Mike Holt Load Calculator: How It Works

The Mike Holt load calculator is a tool based on the load calculation methods taught by Mike Holt, a well-known electrical educator, for sizing residential and commercial electrical services. You enter the square footage, appliance loads, and other demand factors, and it returns the total calculated load in amperes or volt-amperes, which you then use to select the correct service size. This article explains the standard calculation method it uses, how to apply it, and common mistakes to avoid.

Formula

The core formula for a residential load calculation per the National Electrical Code (NEC) is:

Total VA = (Living area in sq ft × 3 VA/sq ft) + Small-appliance and laundry circuits (1500 VA each) + Other loads (appliances, motors, etc.)

Where:

• Living area – the conditioned floor area of the dwelling in square feet (or square meters, but the 3 VA/sq ft factor is based on US practice).
• Small-appliance circuits – two 1500 VA circuits for kitchen countertop receptacles, and one 1500 VA circuit for laundry, totaling 4500 VA for a typical home.
• Other loads – the sum of all other fixed loads, such as an electric range (typically 8000-12000 VA), dryer (5000 VA), water heater (4500 VA), and HVAC equipment. These are added with demand factors per NEC Article 220.
• Total VA – the sum of all loads, in volt-amperes.

To convert VA to amperes at a given voltage, use: Amps = VA / Voltage. For a 240V single-phase service, divide by 240. The result is the minimum service size in amperes.

Worked Example

Using the default values from the calculator above:

1. Living area = 2000 sq ft → 2000 × 3 = 6000 VA
2. Small-appliance + laundry = 4500 VA (as given)
3. Other loads = 5000 VA (as given)
4. Total VA = 6000 + 4500 + 5000 = 15,500 VA
5. Total amps at 240V = 15,500 / 240 = 64.58 A
6. Recommended service size = next standard size above 64.58 A, which is 70 A (if using 10 A increments) or 100 A (standard service sizes).

The calculator rounds up to the next 10 A, giving 70 A. Note that in practice, standard service sizes are 60, 100, 125, 150, 200, etc., so you would choose 100 A for this example.

Understanding the NEC Load Calculation Method

The Mike Holt load calculator is based on the NEC’s standard method for dwelling units, found in Article 220, Part III. This method is used by electricians and engineers to ensure that the electrical service is not undersized, which could cause overheating and breaker trips, nor oversized unnecessarily, which adds cost. The calculation starts with a general lighting load of 3 VA per square foot, then adds specific circuits and appliances with demand factors that reflect that not all loads run at full capacity simultaneously.

For example, the NEC allows a demand factor of 75% for the first 10 kW of a range load, and 65% for the remainder. Similarly, clothes dryers are calculated at 5000 VA each, but if there are more than four, a demand factor applies. The Mike Holt calculator simplifies this by letting you enter the total “other loads� as a single number, but you must apply the demand factors yourself before entering them.

How to Use the Calculator Correctly

To get an accurate result, you need to gather specific information about the dwelling:

• Floor area – measure the living space, excluding garages, porches, and unfinished basements (unless they are heated and finished).
• Small-appliance circuits – count the number of 20-amp circuits feeding kitchen countertop receptacles. The NEC requires at least two, each at 1500 VA. Add one for laundry.
• Fixed appliances – list all large appliances: range/oven, dryer, water heater, dishwasher, disposal, microwave, and HVAC. For each, find the nameplate VA or wattage (watts = VA for resistive loads).
• Demand factors – apply the NEC demand factors to appliances. For example, a dishwasher is counted at 100% of its nameplate rating, while a dryer is counted at 5000 VA (or nameplate if higher).

Once you have the total VA, divide by the service voltage (usually 240V) to get the amperage. Then select a service size that is equal to or greater than this amperage, from standard sizes.

Common Mistakes and Misconceptions

One common mistake is using the square footage of the entire property, including garages and unfinished spaces, which overestimates the load. Another is forgetting to include the small-appliance circuits – the 4500 VA is mandatory for a dwelling, even if you don’t use those circuits. Also, people often confuse VA with watts, but for resistive loads they are the same; for motors and inductive loads, you must use VA (volts × amps) from the nameplate, not just watts.

A frequent misconception is that the calculated load is the same as the actual peak demand. In reality, the NEC calculation is a conservative estimate that includes safety margins. It is not unusual for the calculated load to be higher than the measured peak demand. Conversely, some people think they can simply add up all breaker ratings, but that is incorrect because breakers are sized to protect the wiring, not to reflect the load.

Limitations of the Calculator

This calculator provides a simplified version of the NEC calculation. It does not account for all demand factors, such as the 75% factor for the first 10 kW of range load, or the 65% factor for the remainder. It also does not include optional calculations for heat pumps, electric vehicle chargers, or other high-demand loads that may require additional calculations. For a complete and code-compliant calculation, consult the NEC Article 220 or use a professional electrical designer.

Additionally, the calculator assumes a 240V single-phase service, which is standard in North America. For other voltages or three-phase systems, you would need to adjust the formula accordingly. Always check local codes, as some jurisdictions have amendments to the NEC.

Frequently Asked Questions

Q: What is the difference between VA and watts?
VA (volt-amperes) is the product of voltage and current, while watts is the real power. For resistive loads (like heaters and incandescent lights), they are equal. For inductive loads (motors, transformers), VA is higher than watts due to power factor. The NEC uses VA for load calculations to ensure the service can handle the current draw.

Q: Can I use this calculator for a commercial building?
No, this calculator is for dwelling units only. Commercial load calculations use different methods, such as the optional method for new restaurants or the general method for other occupancies, which consider connected load, demand factors, and diversity.

Q: Why does my calculated load seem so high?
Because the NEC method is conservative. It assumes worst-case conditions, such as all lights on and all appliances running at the same time. In practice, you rarely use everything at once, but the service must be sized to handle the potential load.

Q: How do I find the VA of an appliance?
Look at the nameplate on the appliance. It will list volts and amps, or volts and watts. If only watts are given, use that as VA for resistive loads. If amps are given, multiply volts × amps to get VA. For example, a 240V dryer drawing 20A is 4800 VA.

Q: What is the minimum service size for a small apartment?
Even a small apartment typically requires at least 60A, but the calculation depends on the square footage and appliances. A 500 sq ft apartment with no electric range might calculate to 3000 VA (500×3) + 4500 VA (small-appliance) = 7500 VA, which is 31.25A at 240V, so a 60A service would be sufficient. However, local codes may require 100A.

Q: Does the calculator include the 8 kW range demand factor?
No, it does not. You must apply demand factors to your “other loads� before entering them. For a range, the NEC allows a demand of 8 kW for the first 10 kW of nameplate rating, and 5 kW for the remainder. So a 12 kW range would be entered as 8 + (12-10)×0.65 = 9.3 kW, or 9300 VA.

Related

• Repair Corrupted MOV File: Step-by-Step Fixes
• Container Load Calculator: How to Use It
• Pro Tools Session Disappeared: File Backups Explained

in sqft | Living area | sq ft | 2000 | 0
in la | Small-appliance + laundry circuits | VA | 4500 | 0
in other | Other loads (dryer, range, AC, etc.) | VA | 5000 | 0
out total_va | Total calculated load | VA | (sqft * 3 + la + other) | 0
out total_amps | Total load at 240V | A | total_va / 240 | 1
out service_amps | Recommended service (next standard) | A | ceil(total_amps / 10) * 10 | 0