Capture the full carried load
An upstream ATS may carry more normal-source load than remains connected on generator power. The smaller standby subset does not erase that normal duty.
Load-first sizing · Formulas + worked example
4 Steps to Calculate Automatic Transfer Switch Amperage
Define the load. Calculate current. Apply the right load rules. Verify the complete switch. To calculate automatic transfer switch amperage, start with the loads the ATS must carry in each operating state—not just generator kW or breaker handle ratings. A current result is a sizing input, not a final product approval.
4 documented stepsBoundary, conversion, load treatment and equipment verification.
2 source dutiesReview normal and alternate operation at the actual ATS location.
1 traceable exampleA 480 V case separates the arithmetic from final approval.
No automatic size choiceThe calculator returns amperes, not a catalog recommendation.
Original article: William · Published April 20, 2026 · Updated August 26, 2026.
SENTOP transfer-switch family shown for product context. Current ratings, load suitability and approvals must be verified for the exact model.
01 · DEFINEDraw the load boundaryIdentify what passes through the ATS in normal, standby and return operation.
02 · CONVERTUse the correct current equationKeep voltage, phase, input power and power factor attached to each result.
03 · APPLYDocument the load treatmentUse the governing continuous-load, motor and demand method without double-counting.
04 · VERIFYMatch the complete ATSCheck continuous duty, fault performance, sources, neutral, transition and installation.
The direct answer
Calculate the current requirement. Then verify the rating.
An automatic transfer switch (ATS) connects a load to one of the permitted sources. Its continuous ampere rating must suit the approved load basis and the conditions of the offered assembly. Generator capacity, feeder protection and short-circuit performance remain separate decisions.
Begin with an approved one-line diagram, a simplified map of the power system. Build the downstream load schedule, convert compatible inputs to current, and apply the method required by the project and jurisdiction. Then examine an actual ATS configuration at or above the resulting requirement.
Catalog frame size is not the full answer. A family label or an ampere number does not establish voltage, neutral switching, terminal capacity, fault rating or certification for the ordered configuration.
- ATS continuous current
- The current the specified assembly can carry under its stated thermal, enclosure and installation conditions.
- Calculated load / feeder basis
- The project current requirement after the applicable load, demand and motor treatment. This is not a sum of breaker ratings.
- Generator kW and kVA
- Real and apparent output capability under stated conditions. Running capacity does not by itself establish motor-start performance.
- Withstand and closing rating (WCR)
- Fault withstand/closing capability under specified voltage, duration and protective-device conditions. It is not continuous load current or an interrupting rating.
Planning guidance, not permission for electrical work. Final load calculations, product acceptance and installation require the responsible qualified project team and applicable authority. Do not open equipment, measure live terminals or perform a transfer test to obtain inputs from this article. Use approved records and the original equipment manufacturer (OEM) documentation.
Step 1 / establish the boundary
Define what the ATS carries in every operating state
The same building can have a whole-distribution transfer switch, a smaller critical-load feeder switch, or several coordinated switches. Their load boundaries are not interchangeable.
Record the normal and alternate source voltage, phase, frequency, wire arrangement and supply capacity. Identify the transfer location, service-entrance or feeder role, and the loads connected downstream. Equipment outside that boundary does not automatically belong in this calculation.
Record retained and shed loads
Document which loads transfer, which are blocked or shed, and when they reconnect. Take credit only for a supported, approved load-management scheme.
Check the actual distribution
Keep line-to-line and line-to-neutral voltage distinct. An unbalanced panel needs per-phase or per-leg review; a balanced average can hide the limiting current.
Resolve neutral and location
Confirm the grounding design, switched or unswitched neutral, source protection and installation environment. These determine configuration, not merely arithmetic.
Hold the calculation when the boundary is unknown. A photograph of a 200 A panel main or a generator data plate cannot establish every load carried by the ATS. Resolve the one-line and sequence of operation first.
Application basis: Cummins T011 transfer-switch application manual, covering transfer location, continuous current and system duty.
Step 2 / convert compatible inputs
Use the equation that matches voltage, phase and power
Use documented electrical input current where the applicable method calls for it. Otherwise convert input kVA, or input kW with an appropriate power factor. Do not substitute motor shaft output for electrical input.
Known kVA at the load voltage
I = 1,000 × kVA ÷ VUse the voltage across the stated single-phase load. A mixed 120/240 V panel requires separate leg checks unless balance is established.
Known total three-phase kVA
I = 1,000 × kVA ÷ (√3 × VLL)Use total three-phase apparent power and line-to-line voltage. The result is current in each line under the balanced assumption.
Known kW and input power factor
I = 1,000 × kW ÷ (V × PF)Power factor must describe the same load and operating point as the input kW. Do not assume it is always 0.8 or always 1.
Known total input kW and PF
I = 1,000 × kW ÷ (√3 × VLL × PF)A balanced three-phase relationship does not calculate the heaviest phase of an unbalanced load or the transient current at starting.
I = current in amperes (A) · V = voltage across a single-phase load · VLL = line-to-line voltage · kVA = apparent power · kW = real electrical input power · PF = power factor. For the same operating point, kVA = kW ÷ PF. Once you start with kVA, do not divide by PF again.
Same power, different current: 100 kVA at balanced three-phase 480 V is 120.28 A. At 208 V it is 277.57 A. Neither number includes load-rule adjustments, motor starting, neutral assessment or an ATS catalog selection.
Formula reference: Schneider Electric kVA calculation guidance. Use the stated electrical-input, voltage and balance conditions; nonlinear loads need appropriate input-current, true-PF and harmonic data.
Calculation aid / not a product selector
Convert a known load to preliminary current
This tool performs the equations above only. It does not add demand factors, continuous-load allowances, motor treatment, future capacity or automatic rounding to an ATS size.
The calculator runs locally in your browser and does not send these inputs to SENTOP. It is a reproducible conversion aid, not a design approval or a catalog selection service.
Step 3 / establish the design basis
Apply the required load treatment once, with its conditions
Running-current arithmetic is not automatically the governing design current. Classify the loads, identify the adopted calculation method and record which operating states may coincide.
Confirm the classification
Apply the allowance required by the governing method and equipment conditions. Do not label every load “continuous” or add 25% to every ATS calculation.
Support any reduction
A demand factor is a documented rule or design input, not an arbitrary discount. Do not add mutually exclusive operating states as though they always occur together.
Use the required current basis
The applicable method may call for a specified full-load current rather than the nameplate value. Keep this calculation allowance separate from starting-current analysis.
Retain the full scope
Include the other required motor currents and document future additions separately. Avoid counting a motor again inside a general-load kVA subtotal.
ILLUSTRATIVE FEEDER-STYLE METHOD — NOT A UNIVERSAL ATS FORMULA
Ibasis = 1.25IC + DF × IN + 1.25IM,max + ΣIM,otherIC is continuous non-motor current; IN is noncontinuous non-motor current; DF is its permitted demand factor; motor currents use the basis required by the selected method. Cummins T030 describes this type of mixed motor/non-motor feeder calculation. It illustrates a method, not the code adopted for every project. Do not multiply the assembled result by 1.25 again.
Method reference: Cummins T030, electrical-design guidance. Confirm the current applicable requirements with the qualified designer and authority having jurisdiction (AHJ).
A load allowance is not a blanket ATS derating. Cummins distinguishes transfer-switch continuous rating from the rating limits of an assembly with integral overcurrent protection. Confirm the exact assembly's permitted continuous duty, ambient and protection conditions. Neither “all ATS are 80% rated” nor “every assembly carries 100% in every installation” is a valid shortcut.
Motor starting remains a separate check. A generator can carry the running load yet experience unacceptable voltage or frequency dip during a restart. Review motor/starter or variable-frequency drive (VFD) input data, load steps and the specified transition method. Randomly increasing ATS amperes does not solve generator transient response.
Worked example / transparent arithmetic
A 480 V case yields 210.80 A under stated assumptions
The example shows how to preserve the calculation trail. It is not a reusable code calculation, generator size, verified site condition or approved ATS order code.
Assume a balanced 480Y/277 V, three-phase panel. Its 90 kVA of non-motor load consists of 60 kVA continuous and 30 kVA noncontinuous load. Separate motor currents are 34 A for the largest motor and 42 A for the others, assumed already established by the applicable method. Use the illustrative feeder-style treatment above, with DF = 1.00 and no future-load allowance.
| Load item | Current before treatment | Illustrative treatment | Contribution |
|---|---|---|---|
| 60 kVA continuous, non-motor | 60,000 ÷ (√3 × 480) = 72.1688 A | 72.1688 × 1.25 | 90.2110 A |
| 30 kVA noncontinuous, non-motor | 30,000 ÷ (√3 × 480) = 36.0844 A | 36.0844 × 1.00 | 36.0844 A |
| Largest motor | 34 A assumed calculation basis | 34 × 1.25 | 42.5000 A |
| Other motors combined | 42 A assumed calculation basis | 42 × 1.00 | 42.0000 A |
| Assembled result using unrounded intermediate values | 210.7954 A ≈ 210.80 A | ||
The 90 kVA non-motor subtotal alone is 108.2532 A before continuous-load treatment. The four contributions above are separate; do not add that 108.2532 A subtotal again.
ARITHMETIC OUTPUT210.80 A
The result belongs to these inputs and this illustrative method. Preserve the load categories, full-precision arithmetic and operating assumptions.
PROCUREMENT OUTPUTStill requires approval
A 225 A, 250 A or larger configuration may be a candidate only if actually offered and supported by the final load, source, protection and installation review.
Do not treat 211 A as an automatic 225 A selection. The current basis may change after code review, phase assessment, operating-state review or future-load decisions. Fault duty or terminal requirements may also change the suitable product configuration.
Step 4 / verify an exact configuration
Select on amperes. Approve on the full system.
Once the qualified design establishes the required current, examine the next suitable offered rating. Keep every remaining check attached to the exact model, options and protective-device arrangement.
Check the finished assembly
Verify rated continuous current, ambient/enclosure conditions, integral protection limits and any manufacturer derating. An ampere frame alone is not the approved configuration.
Match WCR and protection
Use the available fault-current study and the applicable WCR at the stated voltage, duration and protective-device conditions. Record the required breaker/fuse model, rating and settings.
Verify both supplies
Check voltage, frequency, phase/wire system, sensing and control power. Review fault contribution from both sources and motors; include any permitted parallel operating state.
Follow the grounding design
Specify switched or unswitched neutral from the approved grounding and protection scheme. Neither a larger ampere rating nor an extra pole resolves an incorrect system design.
Confirm transfer behavior
Open, delayed-open, in-phase-open and closed transition have different conditions. Motor reacceleration, interruption limits, synchronization and utility permission need separate review.
Check lugs and enclosure
Confirm conductor count/material/range, terminal temperature conditions, bending space, cable entry, enclosure, environment and maintenance access for the ordered assembly.
WCR is not an interrupting rating. Do not relabel WCR as kAIC (kiloamperes interrupting capacity). WCR describes specified fault withstand and closing duty, not fault-clearing capability. A transfer mechanism does not acquire overcurrent protection from its WCR label; the appropriate integral or upstream overcurrent protective device (OCPD) must clear the fault within the applicable conditions.
Rating references: Cummins T011 and Eaton power-frame ATS design guide. For relevant US product evaluation, UL Solutions identifies UL 1008 transfer-switch categories. The standard name is not a load-calculation method or proof that a specific SENTOP product is listed. Other markets require their own applicable evidence.
Closed-transition fault duty is a distinct operating case. Temporary source paralleling can change available fault current. That requires the appropriate system study; it does not mean adding the two sources' normal rated amperes together to choose the ATS. See Eaton's transition application guidance.
Where shortcuts break down
Four cases that change how the result is interpreted
The load schedule, operating state and input definition matter as much as the equation. These cases show why one total power value is not always enough.
A smaller generator is not the boundary
The ATS can carry normal-source loads that are later shed for standby operation. Size from the approved duty at its actual location, then verify the source and load-management limits.
A dedicated panel can narrow the scope
Only loads demonstrably downstream of that ATS belong to its load schedule. The whole-building main rating is not automatically the right current basis for this smaller feeder.
The heavier leg can govern
For only 9.6 kVA on one 120 V leg and 4.8 kVA on the other, currents are 80 A and 40 A. Total 14.4 kVA ÷ 240 V gives 60 A and hides the heavier leg. Neutral review is separate.
Output power is not input demand
For an uninterruptible power supply (UPS), VFD or power converter, obtain input current/kVA, true power factor and relevant inrush/harmonic data for the operating mode. Do not size from output kW alone.
Calculation review
Replace a convenient shortcut with a verifiable check
A good review finds what an input leaves out. Keep the calculation, assumptions and model-specific evidence together so another person can reproduce the decision.
| Shortcut | What it misses | Better check |
|---|---|---|
| Use generator breaker amps | Normal-source duty, actual served load and the separate protection basis. | Compare the load calculation, both sources and the one-line location. |
| Add all panel breaker ratings | Breaker sizes are not a simultaneous load schedule. | Use supported load inputs, the adopted demand method and operating states. |
| Multiply everything by 1.25 | Different load categories, already-applied allowances and equipment conditions. | Apply each required treatment once and identify its basis. |
| Use running amps for every event | Motor starts, reacceleration and block-load source response. | Review the actual starting data, sequence and generator transient capability. |
| Match only a kA number | WCR voltage, duration, OCPD conditions and the difference from interrupting rating. | Match the complete tested rating and fault-study requirements. |
| Oversize “for safety” without a plan | Source limits, lug fit, enclosure space, protection and cost. | Document future load and verify the complete larger configuration. |
From calculation to a useful enquiry
Send a sizing basis, not just an ampere number
Prepare these six information groups for a request for quotation (RFQ). SENTOP can review product matching and supply requirements against the project's approved design basis.
- 01 · System and load boundaryOne-line diagram, ATS location, voltage, phase, frequency, wire/neutral arrangement, service or feeder role, and the normal/alternate operating states.
- 02 · Current calculationLoad schedule, input kVA/kW/amps, power factors, phase balance, continuous/noncontinuous classification, motor current basis, demand factors and future-load assumptions.
- 03 · Source and transient dataGenerator rating and output data, source OCPDs, motor/starter/VFD information, restart sequence, load-shed logic and the required voltage/frequency response.
- 04 · Fault-duty and protection evidenceAvailable fault current from all relevant operating cases, source reactance-to-resistance ratio (X/R) where required, OCPD make/model/rating/settings and the accepted coordination study.
- 05 · Exact product and installationTransition, poles, neutral, control functions, bypass/isolation or service-entrance needs, enclosure, conductor/terminal requirements, cable entry and dimensions.
- 06 · Acceptance and supply scopeTarget market and product documents, quantity, destination, approved model/options, inspection and commissioning criteria, settings records, labels and delivery requirements.
Close the loop at commissioning. The approved plan should verify source sensing, transfer and return logic, managed loads, alarms and the final operating state. A successful transfer does not establish the WCR, conductor suitability or correctness of the load calculation by itself.
Frequently asked questions
ATS amperage and sizing FAQ
Use these answers to check assumptions and prepare a clearer requirement. The exact equipment and approved project documents control the final selection.
Can I calculate automatic transfer switch amperage from generator kW?
Not by itself. Generator rated current requires its voltage, phase and rated kVA, or rated kW with the corresponding rated power factor. Load current requires the load's electrical input data and power factor. Compare the served-load calculation, normal-source duty, generator performance and protection before selecting the ATS.
What is the formula for three-phase ATS amps?
For a balanced three-phase load, I = 1,000 × kVA ÷ (√3 × line-to-line volts). If using real electrical input power, I = 1,000 × kW ÷ (√3 × line-to-line volts × PF). Use total three-phase power. The result does not include load-rule allowances, motor starting or product approval.
Should ATS amperage equal the main breaker rating?
Not automatically. An upstream or service-level ATS may carry normal-service load, while a critical-load feeder ATS may have a smaller boundary. The approved one-line, load calculation, source protection, service requirements and exact assembly conditions determine whether the ratings should match.
Do motor loads require a larger ATS?
They can change the required load-current basis, but there is no universal motor multiplier for ATS selection. Use the motor current and load treatment required by the applicable method. Separately verify starting or reacceleration, simultaneous load steps, transfer mode and generator response. A larger ATS does not by itself solve a generator transient problem.
What does WCR mean for ATS selection?
WCR is the withstand and closing rating under specified conditions. Compare it with available fault current at the stated voltage and required duration, and verify the applicable tested protective-device conditions. WCR is not continuous ampere rating and is not an interrupting rating; the appropriate protective device must clear the fault.
Is a 200 A ATS always enough for a 200 A service?
No. It is only a candidate if the exact configuration, approved load basis, continuous-duty conditions, sources, fault duty, service classification, conductors and applicable requirements support the installation. The matching ampere number alone is not approval.
Can load shedding reduce the required ATS ampere rating?
It can reduce the planned connected load in a defined operating state when the adopted rules and an engineered, verified control scheme support that credit. It does not erase larger normal-source duty that passes through the ATS. Review transfer, return, override and failure behavior as part of the approved load-management design.
Who should approve the final ATS amperage?
The responsible qualified electrical designer and installer must coordinate with the ATS and generator manufacturers and the applicable authority, including the utility where required. Retain the approved calculation, source data, fault/protection review, exact order code and commissioning evidence. A supplier enquiry or this calculator is not design approval.
Technical references
Evidence behind the formulas and selection boundaries
Manufacturer application manuals explain methods and rating conditions. They do not establish the locally adopted code or approve another manufacturer's equipment.
- Cummins — T011, Application Manual: Transfer Switches. Continuous current, transfer location, fault duty and system considerations; see section 2.4.1 for continuous rating.
- Cummins — T030, Application Manual: Liquid Cooled Generator Sets. Electrical-design and mixed motor/non-motor feeder-calculation guidance; the illustrated method remains subject to adopted requirements.
- Schneider Electric — kVA calculation formulas. Single-phase and three-phase voltage/current/power relationships.
- ASCO — Transferring Motor Loads Between Power Sources. Motor-transfer inrush and application considerations.
- Eaton — Power-frame ATS Design Guide, DG140003EN. Withstand/closing ratings and protective-device conditions, including the distinction from fault interruption.
- Eaton — Transfer Switch 101. Transition methods and the implications of temporary source paralleling.
- UL Solutions — Switch Certification and Evaluation Services. UL 1008 evaluation categories for relevant automatic and nonautomatic transfer-switch products.
From a number to an approved requirementBring the load basis.
Bring the load basis.
Review the complete ATS.
Share the one-line, calculated current, load categories, generator data, fault-duty requirements and required transition. SENTOP can support model matching and supply documentation; the responsible project team retains authority over electrical design, safety and commissioning.
