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Can I Run This On That? - Electrical Load Checker

Check If Your Circuit Can Handle a Specific Load - Based on NEC Standards

Quick safety check before you plug in that space heater, power tool, or new appliance: can your circuit actually handle it? Enter your circuit breaker size, wire gauge, and the load you want to run. This tool checks both the breaker capacity and wire ampacity, applying the NEC 80% continuous load rule, and gives you a clear pass or fail verdict with a plain-English explanation.

This calculator is deliberately conservative. It follows NEC standards and errs on the side of safety because overloaded circuits cause fires. If a load is marginal, this tool will tell you to upsize - because "close enough" is not a concept that belongs in electrical work.

Perfect for homeowners wondering if they can add another appliance to an existing circuit, DIYers planning workshop wiring, and anyone who wants a quick sanity check before they trip a breaker - or worse.

Pro Tip: Space heaters are continuous loads - a 1,500W heater on a 15A/120V circuit uses 12.5A, which exceeds the NEC 80% rule (12A maximum continuous load on a 15A breaker). Use a 20A circuit for space heaters.

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Can I Run This On That?
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How It Works

  1. Enter Circuit Breaker Size

    Check your electrical panel and enter the amp rating of the breaker feeding the circuit you want to use. Common residential sizes are 15A (lighting), 20A (receptacles), 30A (dryers), and 50A (ranges).

  2. Select Wire Gauge

    If you know the wire gauge, enter it. If you do not, the calculator uses the standard gauge for your breaker size: 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A. The wire gauge determines the maximum safe ampacity.

  3. Enter Your Load

    Enter the wattage or amperage of the appliance, tool, or equipment you want to plug in. Check the nameplate label on the device for exact ratings. For multiple items on the same circuit, enter the total combined load.

  4. See Pass or Fail Verdict

    The calculator checks your load against both the breaker rating and wire ampacity, applies the 80% rule for continuous loads, and displays a clear pass, caution, or fail result with a detailed explanation of why.

Built For

  • Homeowners checking if a 1,500W space heater is safe on an existing circuit
  • Workshop owners adding a table saw or air compressor to an existing garage circuit
  • Tenants checking if a window AC unit can run on an apartment bedroom circuit
  • EV charger feasibility checks before hiring an electrician for a dedicated circuit
  • Kitchen appliance planning: can the existing 20A circuit handle a new countertop oven?
  • Home office setups with multiple monitors, computers, and laser printers on one circuit
  • Checking if an existing circuit is already overloaded before adding anything new

Features & Capabilities

NEC 80% Continuous Load Rule

Applies the NEC requirement that continuous loads (those running 3+ hours) must not exceed 80% of the circuit breaker rating. A 15A breaker can handle 15A for brief periods but only 12A continuously. This is the rule most people do not know about and the one that matters most for heaters, AC units, and grow lights.

Wire Gauge Ampacity Check

Verifies that the wire gauge installed in the circuit can safely carry the load current. Even if the breaker is sized correctly, undersized wire creates a fire hazard because the wire can overheat before the breaker trips.

Voltage Selection

Supports 120V, 208V, 240V, and other common voltages. Automatically converts between watts and amps based on the voltage you select so you can enter whichever unit is shown on the appliance nameplate.

Clear Pass/Fail With Reasoning

Does not just say "yes" or "no." Explains why the circuit passes or fails, shows the actual load as a percentage of capacity, and tells you exactly what would need to change to make a failing load work (upsize breaker, upsize wire, or use a different circuit).

Safety Margins Built In

Calculations include built-in safety margins consistent with NEC practices. When results are borderline, the calculator recommends the safer option because there is no acceptable margin for error in electrical safety.

Multi-Load Support

Enter total combined wattage when you want to check multiple appliances on the same circuit. Helps identify circuits that are safe for each individual load but dangerously overloaded when everything runs at the same time.

Comparison

Circuit Breaker Wire Gauge (Min.) 80% Continuous Max Typical Loads
15A / 120V 14 AWG 12A (1,440W) Lighting, small appliances
20A / 120V 12 AWG 16A (1,920W) Kitchen countertop, bathroom, general receptacles
20A / 240V 12 AWG 16A (3,840W) Window AC, small shop tools
30A / 240V 10 AWG 24A (5,760W) Clothes dryer, water heater
40A / 240V 8 AWG 32A (7,680W) Electric range, large cooktop
50A / 240V 6 AWG 40A (9,600W) Electric range, EV charger, sub-panel
60A / 240V 6 AWG 48A (11,520W) Sub-panel feeder, large equipment

Assumptions

  • Circuit breaker rating assumed to be the nameplate ampere value with no derating for ambient temperature
  • Wire ampacity based on NEC Table 310.16, 75°C column for standard terminations
  • Load assumed to be either continuous (3+ hours, requiring 125% sizing) or non-continuous based on user selection
  • Existing circuit load assumed at zero unless user specifies current draw from other devices on the circuit
  • Single-phase 120V or 240V circuits — does not evaluate three-phase or high-leg delta configurations
  • Standard residential wiring assumed (copper conductors, NM-B cable in walls, THWN in conduit)

Limitations

  • Does not verify actual wire gauge installed in the wall — user must confirm existing wire size
  • Cannot detect shared neutrals, multi-wire branch circuits, or other wiring configurations that affect capacity
  • Does not account for voltage drop on long runs which may reduce effective capacity
  • Motor loads require special consideration (NEC Article 430) beyond simple ampacity comparison
  • Does not evaluate panel bus rating, main breaker capacity, or service entrance limitations
  • Existing circuit breaker condition and age are not factored into the assessment

References

  • NEC (NFPA 70) Article 210 — Branch Circuits
  • NEC Article 220 — Branch-Circuit, Feeder, and Service Load Calculations
  • NEC Table 310.16 — Conductor Ampacities (75°C column)
  • NEC 210.20(A) — Continuous and Noncontinuous Loads (125% rule)
  • NEC 240.4 — Protection of Conductors (overcurrent protection requirements)
  • UL 489 — Standard for Molded-Case Circuit Breakers

Frequently Asked Questions

NEC Section 210.20 requires that the continuous load on a circuit (any load expected to run for 3 hours or more) must not exceed 80% of the overcurrent device (breaker) rating. This means a 20A breaker can only carry 16A continuously. The reason is thermal: breakers and wiring generate heat under load, and sustained operation at 100% can cause overheating, nuisance tripping, and in extreme cases, fire. Space heaters, grow lights, and AC units are all continuous loads.
You have several options: use a different circuit with more capacity, install a new dedicated circuit for the load, split your loads across multiple circuits, or choose a lower-wattage appliance. Never upsize only the breaker without also upsizing the wire - the breaker protects the wire, and a larger breaker on undersized wire creates a serious fire hazard.
Only if the wiring is 12 AWG or larger. If the circuit is wired with 14 AWG (standard for 15A circuits), putting a 20A breaker on it creates a fire hazard because the wire can overheat before the breaker trips. The breaker size must match the wire gauge: 14 AWG gets a 15A breaker, 12 AWG gets a 20A breaker, 10 AWG gets a 30A breaker. This is a fundamental safety rule that cannot be bypassed.
GFCI (Ground Fault Circuit Interrupter) and AFCI (Arc Fault Circuit Interrupter) breakers have the same amperage ratings as standard breakers and follow the same 80% rule. GFCI protection is required in bathrooms, kitchens, garages, outdoors, and near water. AFCI protection is required in bedrooms and living areas. These breakers add ground-fault or arc-fault protection but do not change the circuit capacity.
Most 1,500W space heaters draw 12.5 amps at 120V. Under the NEC 80% rule, a 15A circuit can only carry 12A continuously. Since space heaters run for hours at a time, 12.5A exceeds the 12A continuous limit. The safe option is to use a 20A circuit (which allows 16A continuous) or use a lower-wattage heater. This is one of the most common causes of tripped breakers and a frequent contributor to residential electrical fires.
Common loads at 120V: space heater 12.5A, microwave 8-13A, hair dryer 10-15A, window AC 5-12A, toaster 7-8A, vacuum cleaner 6-12A, desktop computer 1-3A, TV 1-2A, LED light bulb 0.1A, refrigerator 1.5-3A running (8-10A starting). At 240V: clothes dryer 24A, electric range 30-50A, EV charger 32-48A, water heater 18-25A, central AC 15-30A. Nameplate data on the appliance is always the most accurate source.
On a 15A circuit - no. A 1,500W space heater draws 12.5A, leaving only 2.5A for anything else. A microwave draws 8-13A. Together they would trip the breaker immediately. On a 20A circuit - still risky. The space heater alone exceeds the 80% continuous load rule (12A max continuous on a 15A, 16A max on a 20A). The safest answer: space heaters should always be on a dedicated circuit or a circuit with minimal other loads.
Disclaimer: This tool provides conservative estimates based on NEC standards for informational purposes. It does not account for all possible installation variables, existing loads on the circuit, or local code amendments. Always consult a licensed electrician before modifying electrical circuits or adding significant new loads.

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