The source sends power through the main disconnecting and overcurrent device, across the MCC bus, through each branch protective device and starter, and finally to the motors. Follow that path conductor by conductor. The 28.26 kW three-phase motor creates balanced line-current demand; the 13.42 kW single-phase motor adds current only to the conductors that supply it. The main device must protect the feeder and coordinate with the motor branches while remaining within the MCC rating.
Which sizing approach fits this MCC?
| Approach | Use | Limitation |
|---|---|---|
| Convert total kW at unity power factor and add 20%–25% | Preliminary load estimate | Ignores actual voltage, power factor, efficiency, phase distribution, motor starting, and applicable motor-circuit rules |
| Calculate each load, assign it to supply conductors, then apply the governing code | Final feeder and main-device selection | Requires nameplate data, supply topology, conductor data, branch protection, and operating sequence |
| Size the feeder near the MCC bus rating | Planned expansion where source capacity and conductors support it | The bus rating alone does not establish feeder ampacity or the main-device rating |
Use the load-and-code method for the installed equipment. Add documented spare capacity only after calculating the present conductor currents. A 20%–25% adder is a planning heuristic, not a substitute for the motor-feeder rules in NEC Article 430 or the corresponding local IEC-based requirements. Likewise, an MCC bus that is 600 A or more does not automatically call for a 600 A feeder.
Where does the single-phase current enter the path?
Layer one first: identify the actual conductors feeding the 13.42 kW load. Its voltage cannot be inferred from the three-phase system voltage.
| Connection | Voltage used | Conductors carrying added current | Sizing consequence |
|---|---|---|---|
| Line-to-neutral | Measured or nameplate line-to-neutral voltage | One phase conductor and the neutral | One phase becomes more heavily loaded; neutral loading must also be checked |
| Line-to-line | Measured or nameplate line-to-line voltage at that load | Two phase conductors | Two line currents change; calculate all three line currents |
| Separate single-phase source or transformer | Secondary voltage | Secondary conductors; the primary sees transformer input demand | Reflect the load through transformer rating, efficiency, and connection before combining it with upstream currents |
Do not treat the two kW ratings as one balanced three-phase load. The main device carries conductor current, so the controlling value is the most heavily loaded line after the single-phase branch has been assigned. If power factors differ, the exact combined line current is a phasor sum; arithmetic addition is a conservative screening calculation only when both currents are defined on the same supply side and in amperes.
What information is required before calculating current?
| Item | Where to obtain it | Why it controls the result |
|---|---|---|
| Three-phase line-to-line voltage | Supply documentation and measurement | Appears directly in the three-phase current equation |
| Single-phase voltage and connection | Load nameplate and wiring diagram | Determines current magnitude and which lines carry it |
| Nameplate full-load current | Each motor nameplate | Provides the equipment operating-current reference |
| Power factor and efficiency | Nameplate or manufacturer data | Required when converting shaft-output kW to electrical input current |
| Starting method and simultaneous operation | Control narrative and starter configuration | Determines the starting-current and voltage-drop case |
| Feeder conductor ampacity and installation conditions | Drawings and field inspection | The protective device must protect the installed conductors |
| MCC bus and short-circuit ratings | MCC nameplate | Set upper equipment limits and interrupting-duty requirements |
| Branch protective devices | One-line diagram and device labels | Required for feeder-protection and coordination checks |
The stated 28.26 kW and 13.42 kW ratings are insufficient by themselves. Establish whether each is mechanical output or electrical input. Motor kW commonly represents rated mechanical output, in which case efficiency belongs in the conversion.
How are the two motor currents calculated?
For the three-phase motor, using line-to-line voltage:
I3φ = 28.26 × 1000 / (√3 × VLL × PF3 × η3)
For the single-phase motor:
I1φ = 13.42 × 1000 / (V1φ × PF1 × η1)
Use decimal values for PF and efficiency η. If a listed kW value is electrical input power, omit efficiency from that equation. If both loads were electrical-input kW at unity power factor, the screening expressions would reduce to:
I3φ = 16,316 / VLL
I1φ = 13,420 / V1φ
Those expressions still cannot produce amperes until both voltages are known. The statement that kVA equals kW applies only at unity power factor. A motor with non-unity power factor draws more current than the unity-power-factor shortcut predicts for the same electrical kW.
After calculating the branch currents, build a line-current schedule. Start with the balanced three-phase current on L1, L2, and L3; then add the single-phase contribution to its actual conductors. Retain the highest resulting line current as the load-distribution case for conductor and device evaluation.
How should the main breaker or fuse be selected?
- Record the supply topology,
VLL, single-phase voltage, frequency, and grounding arrangement. - Read each motor’s nameplate current, kW definition, power factor, efficiency, and starting method. Reconcile calculated current with the nameplate value rather than replacing nameplate data with a unity-power-factor estimate.
- Map every branch to
L1,L2,L3, and neutral where present. Calculate the heaviest normal operating line. - Apply
NEC Article 430when the installation is governed by the NEC, or the applicable local IEC-based motor and feeder rules. Use the prescribed current basis and protective-device method from that governing document; a blanket 20%–25% margin does not complete this step. - Check feeder conductor ampacity after applying the installation’s temperature, grouping, termination, and other required corrections.
- Check motor starting. The main device must ride through permitted starts without nuisance operation while branch devices continue to clear branch faults selectively where the design requires coordination.
- Confirm that the selected device voltage rating, interrupting rating, and setting or fuse class suit the available fault duty and equipment ratings.
- Confirm that the main rating does not exceed the MCC main, bus, terminals, or feeder conductors. Treat expansion capacity as a separate design input supported by the source and conductor capacity.
How is the completed selection verified?
- Review the one-line diagram against the installed phase assignments and device labels.
- Recalculate
L1,L2, andL3for every permitted operating combination, including the combination that places the single-phase load on the most heavily loaded line. - Evaluate the worst permitted starting sequence for voltage drop and main-device operation.
- Compare the device selection with feeder ampacity, MCC ratings, branch protection, and available fault current.
- After commissioning, measure all line currents under the highest practical operating load. Investigate phase imbalance, current above the design basis, or unexpected neutral current before accepting the installation.
FAQ
Can I add the 13.42 kW load to the 28.26 kW load?
Add electrical demand only after converting each load with its own voltage, power factor, and efficiency. Then assign the single-phase current to its actual line conductors instead of treating 41.68 kW as a balanced three-phase load.
Does kW equal kVA for these motors?
Only at unity power factor. If the kW rating is mechanical output, efficiency must also be included when calculating electrical input current.
Can I size the main breaker 20%–25% above total current?
Use that only as a preliminary estimate. Final selection must follow NEC Article 430 or the applicable local IEC-based rules and must account for conductors, starting, branch protection, and fault duty.
Does a 600 A MCC bus require a 600 A feeder?
No. The bus rating is an equipment limit, not the calculated feeder demand. A 600 A feeder requires conductors, source capacity, terminals, protection, and fault ratings suitable for that design.
Can I verify the main-device size from kW alone?
No. Record both voltages, connection, nameplate currents, power factors, efficiencies, starting methods, and phase assignment; then perform the final verification by measuring L1, L2, L3, and neutral where fitted under the highest practical operating load.