🧮 TradeCalcsPRO

Generator Transfer Switch Interlock Planner

Plan generator interlock breaker sizing, essential load compatibility, and main panel checks using NEC 702 continuous load guidelines — with PASS or FAIL results for manual transfer switch configurations.

Generator & Panel Details

W
A
A

Live Results

Recommended Interlock Breaker

40A

Generator Amps

31.25A

Continuous Load (125%)

39.06A

Compatibility Checks

Essential LoadPASS
Interlock BreakerFAIL
Main BreakerPASS

Input Summary

Generator Watts7,500 W
Generator Voltage240 V
Main Breaker Size200 A
Selected Interlock Breaker30 A
Essential Load Amps25.00 A
Manual TransferYes

Interlock planning follows NEC 702 guidelines. Always verify final design with local electrical code and manufacturer specifications.

How to Use This Generator Interlock Planner

  1. Enter generator wattage and voltage. Input the generator's rated wattage from the nameplate and select 120 V or 240 V output. Most portable generators for whole-home backup use 240 V to feed a double-pole breaker; smaller units may be 120 V only. Wattage and voltage together determine the maximum amperage the generator can deliver.
  2. Enter main panel breaker size. Input the amp rating of your main service disconnect or main breaker — typically 100 A, 150 A, or 200 A for residential panels. The calculator checks whether the main breaker is large enough to accommodate the recommended interlock breaker without exceeding service capacity.
  3. Select interlock breaker size (optional). Choose the double-pole breaker size you plan to install for the generator inlet or backfeed circuit. Common interlock kit sizes are 30 A, 40 A, or 50 A. If provided, the calculator runs a PASS or FAIL check against the NEC 125% continuous load recommendation.
  4. Enter essential load amps (optional). Input the total amperage of circuits you plan to power during an outage — refrigerator, sump pump, lighting, and critical outlets combined. Compare this against generator amperage to confirm the unit can support your essential load list without overload.
  5. Review PASS/FAIL results and warnings. Check recommended interlock breaker size, continuous load amps, and all compatibility checks. Review warnings for small generators, large breaker requirements, and automatic transfer switch considerations. Confirm final design with local code and manufacturer interlock kit specifications.

Formulas & Example

Generator interlock breaker sizing converts rated wattage to amperage, applies the NEC 125% continuous load factor, and selects the smallest standard breaker that meets or exceeds the result.

Generator Amps = Generator Watts ÷ Generator Voltage
Continuous Amps = Generator Amps × 1.25
Recommended Interlock Breaker = smallest standard breaker ≥ Continuous Amps

Essential Load PASS if Essential Load Amps ≤ Generator Amps
Interlock PASS if Selected Breaker ≥ Recommended Breaker
Main Breaker PASS if Main Breaker ≥ Recommended Breaker

Worked Example

7,500 W generator at 240 V with a 200 A main breaker:

Generator Watts = 7,500 W
Generator Voltage = 240 V

Generator Amps = 7,500 ÷ 240 = 31.25 A
Continuous Amps = 31.25 × 1.25 = 39.06 A
Recommended Interlock Breaker = 40 A

Essential Load = 25 A → 25 ≤ 31.25 → PASS
Selected Interlock Breaker = 30 A → 30 < 40 → FAIL
Main Breaker = 200 A → 200 ≥ 40 → PASS

In this example the 30 A interlock breaker is undersized — upsize to 40 A to match generator capacity. Pair this tool with the Generator Sizing, Load Calculation, Breaker Size, and EV Charger Load calculators for complete backup power planning.

Frequently Asked Questions

What is a generator interlock?▾
A generator interlock is a mechanical or listed device that prevents the main breaker and generator breaker from being ON at the same time. It typically mounts on the panel cover and slides a plate so only one breaker can be in the ON position — eliminating dangerous backfeed into utility lines. Interlock kits are a cost-effective alternative to standalone transfer switches for manual generator connections when installed per NEC Article 702 and local authority requirements.
How do I size an interlock breaker?▾
Size the interlock breaker at or above the generator's continuous load current. Convert generator watts to amps (watts ÷ voltage), then multiply by 1.25 per the NEC continuous load rule. Select the next standard breaker size at or above that value — for example, 7,500 W at 240 V equals 31.25 A, continuous load is 39 A, and the recommended interlock breaker is 40 A. The breaker must also match your generator cord, inlet rating, and interlock kit maximum size.
What loads can a generator support?▾
A generator supports loads up to its rated wattage divided by system voltage — not the full panel capacity. A 7,500 W generator at 240 V delivers about 31 A, enough for essential circuits like refrigeration, lighting, a sump pump, and selective outlets — not an entire 200 A service simultaneously. List essential loads, add running watts, and compare total amperage against generator output. Motor starting surge may require temporarily staggering large loads.
What is NEC 702?▾
NEC Article 702 covers optional standby systems — generators and transfer equipment that supply power to selected circuits when normal utility power is unavailable. It addresses disconnecting means, overcurrent protection, conductor sizing, transfer switching, and interconnection methods. While this calculator applies general continuous load sizing principles from the NEC, always verify your specific installation against the current NEC edition and local amendments.
What is the difference between manual and automatic transfer?▾
A manual transfer switch or interlock requires you to start the generator, move the interlock or switch to generator position, and then energize selected circuits. An automatic transfer switch (ATS) detects utility loss, starts a standby generator, and transfers load without operator action — common with permanently installed standby units. ATS installations require different wiring, control circuits, grounding, and often permitting. This planner is oriented toward manual interlock configurations.

Related Tools

Explore more free calculators. Back to home

ADVERTISEMENT ZONE // CONTAINER_LEADERBOARD728 x 90