Load Calculation Online: How It Works

An online load calculation estimates the electrical load of a building or circuit—how much power (in watts or amperes) the lights, outlets, and appliances will draw—so you can size wires, breakers, and panels correctly. You enter the square footage, the number of circuits, and the major appliances, and the calculator returns the total load in amperes or volt-amperes, which you then compare to your panel rating. This page explains the standard method behind these tools, what the results mean, and where they fall short.

Formula

The most common residential method is the optional calculation from the National Electrical Code (NEC, Article 220.82). It works like this:

• General lighting and receptacle load: 3 volt-amperes (VA) per square foot of conditioned floor area.
• Small-appliance and laundry circuits: 1,500 VA each (the NEC assumes two for a kitchen, plus one for laundry).
• Add the nameplate rating of fixed appliances (range, water heater, dryer, dishwasher, disposal, etc.).
• Then apply a demand factor: the first 10 kVA at 100%, the remainder at 40%.

In symbols: Total VA = (3 × floor area) + (1,500 × number of small-appliance/laundry circuits) + appliance VA. To get amperes, divide by the supply voltage (typically 240 V in North America). The calculator above uses the simplified version without the demand factor; for a real panel sizing you must apply the 40% factor to everything above 10 kVA.

Worked Example

Using the defaults: 2,000 ft², 2 small-appliance/laundry circuits, 4,500 VA of major appliances, 240 V supply.

1. Lighting load: 2,000 × 3 = 6,000 VA
2. Circuit load: 2 × 1,500 = 3,000 VA
3. Appliance load: 4,500 VA
4. Total: 6,000 + 3,000 + 4,500 = 13,500 VA
5. Current: 13,500 ÷ 240 = 56.25 A

The calculator shows 13,500 VA and 56.3 A. In practice, the NEC demand factor would reduce the first 10,000 VA to 10,000 and the remaining 3,500 VA to 1,400, giving 11,400 VA and 47.5 A—a meaningful difference. Always use the full method for a real panel.

When to Use an Online Load Calculator

These tools are for preliminary sizing: checking whether a new panel is big enough, estimating the cost of a service upgrade, or deciding between a 100 A and 200 A panel. They are also handy for adding a single circuit—say, a new EV charger or a hot tub—to see if the existing panel has headroom. For a new build or a major renovation, you should still have a licensed electrician do the formal calculation, because local codes and the exact appliance list matter.

The result is a snapshot, not a guarantee. A load calculation tells you the expected demand under worst-case assumptions, not what you will actually draw at any moment. Most homes never run every appliance at once, but the code assumes they might.

How to Choose the Right Calculator

Not all online load calculators are equal. Look for one that:

• Uses the NEC optional method (or clearly states which code it follows).
• Asks for floor area, circuit count, and appliance list—not just square footage.
• Applies the demand factor automatically, or at least explains it.
• Lets you input the voltage (120/240 V for North America, 230 V for much of Europe, etc.).

If the calculator only multiplies square footage by 3 VA and stops, it is incomplete. The demand factor is the part that keeps your panel from being oversized (and expensive) or undersized (and dangerous).

Common Mistakes and Misconceptions

The biggest error is confusing watts and volt-amperes. For resistive loads (heaters, incandescent lights) they are the same, but for motors and electronics the power factor makes VA higher than watts. Load calculations use VA because that is what the breaker and wire must carry.

Another mistake: forgetting the 40% demand factor on the portion above 10 kVA. Without it, you overestimate the load by a lot—in the example above, 13,500 VA vs. 11,400 VA. That can push you into a larger, more expensive panel than needed.

Also, do not use the maximum current of an appliance for the load; use the nameplate rating, which is usually lower. And never mix 120 V and 240 V loads in the same amperage column—convert everything to VA first, then divide by the supply voltage.

Limits of the Formula

The 3 VA per square foot figure is a conservative average. A home with electric heat, a pool, or a workshop will exceed it. The formula also ignores power factor and harmonics, which matter for motor-driven equipment. And it assumes a balanced load across the two legs of a 120/240 V system; if you put all the heavy loads on one leg, the neutral and that leg can be overloaded even when the total is fine.

For commercial or industrial buildings, the residential optional method does not apply. Those require a more detailed calculation per NEC Article 220, often using demand factors from tables that vary by occupancy type. Online calculators for those are less common and less reliable, so a professional study is the norm.

Cost and Safety Implications

An undersized panel is a fire hazard—breakers trip, wires heat up, and insulation degrades. An oversized panel costs more but is not dangerous. The load calculation is the tool that finds the safe middle. If your calculator says you need 47 A and you have a 100 A panel, you have room; if it says 95 A, you are at the edge and should consider a 200 A service.

Never attempt to change a panel or add a circuit based solely on an online calculation. The math is necessary but not sufficient—your local code, the condition of the wiring, and the actual conductor sizes all matter. A licensed electrician will do the official calculation and pull the permit.

Frequently Asked Questions

What is the difference between a load calculation and a load test?

A load calculation is a paper estimate of what a circuit will draw under worst-case conditions. A load test measures what it actually draws, usually with a clamp meter or a data logger. You do a calculation before installing; you run a test after to verify.

Can I use an online calculator for a 3-phase system?

Yes, but the formula changes: total VA = √3 × line voltage × line current. The residential method above assumes single-phase. Most online calculators that mention “3-phase� use that formula, but you must enter the correct line-to-line voltage (e.g., 208 V or 480 V) and divide by √3, not 1.

Why does my electrician’s load calculation differ from the online one?

Electricians often use the full NEC Article 220 method, which includes specific demand factors for ranges, dryers, and motors that the simplified optional method does not. They also account for the actual appliance list, not a generic “major appliance� lump. Differences of 10–20% are normal.

How many square feet does a 100 A panel support?

Using the optional method: 100 A × 240 V = 24,000 VA. Subtract two small-appliance circuits (3,000 VA) and a typical appliance load (say 5,000 VA), leaving 16,000 VA for lighting. Divide by 3 VA/ft² gives about 5,300 ft². But that ignores the demand factor and assumes no electric heat or large loads, so real-world capacity is usually less.

Is the load calculation the same as the service size?

No. The load calculation gives the expected demand. The service size (the main breaker and wire) must be equal to or larger than that demand, but it is also chosen based on future expansion, voltage drop, and code minimums. You calculate the load, then pick the next standard service size above it.

Related

• Ring Doorbell Alternatives: Is Now the Time to Ditch?
• Radiator Watts Calculator: How Many BTU Do I Need?
• Recover Unsaved MS Project File: 6 Ways

in sqft | Conditioned floor area | ft² | 2000 | 0
in la | Small-appliance & laundry circuits | count | 2 | 0
in app | Major appliance load | VA | 4500 | 0
in volt | Supply voltage | V | 240 | 0
out total | Total load | VA | sqft * 3 + la * 1500 + app | 0
out amps | Total current | A | total / volt | 1