Load Schedule Calculator: How to Use It

A load schedule calculator works out the total electrical demand of a building or panel by adding up the loads of individual circuits and applying demand factors. You enter each circuit’s load in watts or amps and its type (lighting, receptacle, motor, etc.), and the calculator returns the total connected load and the estimated demand load in amperes or kilowatts. The result helps you size feeders, panels, and service equipment so they don’t overheat or trip.

Formula

The core formula for a simple load schedule is:

Demand Load = (First 3000 VA × 100%) + (Remaining VA × 35%)

For a general-purpose dwelling, the National Electrical Code (NEC) allows this demand factor for lighting and receptacle loads. In the calculator above:

• Total connected load = sum of all circuit loads (watts, assuming a power factor of 1.0 for simplicity).
• Demand load = first 3000 W at 100%, plus 35% of anything above 3000 W.
• Amperes at 240V = demand load ÷ 240 (for a typical US split-phase service).

For commercial or industrial panels, the demand factors come from the applicable electrical code (e.g., NEC Table 220.42 for lighting, Table 220.44 for receptacles). This calculator uses the residential general lighting demand factor as a common example—your actual project may need a different factor.

Worked Example

Using the default values from the calculator: Circuits of 1500 W, 1200 W, 1000 W, 800 W, and 600 W.

1. Total connected load = 1500 + 1200 + 1000 + 800 + 600 = 5100 W
2. First 3000 W at 100% = 3000 W
3. Remaining = 5100 − 3000 = 2100 W
4. 35% of remaining = 2100 × 0.35 = 735 W
5. Demand load = 3000 + 735 = 3735 W
6. Amperes at 240V = 3735 ÷ 240 = 15.6 A (rounded to one decimal)

The calculator’s first output shows 3735 W for the demand, and 15.6 A for the current—matching the manual calculation.

Why Use a Load Schedule?

A load schedule is a table that lists every circuit in a panel, its load, and the demand after applying code factors. It serves several purposes:

• Panel sizing: ensures the panel’s busbar rating and main breaker are large enough for the computed demand.
• Feeder sizing: determines the conductor ampacity needed from the service to the panel.
• Voltage drop checks: with the load current, you can calculate voltage drop over long runs.
• Generator or transformer sizing: for backup power, you need the demand load, not the connected load.

Without a load schedule, you’d be tempted to size for the sum of all breakers (connected load), which is almost always larger than the actual demand—wasting money on oversized equipment.

How to Build a Load Schedule

Whether you use a calculator or a spreadsheet, the process follows the same steps:

1. List every circuit in the panel, with its breaker size and the loads it feeds (e.g., lighting, receptacles, appliances, motors).
2. Determine the load per circuit in volt-amperes (VA). For receptacles, use 180 VA per outlet (NEC 220.14(I)) or the actual connected equipment.
3. Apply demand factors per the code for each load category. For example, lighting in a dwelling uses the 35% factor above; electric range uses a specific table; motors use 125% of the largest motor.
4. Sum the demand loads to get the total demand on the panel.
5. Convert to amperes by dividing by the system voltage (e.g., 240V for split-phase, 208V for three-phase).

Many engineers use Excel templates with formulas like the one above, but a dedicated calculator speeds up the process and reduces arithmetic errors.

Common Mistakes and Misconceptions

Here are frequent errors when using a load schedule calculator:

• Using connected load instead of demand load: The sum of all breakers is not the actual draw. Demand factors exist because not all loads run at full capacity simultaneously.
• Ignoring the largest motor: For motor loads, NEC requires adding 25% of the largest motor’s current to the total—this accounts for starting surge.
• Mixing watts and volt-amperes: For resistive loads (heaters, incandescent lights), watts = VA. For inductive loads (motors, fluorescent ballasts), VA = watts ÷ power factor. Using watts directly understates the load.
• Wrong voltage: A 120V circuit draws twice the current of a 240V circuit for the same wattage. Always use the correct system voltage for the amperage calculation.
• Rounding too early: Round only the final result, not intermediate values, to avoid significant errors.

A common misconception is that the main breaker size must equal the sum of all branch breakers. In reality, the main breaker is sized for the computed demand load, which is usually much less. That’s why a 200A main panel can have branch breakers totaling 400A.

Limitations of a Simple Calculator

This calculator handles the general lighting demand factor, but real load schedules are more complex. It does not account for:

• Specific appliance demand factors (ranges, dryers, water heaters) which have their own NEC tables.
• Motor loads with their 125% rule and starting currents.
• Power factor for inductive loads—the calculator assumes 1.0.
• Continuous vs. non-continuous loads (continuous loads require a 125% factor).
• Three-phase systems where the current formula is different (I = VA ÷ (V × √3)).

For a full residential load calculation, you’d need to include the 3000 VA for the first appliance circuit, 1500 VA for each additional circuit, and the laundry circuit—this calculator is a simplified educational tool, not a substitute for a licensed electrician’s design.

Frequently Asked Questions

What is the difference between connected load and demand load?

Connected load is the sum of all loads that could be turned on at once, while demand load applies diversity factors to reflect that not everything runs simultaneously. For example, a house might have 40 kW of connected loads, but the demand load might be only 15 kW, allowing a smaller service.

How do I calculate load schedule for a three-phase panel?

For three-phase, sum the VA loads per phase, then divide the total VA by (line-to-line voltage × √3). For example, 50,000 VA at 208V three-phase gives 50,000 ÷ (208 × 1.732) ≈ 139 A. Demand factors still apply before this calculation.

What is the 125% rule for continuous loads?

NEC requires that continuous loads (operating for 3 hours or more) be sized at 125% of the actual load to prevent overheating. So a 16A continuous load needs a 20A circuit. This applies to feeders and breakers, not just branch circuits.

Can I use this calculator for a commercial building?

No, this calculator uses the residential general lighting demand factor. Commercial buildings have different demand factors based on occupancy type and square footage, found in NEC Chapter 220. For commercial work, use a professional load calculation service or software.

Why does my load schedule show more amps than my panel rating?

If your computed demand exceeds the panel rating, you need to either reduce the load (e.g., by using energy-efficient equipment), redistribute circuits to another panel, or upgrade the service. This is a red flag that must be addressed before installation.

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in a | Circuit 1 load | W | 1500 | 0
in b | Circuit 2 load | W | 1200 | 0
in c | Circuit 3 load | W | 1000 | 0
in d | Circuit 4 load | W | 800 | 0
in e | Circuit 5 load | W | 600 | 0
out total | Total connected load | W | a+b+c+d+e | 0
out demand | Estimated demand (first 3kVA at 100%, rest at 35%) | W | min(total,3000) + max(total-3000,0)*0.35 | 0
out amps | Demand at 240V | A | demand/240 | 1