MCCB in Main LV Distribution Boards

What is the role of an MCCB in a main LV distribution board? In a main low-voltage distribution board, molded case circuit breakers (MCCBs) most often serve as the incomer, or as outgoing feeder breakers downstream of an air circuit breaker (ACB), switching and protecting current in the 100 A to 1600 A class per IEC 60947-2. Get the incomer/feeder pairing wrong and the board either nuisance-trips on inrush or fails to clear a fault before the busbar's short-time rating is exceeded. This article covers incomer versus feeder duty, coordinating an MCCB incomer with an upstream ACB, discrimination down to sub-boards, matching busbar bracing to breaker Icu, and the frame and current classes panel builders actually specify.

Incomer Duty vs Outgoing Feeder Duty

An MCCB incomer sees the full connected load of the board, or as much of it as diversity allows, and its trip settings have to sit above every downstream device it protects. An MCCB feeder sees one sub-board or one large load — a motor control center tap, a sub-distribution board, a transformer secondary — and its settings sit below the incomer's, with margin for discrimination. For a step-by-step approach to that sizing sequence, the MCCB selection checklist walks through it in order.

Both jobs use the same hardware family. What changes is the trip unit and the setting range. On an ABB Tmax XT incomer rated 400-800 A, an Ekip Touch or Hi-Touch electronic trip gives LSIG protection (long-time, short-time, instantaneous, ground fault) with adjustable time bands — the short-time delay is what lets the incomer ride through a downstream fault long enough for the feeder breaker to clear first. A feeder on the same board, sized 63-250 A, can run a simpler Ekip Dip dip-switch trip or even a TMD thermal-magnetic unit if it has nothing downstream to coordinate with.

Same logic applies across Schneider ComPact NSX and Siemens Sentron 3VA molded case circuit breakers: the incomer usually carries the electronic trip (Micrologic 5/6/7, or ETU4xx/6xx/8xx) because only electronic units offer the short-time delay needed for time-graded discrimination. Thermal-magnetic trips on feeders are fine when the feeder is the last device before the load, with nothing downstream to coordinate with.

Coordinating the MCCB Incomer With an Upstream ACB

Many main LV boards aren't fed directly from a transformer — they're fed from a switchboard where an ACB (ABB Emax2, Schneider Masterpact MTZ, Siemens 3WA) sits between the transformer and the MCCB incomer. The ACB carries a short-time withstand rating (Icw) that MCCBs generally lack, or carry only at a fraction of Icu. That difference is the whole point of the pairing: the ACB can hold a fault current for a set time, typically 0.1-1 s, without opening, while the MCCB incomer or feeder below it clears the fault instantaneously or on a short delay.

This is why the ACB almost always sits at a higher protection tier even though its rated current can be lower than the combined MCCB feeders below it — its job is time-grading, not raw current capacity. Set the ACB's short-time pickup and delay so it stays closed through anything the MCCB tier below can clear on its own, and only opens for faults that tier fails to clear.

What we see in the field: panel builders sometimes try to skip the ACB tier to save panel depth and cost, running the MCCB incomer straight off the transformer. That works on smaller boards, but once the prospective fault current at the incomer approaches the top of the MCCB's breaking-capacity class, the ACB tier comes back — not for current rating, but because it is the only device with an Icw high enough to ride through a downstream fault without instantaneous tripping.

Selectivity Down to Sub-Boards

Discrimination between the MCCB incomer and any downstream MCCB feeder falls into two categories: total discrimination, where the downstream breaker clears the fault at every current up to its full breaking capacity without the incomer tripping, and partial discrimination, where that holds only up to a stated current threshold (Is) — above it, both breakers may trip. The mechanics are covered in more depth in MCCB discrimination and selectivity.

Total discrimination between two MCCBs of the same family is achievable at moderate current ratios, but current-limiting MCCBs are fast — often sub-half-cycle at high fault currents — which makes pure time-grading difficult once both devices in the pair are MCCBs. Manufacturers publish discrimination tables for their own verified combinations rather than a general formula, because the result depends on the specific trip curves of both devices, not just their current ratings.

In practice, going from an MCCB incomer to an MCCB sub-board feeder rarely achieves total discrimination at the high end of the incomer's range — this depends on the exact trip curve pairing, and some engineers push for zone-selective interlocking (ZSI) between electronic trips instead of relying on current-based grading alone. ZSI lets the downstream device signal upstream "I'm handling this," holding the incomer's instantaneous function back without adding a time delay for every fault.

Busbar Bracing vs Breaker Icu

The busbar's short-circuit withstand rating and the incomer breaker's Icu both have to clear the prospective fault current at that point in the board — but they're not the same number, and they don't automatically match. A busbar rated for 50 kA rms/1 s paired with an 85 kA Icu incomer is a mismatch in the other direction: the breaker will interrupt the fault, but if it doesn't do so fast enough, the busbar sees current above what it was braced for during the interruption time.

The same I_k″ value that sizes the breaker's Icu also has to sit under the busbar's rated peak withstand current and rms short-time withstand current. On a main board fed by a large transformer, I_k″ at the incomer can run well above 65 kA — pushing toward the top classes on an ABB Tmax XT7 (V class, roughly 200 kA at 415 V) or a Siemens 3VA2 high-frame ETU. If the busbar system wasn't specified for that fault level, upsizing the breaker's Icu doesn't fix anything; the busbar becomes the limiting component.

This is where panel builders push back on generic "upgrade the breaker" fixes. A higher Icu incomer costs more and does nothing if the copper or the bus-support system underneath it hasn't been re-verified for the new fault level — type-tested assemblies specify busbar bracing as part of the verified combination, not as an independent spec a builder can swap freely.

Typical Frame and Current Choices

Incomer sizing on a main LV board usually starts from the transformer or upstream ACB rating and applies a diversity factor for the connected feeder load, rather than summing every feeder's full nameplate current. A board with six 100-250 A feeders rarely needs an 800 A+ incomer if the feeders aren't all loaded simultaneously — but the incomer's breaking capacity still has to clear the full prospective fault current, regardless of diversity on the load side. The full frame-and-fault-current sizing sequence is laid out in the MCCB engineering guide.

Frame choices in practice: an ABB Tmax XT5 (400/630 A) or XT6 (630/800 A) incomer with Ekip Hi-Touch covers most mid-size main boards; larger installations move to XT7, to 1600 A. Schneider boards commonly use NSX400 or NSX630 with Micrologic 6 or 7 at the incomer for LSIG plus earth-fault protection. Siemens boards use 3VA2 frames with ETU for the same reason — only the electronic trip range gives the ground-fault and short-time functions an incomer needs.

Outgoing feeders scale down from there: 100-250 A frames (ABB XT2/XT3, Schneider NSX100/160/250, Siemens 3VA1 or lower 3VA2) cover most sub-board and MCC taps, with thermal-magnetic trips where the feeder doesn't need to coordinate with anything downstream, and electronic trips where it does.

Rated voltage rarely drives the frame choice on a 400/415 V board — the relevant MCCB families from all three brands cover that range comfortably. Breaking capacity class drives it instead: a board fed by a large transformer with low source impedance pushes the incomer toward the H, S, or L class on ABB, the S or L class on Schneider, or the higher 3VA2 frames on Siemens, independent of the current rating itself. See MCCB voltage and current ratings for the full frame/current matrix, and Icu, Ics, and Icw ratings for how the breaking-capacity classes are defined.

Conclusion

An MCCB's job in a main LV distribution board changes with its position: incomers need electronic trips, adjustable short-time delay, and a breaking capacity matched to the prospective fault current at the board, while feeders need only enough protection and discrimination margin to clear their own load without tripping the incomer above them. Neither setting is correct in isolation. Incomer, feeder, upstream ACB, and busbar bracing all have to be sized against the same fault-current study, not selected as separate line items.