How to Size ABB Emax 2 Circuit Breaker for a Distribution Panel
What is ABB Emax 2 circuit breaker sizing? ABB Emax 2 circuit breaker sizing is the engineering process of matching a 400–6300 A, IEC 60947-2-compliant air circuit breaker frame and Ekip trip unit to a distribution panel's verified load current, prospective short-circuit current (Icw/Icu), and ambient derating conditions. Undersizing the frame risks thermal overload and contact welding under fault conditions, while an oversized or misconfigured Ekip protection curve can fail to clear faults within the coordination window, exposing downstream equipment and voiding selective protection. This guide covers design load current calculation, prospective short-circuit current determination, Emax 2 frame selection, thermal derating for enclosed panels, and Ekip trip unit configuration.
If you have ever opened a switchgear cubicle in a hot Saudi cement plant in July and watched the temperature inside the panel climb past 55 °C, you already know that nameplate ratings are only the starting point. The Emax 2 is a forgiving breaker — but only if you size it for the actual conditions, not the catalog conditions.
What Sizing an Emax 2 Actually Means
Sizing an ABB Emax 2 is not picking the next-largest frame from a catalog. It is a four-axis decision: rated uninterrupted current (Iu), service breaking capacity (Ics), trip unit protection functions, and the mechanical/thermal envelope of the panel. Get one of those wrong and the breaker either trips on a Monday morning when the chiller restarts, or it sits there during a fault and lets the busbar evaporate.
In our experience, procurement teams default to matching nameplate amps to the transformer LV current and call it done. That works for a feeder pillar in a warehouse. It does not work for a 2 MVA transformer feeding a paper mill where the variable-speed drives push 3rd and 5th harmonic currents that load the breaker thermally well above what the RMS ammeter shows.
The Four Questions You Have to Answer Before You Pick a Frame
Before opening the ABB catalog, the engineer needs four numbers on the desk: the calculated load current (IB), the prospective short-circuit current at the point of installation (Icc), the ambient temperature inside the panel (not the room), and the duty profile (continuous, cycling, motor inrush dominated). Skip any of these and you are guessing.
For complete technical data and selection criteria of the ABB Emax 2 air circuit breaker family, refer to the ABB SACE Emax 2 product documentation, which provides catalog ratings, derating curves, and Ekip trip unit configuration guidance compliant with IEC 60947-2.
Step 1: Calculate the Design Load Current
The first calculation in any ABB Emax 2 sizing exercise is straightforward but engineers often overlook the diversity factor. A distribution panel does not see the sum of all connected loads at full power simultaneously — except in the rare case of a process line where every motor must run together. For a typical plant MCC or sub-distribution board, applying a diversity factor of 0.7–0.85 against the connected load is realistic.
Formula: Design load current for three-phase distribution panel — Source: IEC 60364-5-52, Annex B
IB = (Ptotal × kd) / (√3 × Un × cos φ × η)
| Symbol | Description | Unit |
|---|---|---|
| IB | Design load current | A |
| Ptotal | Total connected active power | W |
| kd | Diversity factor (0.6–1.0) | — |
| Un | Nominal line-to-line voltage | V |
| cos φ | Power factor at full load | — |
| η | Combined efficiency of downstream loads | — |
Worked example. A 1250 kW pumping station at 400 V, cos φ = 0.85, η = 0.94, diversity 0.8. The result: IB = (1,250,000 × 0.8) / (1.732 × 400 × 0.85 × 0.94) = 1808 A. That points to an Emax 2 E2.2 frame, not E1.2, even though a quick "1250 kW divided by 400 V times root three" calculation would land you near 1800 A and tempt you to specify an 1800 A breaker that does not exist as a standard rating.
The next standard rating up is 2000 A, which means an ABB 1SDA071021R1 E2.2B 2000 Ekip Dip LI 3P with horizontal rear (HR) terminals — and you set the L threshold of the trip unit at I1 = 0.9 × In = 1800 A so the protection actually matches the calculated load.
Step 2: Determine the Prospective Short-Circuit Current
The second number — Icc at the installation point of the ABB Emax 2 — drives the breaking capacity selection. Get the transformer's secondary short-circuit current from its nameplate (or calculate it from impedance), then subtract the cable impedance from transformer to panel.
For a 2000 kVA, 400 V transformer with Ucc = 6%, the maximum three-phase symmetrical fault at the LV terminals is approximately:
Icc ≈ Sn / (√3 × Un × Ucc) = 2,000,000 / (1.732 × 400 × 0.06) = 48,113 A ≈ 48 kA
That is the fault level at the transformer terminals. By the time the fault propagates through 15 m of 2 × 240 mm² copper cable to the main distribution panel, you are typically looking at 38–42 kA. An Emax 2 B-version with Icu = 42 kA at 400 V is the natural fit. For longer runs, the C-version (Icu = 50 kA) gives margin; for direct-coupled bus duct from a larger transformer, you may need the N-version (66 kA) or H-version (100 kA).
A common mistake is using the transformer terminal Icc to spec every breaker in the building. The fault current decays through cable impedance — the further downstream you go, the lower the prospective Icc, and the cheaper the breaker. We have seen specifications where a sub-distribution board 60 m from the main switchboard was specified with 100 kA breakers when the actual fault level at that point was 22 kA. That is procurement waste at scale.
Why Service Breaking Capacity Matters More Than Ultimate
Icu is what the breaker can interrupt once before going to the workshop. Ics is what it can interrupt repeatedly and stay in service. For a distribution panel feeding production-critical loads, you want Ics ≥ expected Icc — not just Icu ≥ Icc. The Emax 2 B-version's 100% Ics/Icu ratio is the reason it dominates in mining, oil & gas, and data center applications. For a deeper comparison of how this stacks up against Schneider's offering, see the ABB Emax 2 vs Schneider MasterPact MTZ technical comparison.
Step 3: Select the Frame Size
The ABB Emax 2 family splits into four physical sizes — E1.2, E2.2, E4.2, and E6.2 — with rated currents up to 1600 A, 2500 A, 4000 A, and 6300 A respectively. For most distribution panels in the 630–2000 A range, you are choosing between E1.2 and E2.2.
| Criteria | E1.2B 1000 A | E1.2B 1600 A | E2.2B 2000 A |
|---|---|---|---|
| Reference SKU (Ekip Dip LI, 3P, fixed) | 1SDA070781R1 | 1SDA070861R1 | 1SDA071021R1 |
| Rated current Iu at 40 °C | 1000 A | 1600 A | 2000 A |
| Icu / Ics at 415 V (B-version) | 42 / 42 kA | 42 / 42 kA | 42 / 42 kA |
| Width (3-pole, fixed) | 324 mm | 324 mm | 404 mm |
| Depth (with terminals) | 302 mm | 302 mm | 302 mm |
| Operational endurance | 12,500 ops | 12,500 ops | 10,000 ops |
| Typical use | Sub-MDB, MCC incomer | MDB on 1 MVA Tx | MDB on 2 MVA Tx |
Notice that the E1.2 frame holds the same physical dimensions across 630, 800, 1000, 1250, and 1600 A ratings. That is a deliberate ABB design choice — you can swap an 1SDA070701R1 (630 A) for an 1SDA070861R1 (1600 A) in the same panel cutout when load grows, without redesigning the cubicle. This is one of the most underused planning tools in panel design. For full dimensional and rating data, the Emax 2 technical specifications reference covers every frame.
Step 4: Apply Thermal Derating for the Real Panel
This is where most sizing exercises fall apart. The catalog rating for the ABB Emax 2 is at 40 °C ambient inside the enclosure with the breaker in free air. Inside an IP54 distribution panel in a Gulf-region MCC room, the internal temperature can reach 55–60 °C. ABB publishes derating curves in the technical catalog (1SDC200023D0205); use them.
Rule of thumb that holds up in the field: for every 10 °C above 40 °C internal ambient, derate In by 5–7%. So an E1.2B 1600 A inside a 55 °C panel is realistically a 1450 A breaker. If your calculated load is 1500 A, you have just sized too tight and the breaker will run hot, the contacts will oxidize, and within two years you will have intermittent thermal trips that nobody can diagnose because the trip unit log shows "L tripped at 102% In" — which looks fine on paper.
Step 5: Configure the Ekip Trip Unit
The ABB Emax 2 ships with the Ekip electronic trip unit family — Dip, Touch, Hi-Touch, and G versions. For a standard distribution panel feeder, Ekip Dip LSI (Long-time, Short-time, Instantaneous) covers most needs. For motor-dominant panels, you want LSIG with ground-fault protection. For incomers requiring zone selective interlocking, you need Ekip Touch with the dedicated ZSI module.
The L (long-time) function is the thermal overload protection. The S (short-time) function handles short circuits with intentional time delay for selectivity. The I (instantaneous) function handles bolted faults with no delay. Setting these correctly is where engineering judgment separates good panels from bad ones.
Setting L Protection
I1 (the L threshold) should equal IB / In, rounded to the nearest available step. For our 1808 A example on a 2000 A frame: I1 = 1808/2000 = 0.9. The trip time at 6 × I1 (the standard reference) determines selectivity with downstream MCCBs — typically set to 12 s for an incomer to give the downstream 1 s curve enough time to clear.
Setting S Protection
I2 (the S threshold) is usually set at 5–8 × I1 for distribution panels. This catches short circuits but lets motor inrush pass. For a panel feeding multiple motors, set the S delay (t2) to 0.2–0.4 s with I²t = ON to coordinate with downstream selectivity coordination requirements per IEC 60947-2 Annex A.
Setting I Protection
I3 (the instantaneous threshold) should be set above the highest expected through-fault current that the downstream protection can clear, typically 10–12 × In. Setting it too low destroys selectivity — the incomer will trip simultaneously with the feeder, and the whole panel goes dark for a fault on a single cable. We have seen this in petrochemical plants where a contractor set I3 = OFF "to be safe" — and a transformer-side fault took out the entire MCC.
Step 6: Verify Selectivity with Downstream Devices
A distribution panel almost never operates in isolation. The ABB Emax 2 incomer must coordinate with downstream air circuit breakers, MCCBs (Tmax XT or T-Max), miniature circuit breakers, and residual current devices on outgoing feeders.
ABB publishes selectivity tables (the famous DOC software output) that pre-verify common combinations. For example, an E1.2B 1600 A with Ekip Dip LSI is fully selective with Tmax XT3N 250 A up to 42 kA when the time delays are set correctly. Use these tables as the starting point — but always verify with a dynamic time-current curve plot in DOC or ETAP for non-standard configurations.
In practice, what we see in the field is that selectivity is verified at design, then the commissioning team adjusts I3 "to make it stop tripping" and never updates the documentation. Six months later, a fault clears at the wrong level. Document every trip unit setting in the panel record, photograph the Ekip display, and lock the cover.
Step 7: Account for Special Applications
Data Centers and N+1 Redundancy
Data center MDBs operate at 40–60% load even at full IT utilization, but they need to handle the transient load swing during a UPS bypass transfer or generator pickup. Size the ABB Emax 2 for the peak transfer current, not the steady-state load. The full methodology is laid out in our Emax 2 in data center MDB design reference.
Motor-Dominant Distribution Panels
If the panel feeds an MCC where direct-on-line motor starting accounts for more than 30% of the connected load, the inrush current can hit 6–8 × FLC for 100–300 ms. The Ekip trip unit handles this if you enable the "motor protection" curve and set t2 ≥ 0.4 s. A common mistake: engineers select the LI version (no S function) for cost savings, then have to replace it with LSI when nuisance tripping starts. Spec the 1SDA070782R1 Ekip Dip LSI from day one if motors are involved. For a deeper look at why these trips happen, the Emax 2 nuisance tripping diagnostic guide covers root causes.
Harmonic-Heavy Loads
VFDs, LED lighting, and switching power supplies generate significant 3rd, 5th, and 7th harmonic currents. The Ekip trip unit measures true RMS, so the L protection responds correctly — but the breaker contacts and busbars heat up more than the RMS reading suggests due to the skin effect at higher frequencies. Apply an additional 10% derating for THDi > 15%.
Step 8: Mechanical and Panel Integration
The ABB Emax 2 has to physically fit, with proper clearance for arc venting, terminal access, and removal of the racking trolley if it is a draw-out version. IEC 61439-1 requires verified temperature rise for the assembly — meaning you cannot just install a 2000 A breaker in any cubicle and assume it will pass type testing. The panel builder (e.g., ABB SACE MNS or third-party assembler) needs the breaker's heat dissipation data, which for an E2.2 2000 A is approximately 380 W at full load.
For fixed-mount versions (suffix F), the terminals are bolted to the breaker permanently. For draw-out (suffix W), thebreaker plugs into a fixed cassette and can be racked out for maintenance without disconnecting cables. Draw-out costs 30–40% more but pays for itself the first time you need to do a five-year contact inspection on a critical incomer at 2 a.m.
Terminal orientation matters more than people think. The HR (horizontal rear) terminals on an 1SDA070981R1 E2.2B 1600 HR are designed for direct busbar coupling at the back of the panel — the standard configuration for switchboards built to ABB's MNS or comparable platforms. Front-flat (F F) terminals on the 1SDA070821R1 E1.2B 1250 suit cable connections from below in compact distribution panels. Specify the wrong orientation and the panel builder either cannot accept the breaker or has to fabricate custom transition pieces that compromise the IP rating.
Arc Flash and Working Distance
IEEE 1584-2018 calculations for incident energy depend heavily on clearing time. An Emax 2 with Ekip Touch and instantaneous protection set at 8 × In typically clears a 25 kA fault in under 50 ms — yielding incident energy below 8 cal/cm² at 600 mm working distance for a 400 V system. Drop the I3 setting and you cut the incident energy roughly in half. This is the engineering case for never setting I3 = OFF on an incomer.
Step 9: Specify Auxiliary Functions and Communication
The Ekip trip unit on the ABB Emax 2 accepts modules for Modbus RTU, Modbus TCP, Profibus DP, Profinet, EtherNet/IP, and IEC 61850. For a modern industrial distribution panel feeding a SCADA system, specify at least Ekip Com Modbus RTU — it gives you remote breaker status, current/voltage measurements, and trip event logs without adding external metering. The marginal cost is small compared to the value during fault investigation.
Auxiliary contacts (AUX), shunt trips (YO), undervoltage releases (YU), and motor operators (MOE/MOP) are configurable at order time. A common procurement mistake is to order the breaker first and the accessories later — ABB's lead times for retrofit accessories can run 8–12 weeks. Specify the full configuration in the original order.
Worked Example: Sizing an Emax 2 for a Cement Plant Sub-Station
Consider a real project we worked on: a grinding mill sub-station in a cement plant, 1600 kVA transformer at 400 V, feeding a single distribution panel that supplies a 1100 kW main mill motor (VFD-driven), three 75 kW auxiliary motors (DOL-started), and a 200 kVA lighting and small-power transformer.
Connected load calculation:
Mill motor at full load: 1100 kW / (0.94 × 0.95) = 1232 kW input. Auxiliaries: 3 × 75 / 0.92 = 245 kW. Lighting transformer: 200 × 0.8 = 160 kW. Total = 1637 kW.
Design current: IB = 1,637,000 / (1.732 × 400 × 0.88) = 2685 A. Apply diversity 0.85 (mill rarely runs simultaneously with peak auxiliary load): 2685 × 0.85 = 2282 A.
Frame selection: Next standard rating above 2282 A is 2500 A. Select E2.2B 2500 A at 40 °C. Internal panel ambient measured at 50 °C — apply 6% derating: usable In = 2350 A. Margin over IB: 2350/2282 = 1.03. Tight but acceptable for this duty profile.
Short-circuit calculation: Transformer Icc at terminals = 1600 kVA / (1.732 × 400 × 0.0625) ≈ 37 kA. Cable run 8 m, 2×185 mm² Cu — fault at panel ≈ 33 kA. Specify Icu ≥ 42 kA: B-version is sufficient.
Trip unit: Ekip Touch LSIG (G for ground fault, required because of the 200 kVA isolation transformer downstream). I1 = 0.92 × 2500 = 2300 A. I2 = 6 × I1, t2 = 0.3 s with I²t ON. I3 = 10 × In = 25 kA (above downstream MCCB clearing range, below breaker Icw).
Communication: Ekip Com Modbus TCP for integration with the plant DCS.
This is the level of detail every Emax 2 specification should contain. Anything less and you are leaving decisions to the panel builder.
Common Sizing Mistakes We See in the Field
After two decades of plant audits, the same errors recur. Engineers size for nameplate kW without considering the service factor on motors. Procurement substitutes a B-version for a C-version "because the price was better" without checking that the fault level still fits. Panel builders install the breaker in a cubicle that was sized for a smaller frame and the temperature rise test fails. The Ekip trip unit ships with factory defaults (I1 = In, I3 = OFF) and nobody changes them at commissioning.
The Emax 2 is not the problem in any of these cases. The breaker is engineered to a high standard. The problem is treating sizing as a single calculation rather than a system-level decision that touches load analysis, fault calculation, thermal design, selectivity, and lifecycle planning. For the underlying physics of how these breakers actually interrupt fault current, the introduction to the ABB SACE Emax 2 features and benefits is worth a read.
Procurement Considerations
From a procurement standpoint, three factors drive total cost of ownership: lead time, spare parts strategy, and accessory configuration. ABB Emax 2 standard configurations from European stock typically ship in 4–6 weeks; non-standard combinations (special trip unit firmware, IEC 61850 modules) can run 12–16 weeks. Plan accordingly — especially for greenfield projects where the breaker is on the critical path for switchgear factory acceptance testing.
Spare parts: keep at least one trip unit and one set of arc chutes per frame size on site for installations where downtime cost exceeds €10,000/hour. The Ekip trip unit is field-replaceable in under 30 minutes; arc chute replacement after a hard fault is a 2-hour job.
For panels feeding sensitive control circuits, complement the Emax 2 incomer with downstream protective relays for specialized functions (under-frequency, reverse power, neutral displacement) that the Ekip trip unit does not natively cover.
Related Reading
- ABB Emax 2 Full Technical Specifications: Current Ratings, Breaking Capacity and Dimensions
- ABB Emax 2 Selectivity Coordination: Calculation Method and Selectivity Tables
- ABB Emax 2 in Data Centers: MDB Design, Redundancy and Uptime Considerations
- ABB Emax 2 Nuisance Tripping: Root Causes, Diagnostic Steps and Fixes
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Frequently Asked Questions
What is the difference between Emax 2 B, C, N, H, and L versions?
The letter designates the breaking capacity class at 415 V AC. B = 42 kA (standard distribution), C = 50 kA, N = 66 kA, H = 100 kA, L = 130 kA (special applications). All versions in the Emax 2 family share the same physical dimensions and trip unit options — you are only paying for arc chute capacity and contact material upgrades. For most distribution panels fed by transformers up to 2.5 MVA, the B-version is sufficient.
Can I use an Emax 2 frame larger than required and just set the trip unit lower?
Yes, this is a legitimate strategy and one we often recommend for installations with planned capacity growth. An E1.2B 1600 A set at I1 = 0.4 (640 A effective) gives you the same protection as an E1.2B 800 A but with future headroom. The cost premium is 15–25%, far less than a panel rebuild.
How do I size the Emax 2 for a panel feeding mostly variable-frequency drives?
Apply two corrections: an additional 10–15% derating for harmonic content (THDi typically 25–35% on the supply side of a 6-pulse VFD), and verify that the Ekip trip unit's I2t curve coordinates with the VFD's input fuse or DC-bus protection. The Ekip trip unit measures true RMS, so basic protection works correctly, but selectivity with VFD internal protection requires careful curve plotting. Consult the Emax 2 selectivity coordination methodology for the procedure.
What ambient temperature should I use for sizing — the room or the panel interior?
Always the panel interior, measured at the level of the breaker terminals during normal operation. The room temperature plus 10–20 °C is a reasonable estimate for IP54 enclosures with natural ventilation, but for definitive numbers commission the panel builder to perform a temperature rise verification per IEC 61439-1 Clause 10.10. We have seen rooms at 35 °C with panel interiors at 58 °C — the difference matters.
Do I need ground-fault protection (the G in LSIG) on every Emax 2?
No. Ground-fault protection is required for incomers on TN-S systems above 400 A per local codes (NEC Article 230.95 in North America, equivalent IEC requirements vary by country) and for circuits where earth-fault current cannot be reliably detected by phase overcurrent protection. For most three-phase distribution panels, LSI is adequate; specify LSIG when feeding step-down transformers, long cable runs, or systems with high impedance grounding.
How often should the Emax 2 trip unit settings be re-verified?
At commissioning, after every fault clearance, after any downstream load change exceeding 20%, and on a five-year preventive maintenance cycle minimum. The Ekip trip unit retains its settings through power cycles, but the configuration should be documented in the panel logbook with date, signature, and rationale for each setpoint.
Conclusion
Sizing an ABB SACE Emax 2 for a distribution panel is not a single calculation — it is a methodology that integrates load analysis, short-circuit calculation, thermal derating, trip unit configuration, selectivity verification, and panel mechanical integration. Get any one of these wrong and the breaker either fails to protect the installation or causes operational headaches that no amount of field tuning will fix.
The good news is that the methodology is repeatable. Calculate IB with realistic diversity. Determine Icc at the actual installation point, not the transformer terminals. Pick the next standard frame above IB, apply temperature derating, and tune the Ekip trip unit thresholds to match the real load. Verify selectivity with downstream devices using ABB's published tables or DOC software. Document every setting.
For procurement teams, the Emax 2 family rewards careful upfront specification: order the full configuration with the right trip unit, communication module, and accessories from the start, and you avoid the 8–12 week lead times that retrofit changes incur. For engineering teams, the breaker's wide rating range within shared frame dimensions — from the 630 A E1.2B through the 800 A, 1000 A, and up to 1600 A on the same footprint — is a planning tool worth using.
For the complete selection methodology including maintenance schedules, retrofit options, and long-term lifecycle planning, refer to the ABB SACE Emax 2 air circuit breaker engineering guide. The frame on the catalog page is the easy part. Sizing it for your actual installation — that is the engineering.
