mm in the general case, rising to 800 mm when there is a VFD or soft starter (deep drawers).

Practical design considerations

• Always apply demand and diversity: sizing from the sum of rated currents inflates the main breaker, the busbar and the column count.
• Respect the 2X floor of electronic starters and the 800 mm depth — a drive does not fit in a DOL bay.
• Check the kA first: the short-circuit rating is the first safety verification; always round up.
• Reserve space: at least one free 1X drawer, ideally 20 % of the column, for the next load.
• Count the incoming cubicle in the usable height: the control transformer, supply and terminal blocks consume real height and cannot be forgotten.

Following this chain — drawer size by starter, components by commercial series, demand current, main breaker and busbar, short-circuit rating and column packing — yields an MCC that is numerically rigorous and meets IEC 61439 on site.