Answer: Enter your values and the Electrical Load Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
Total amps, breaker size, and service capacity check
This electrical load calculator adds up the wattage of the appliances you plan to run, converts the total to amps, recommends the next standard breaker size with the required 25% safety margin, and shows what percentage of your whole-house service the load represents. It is the quick version of the load-addition math an electrician does before adding a circuit or sizing a subpanel.
The math is grounded in Ohm's law — watts ÷ volts = amps — and in two code conventions: continuous loads (anything running 3 hours or more, like space heaters) must be sized at 125% of their nameplate current per the National Electrical Code (NEC 210.20), and residential services are commonly loaded to no more than 80% of their rating, which is the same 125% rule viewed from the other side.
This page explains each formula, provides a reference table of common appliance wattages drawn from manufacturer nameplate ranges and U.S. Department of Energy appliance energy data, works through an example, and answers the questions that come up most when sizing circuits.
Amps: total watts ÷ circuit volts (120 V for standard receptacles and lighting, 240 V for heavy appliances). Breaker: the smallest standard size — 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, or 100 A — that is at least 125% of the calculated amps. The 125% multiplier is the NEC continuous-load rule and also leaves headroom so the breaker doesn't trip under normal operation.
Service check: the tool divides total watts by 240 V (residential service is split across two 120 V legs) to get service amps, then expresses that as a percentage of your service size (100, 150, 200, or 400 A). Above 80% sustained it flags an upgrade consult — a conservative planning threshold, not a hard code limit, since real services also benefit from load diversity (not everything runs at once).
For whole-house load calculations used to size a new service, the NEC offers the standard calculation (Article 220) with demand factors; this tool is a simpler circuit-level estimator. A practical note on wire: breaker size and conductor size travel together under NEC tables — a 20 A circuit needs 12 AWG copper, 15 A needs 14 AWG, and 30 A needs 10 AWG. The calculator sizes the breaker; the wire must match. Also remember that a circuit's safe continuous load is 80% of its breaker rating, which is the same arithmetic as the 125% sizing rule applied from the other direction.
A note on wire: breaker size and conductor size travel together under NEC ampacity tables — a 20 A circuit needs 12 AWG copper, 15 A needs 14 AWG, and 30 A needs 10 AWG. The calculator sizes the breaker; the wire must match it. A circuit's safe continuous load is 80% of its breaker rating, which is the same arithmetic as the 125% rule seen from the other side, so both conventions produce identical answers.
The table below lists typical running wattages used for planning. Nameplate values vary by model — always check the label or manual on your actual appliance, and note that motor-driven devices (fridges, compressors) draw a brief startup surge well above running watts.
Use the running wattage for the sustained-load calculation, and if several large loads share a circuit, add them at full value: the calculator's 125% sizing already covers the continuous-load margin on the total. Startup surges deserve a final caution: refrigerators, air conditioners, compressors, and anything with a motor briefly draws 2–5× its running watts while spinning up. That surge is why a fridge sharing a 15 A circuit with other loads can trip a breaker that looks fine on paper. Manufacturers publish both running and locked-rotor amps — use running watts for this calculator and check the surge figure against the breaker's trip curve.
| Appliance | Typical Watts | Amps @120 V | Common Circuit |
|---|---|---|---|
| LED lighting (per room) | 100–300 | 1–3 | 15 A lighting |
| Television / media | 100–600 | 1–5 | 15 A shared |
| Refrigerator | 100–800 | 1–7 | 15–20 A dedicated |
| Space heater | 750–1,500 | 6–13 | 20 A |
| Microwave | 600–1,200 | 5–10 | 20 A |
| Window AC (8k BTU) | 900–1,200 | 8–10 | 20 A |
| Portable heater + TV + lights | 2,400 | 20 | 25 A (per 125% rule) |
| Electric water heater | 4,000–4,500 | 17–19 @240 V | 30 A dedicated |
| Electric dryer | 3,000–5,000 | 13–21 @240 V | 30 A dedicated |
Three loads — a 1,500 W space heater, 300 W of LED lighting, and a 600 W TV — total 2,400 W. On a 120 V circuit that is 2,400 ÷ 120 = 20 A. Applying the 125% rule: 20 × 1.25 = 25 A, so the tool recommends a 25 A breaker (or splitting the loads across two 20 A circuits, which is the more common field solution).
Against a 200 A service: 2,400 ÷ 240 = 10 A, which is 5% of capacity — no concern. Repeat with your own list: enter each appliance's watts, and the tool recomputes breaker size and service percentage instantly. For heavier equipment the same steps scale directly. An electric dryer at 5,000 W on 240 V draws 20.8 A; at 125% that is 26 A, hence the standard 30 A dedicated circuit with 10 AWG conductors. A 4,500 W water heater at 240 V draws 18.75 A — sized to 23.4 A, again landing on a 30 A breaker. These pairings are why appliance circuits follow such a uniform pattern across houses.
Heavy equipment scales the same way. An electric dryer at 5,000 W on 240 V draws 20.8 A; sized at 125% that is 26 A, hence the standard 30 A dedicated circuit with 10 AWG conductors. A 4,500 W water heater at 240 V draws 18.75 A — sized to 23.4 A, again landing on a 30 A breaker. These pairings are why appliance circuits look so uniform from house to house.
How do I convert watts to amps?
Divide watts by volts. On a standard 120 V North American circuit, amps = watts ÷ 120, so a 1,500 W space heater draws 12.5 A. On a 240 V circuit, divide by 240.
What size breaker do I need for my appliance?
Size the breaker at 125% of the running current — the NEC rule for continuous loads. A 1,500 W / 120 V heater draws 12.5 A; 12.5 × 1.25 ≈ 15.6 A, so use a 20 A breaker with 12 AWG wire. The calculator applies this automatically and picks the next standard size.
Why does the calculator add 25% to my load?
The National Electrical Code requires continuous loads — anything running 3 hours or longer — to be sized at 125% of current (NEC 210.20). The margin also prevents nuisance trips from voltage dips and motor startup surges.
How do I know if my electrical service is big enough?
Divide total watts by 240 to get service amps, then compare to 80% of your service rating. A 2,400 W load is 10 A — just 5% of a 200 A service. If sustained loads push past 80%, or you're adding EV charging, a heat pump, or a workshop, consult an electrician about a load calculation under NEC Article 220.
Can I put a space heater and a TV on the same circuit?
Combined they draw about 2,100 W = 17.5 A on a 120 V circuit. That fits a 20 A breaker at up to the 16 A continuous limit (80% rule), so it does NOT fit — the heater needs its own circuit. Heaters are best on a dedicated or lightly loaded circuit.