Electrification involves replacing fossil-fuel devices, from furnaces and boilers to cars and lawnmowers, with their electric equivalents. The goal, as David Roberts stated in his widely-read Vox essay, is to “electrify everything.” Electrification has emerged as a major tool for fighting climate change. It results in immediate drops in CO2 emissions, even in regions where electricity is generated largely by fossil fuels. It provides a clear path to carbon-neutrality; as the share of wind- and solar-generated electricity continues to grow, emissions from electric generation will drop and ultimately approach zero. By eliminating pollutants like ozone and carbon monoxide from tailpipes, gas cookstoves, and malfunctioning heating equipment, electrification also improves air quality and human health.
Many new homes are being designed as all-electric from the start, and some municipalities are banning gas in new construction. In these houses, the electric service is typically at least 200A and is sized to accommodate electric heating, air conditioning, hot water, cooking, clothes drying, and possibly a car-charging station. The challenge occurs in older homes, many of which have only 100A or 150A service. Full electrification of these houses will usually require a service upgrade. But homeowners can often get started on a stepwise path—for example, replacing a natural gas water heater or installing a small ductless heat pump—without upgrading their electric service.
Determining capacity of current service
Practitioners of electrification—architects, engineers, energy auditors, and technicians—need to be able to assess the adequacy of the existing electric service. Overtaxing a service with new loads can create a safety hazard; it can also lead to nuisances like dimming lights, tripped breakers, and extra wear and tear on motors and electronics. But specifying a service upgrade when one isn’t needed also has its downside. The extra cost, which typically runs in the $1500−$3000 range, may deter a homeowner from taking the first steps toward electrification. In the case of an emergency replacement, the delays associated with a service upgrade may also be a dealbreaker. Most equipment replacements occur as end-of-lifecycle events; if this brief, critical window for electrification is missed, the homeowner may find themselves locked into fossil fuel equipment for another 10−20 years.
The National Electrical Code (NEC 220.83) describes the steps to determine if an electrical service can safely accommodate new loads. My goal here is to familiarize readers with the service calculation process and required inputs. If you plan to perform these calculations, I’d strongly recommend reading the relevant sections of the NEC. If you’re not comfortable making the final call, or if your local building department requires it, have the calculations done by a licensed electrician or engineer.
The calculations divide loads into two categories: General Loads, and Heating and Air-Conditioning Load. The size of the electric service is determined by adding the two. Loads are calculated in volt-amperes (VA), which is the product of rated voltage (V) and current (A). For purely resistive loads like space heaters, VA is equivalent to power in watts. For inductive loads like motors, in which peak current draw occurs out of phase with peak voltage, VA may be higher than the true power (in watts) consumed by the device.
A few preliminaries
Before diving into the service load calculations (described below), I do a quick visual inspection of the panel. I also try to take some clear photos showing both the breakers and the door label; these can be useful if I have questions after I leave the site. At this time, I’ll note the amperage of the main breaker and the number of empty slots available. If the panel is full or nearly full, I’ll evaluate whether it will be possible to free up space by installing tandem breakers. Not all manufacturers allow tandem breakers, and some limit the number of tandem breakers that can be installed.
I also make note of the age and condition of the panel, checking for corrosion, missing parts, and other damage. If the panel or breakers are an older type, I’ll check online for recalls. Problems noted during the visual inspection may tilt the scales toward a panel upgrade, even if the existing capacity is sufficient for the proposed project. (Although old-style panels with fuses sometimes pass the criteria listed in NEC 220.83, they do not meet modern standards for safety or reliability and should always be upgraded.)
Determining general loads
The following steps walk you through the NEC 220.83 service calculations. Numbers correspond to sections in the worksheet shown below.
- Calculate lighting and general use receptacle loads based on square footage. Use exterior dimensions, but do not include garages, open porches, or basements that will not be finished in the future. Multiply the finished square footage by 3 VA/sq. ft.
- Tally laundry and small appliance branch circuits. Add 1500 VA for each 2-wire, 20-ampere small-appliance branch circuit. These are circuits like kitchen countertop circuits to which no permanently installed light fixtures (other than appliance lights) are connected. Add an additional 1500 VA for each laundry branch circuit. By code, each dwelling unit must have at least two 20A small-appliance branch circuits and one laundry circuit, so the minimum value allowed for Line 2 is 4500 VA.
- Tally fixed appliances. Next, list the nameplate rating, in VA, of all fixed appliances. These are defined as “all appliances that are fastened in place, permanently connected, or located to be on a specific circuit.” The code specifically mentions ranges, ovens, and cooktops; electric water heaters, and clothes dryers that are not connected to the laundry branch circuit (i.e. electric dryers with their own dedicated 240V circuit—these are tallied as the larger of 5000 VA or the nameplate VA). A complete tally will include other fixed appliances and motors such as well pumps, sump pumps, garage door openers, and hot tubs.
4. Sum general loads and apply a demand factor. The code recognizes that not all lights and appliances will be used at once, and derates the general load total accordingly. The first 8000 VA of general loads are counted at 100%, but additional general loads are counted at only 40%.
5. Heating and air-conditioning load. The code assumes that heating and cooling loads do not occur at the same time, and so only counts the larger of the two following loads:
- The full nameplate VA rating of the air conditioning system (full load amps x Volts for the outdoor condensing unit, plus the same for the air handler)
- The full nameplate VA ratings of the electric central heat (i.e. heat pump) plus central electric backup heat plus electric baseboards or space heaters, if present.
6. Total VA and service amps. Calculate the General Load and Heating and Air-Conditioning Load with the proposed equipment included. Do not include any equipment that will be removed as part of the proposed upgrade. The total load in VA is calculated by adding the adjusted General Loads to the maximum Heating and Air-Conditioning Load. The required service rating in amps is calculated by dividing total load by the service voltage (typically 240V). If the capacity of the existing service (as indicated by the main breaker) exceeds the required load, the project can proceed.
What if the existing service doesn’t meet the proposed load?
When the existing service is too small to accommodate the proposed electrification project, a service upgrade will be required. At this point, you need to have a conversation with the homeowners to see if they want to take on the additional expense. It’s worth mentioning that electric panels, like other building systems, have a finite life expectancy. A service upgrade may represent an improvement in safety and reliability and may also make possible other quality-of-life enhancements such as additional lighting or outdoor receptacles. Offering financing for the whole package of improvements may help overcome the cost barrier. Although a service upgrade requires a substantial investment, it will move homeowners forward on the path toward full electrification.
-Jon Harrod is founder of Snug Planet, a contracting company in Ithaca, N.Y., whose mission is to reduce building energy use in ways that make sense for people and the planet. Jon holds multiple certifications from the Building Performance Institute and has published numerous articles on energy efficiency and green building.