Outlet Load Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Circuit breaker size: The amp rating of the breaker protecting the branch circuit that feeds the outlet.

Outlet voltage: The voltage used to convert watts to amps. Most standard US household receptacles are entered as 120 V unless you are checking a true 240 V outlet.

Long-running load: A load expected to run at its maximum current for 3 hours or more.

Short-use load: A load used for a shorter time, such as many tools, vacuums, or kitchen appliances.

Power factor for watt entries: A number from 0.01 to 1.00 used when converting watts to amps for some AC loads. Lower power factor means more amps for the same watts.

Breaker check load: The mixed-load value compared with the usable breaker capacity: 125% of Long-running load plus 100% of Short-use load.

Room left on this check: Usable breaker capacity minus Breaker check load. Positive means room left; negative means over the selected limit.

Mistake: Using the outlet faceplate as Circuit breaker size when the breaker is different.
Fix: Check the panel label for the breaker that feeds that outlet, then choose that Circuit breaker size.

Mistake: Putting a heater that runs all afternoon under Short-use load.
Fix: Enter it under Long-running load if it may run at full load for 3 hours or more.

Mistake: Mixing watts and amps after choosing What numbers do you have?.
Fix: If you choose Watts, fill Long-running load and Short-use load in W; if you choose Amps, fill those same labels in A.

Mistake: Leaving Power factor for watt entries at 1.00 for a motor when the label gives a lower value.
Fix: Enter the known Power factor for watt entries so the amp estimate is not too low.

Mistake: Reading Circuit capacity used below 100% as permission to add any device.
Fix: Also check More long-running load room and Important note, especially for motors, power strips, extension cords, or shared circuits.

• This is a planning check, not approval to change wiring, replace breakers, ignore manufacturer instructions, or ignore local electrical code.
• The outlet is usually limited by the branch circuit breaker and the wiring on that circuit, not only by the outlet faceplate.
• The check assumes the entered loads can run at the same time. If devices cycle on and off, the real current can be lower or higher at a given moment.
• Startup current is not modeled. Motors, compressors, pumps, welders, and some tools can draw much more current when starting.
• Extension cords, multi-outlet power strips, plug adapters, damaged cords, medical equipment, EV charging equipment, generators, and dedicated appliances can have extra rules or lower safe limits.
• Power factor only affects watt-entry mode. If you do not know it, the default 1.00 can understate amps for some motors and electronics.
• Extra room to leave reduces the usable breaker capacity for this check. It is a cushion chosen by the user, not a code rule.

What the calculator checks

The calculator separates loads that may run for 3 hours or more from loads used briefly. For the main check, it compares 125% of the Long-running load plus 100% of the Short-use load with the usable breaker capacity. This mixed continuous and noncontinuous load method is more specific than applying a flat 80% shortcut to every load [1].

Formulas used

usable_capacity_amps = breaker_amps * (1 - safety_margin_percent / 100)

load_amps = load_watts / (outlet_voltage * power_factor)

actual_load_amps = continuous_load_amps + short_load_amps

breaker_check_amps = 1.25 * continuous_load_amps + short_load_amps

capacity_used_percent = 100 * breaker_check_amps / usable_capacity_amps

remaining_capacity_amps = usable_capacity_amps - breaker_check_amps

long_running_room_amps = ((usable_capacity_amps - short_load_amps) / 1.25) - continuous_load_amps

continuous_limit_amps = usable_capacity_amps * 0.80

continuous_limit_watts = continuous_limit_amps * outlet_voltage * display_power_factor

Worked example

For a 20 A breaker, 120 V outlet, 1500 W long-running heater, 300 W short-use device, power factor 1.00, and 0% extra room, the long-running current is 1500 / (120 * 1.00) = 12.5 A. The short-use current is 300 / (120 * 1.00) = 2.5 A. The breaker check load is 1.25 * 12.5 + 2.5 = 18.125 A. Capacity used is 100 * 18.125 / 20 = 90.625%, so Room left on this check is 20 - 18.125 = 1.875 A.

How to read the result

Load check result is based on whether Breaker check load is at or below usable breaker capacity. Circuit capacity used at or below 100% fits the selected check. Room left on this check is signed, so a negative value means the load is over the selected limit. More long-running load room is also signed; a negative value means the long-running load would need to be reduced if the short-use load stays the same.