GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Green Building Blog

Does Your Electrification Project Require a Service Upgrade?

Step-by-step calculations for determining the capacity of your current service

Photo by Matthew Millham

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. 

Electrical panel
Older panels, like this one at the author’s house, are often sized for 100-150A.

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.

Worksheet
Worksheet for NEC 220.83 service load calculations.
  1. 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.
  2. 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.
  3. 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.
Nameplate from an electric cooktop
Nameplate from an electric cooktop. The cooktop is rated at 3.4kW (=3400 VA) on a 240V circuit.

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%.

No Nameplate Instructions

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) 

or

  • 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.

10 Comments

  1. Charlie Sullivan | | #1

    For people facing the possible need to upgrade service, there are a few more options to consider beyond what's discussed here: monitoring for actual peak load in advance of installing new equipment, and active load management to prevent major loads from operating simultaneously. I'll describe each below, but first, thanks for this excellent and detailed article anticipating an issue that will become more and more important!

    NEC 220.87 offers the option of using the actual measured peak demand rather than the calculations laid out in this article. That requires monitoring over a 1-year period, or monitoring over 30 days and adjusting for seasonally varying loads. For my own all-electric house with 100-A service, I initially expected that doing this would indicate that we had plenty of capacity to add a 30 A, 240 V EV charging setup (EVSE), but after many months of staying below 46 A, we happened to run the electric dryer (which we almost never use) at the same time as the oven, the heat pump, and a bunch of other stuff, and got up above 80 A, leaving no room to add anything. The actual peak was remarkably similar that what I calculated as outlined above, indicating that this method works pretty well. So in my case, doing this only confirmed the result from the calculation, but that might not always be the case--monitoring for a month or a year might help some people avoid the need for an upgrade, or might even point to the need to upgrade service even though the calculations didn't call for it. Some people may want to start monitoring in advance of taking steps to electrify everything, to get that full year of data and be prepared to move forward.

    The other opportunity is active load management: smart controls that will prevent low-priority loads from operating when there isn't sufficient capacity to serve them. For example, if you are running the oven and the dryer, such a system would tell the EV charging system to wait and charge after those are finished. A water heater is another classic example of a load that could be delayed. This type of control system is not yet widely available or used, but the Elmec "EVDuty" smart EVSE has an option for a current sensor. With that, the system then operates even more intelligently than I described: it can adjust the EV charge rate to just take the amount of current that is available and never overload the panel, but rarely need to shut off charging completely. https://evdutystore.elmec.ca/products/smart-current-sensor-evccs200

    There are also more general purpose "load shedding controllers" that could be set up to control EV charging and/or other loads such as a water heater. The disadvantage of such a controller is that it is less of a turn-key system than the Elmec EVDuty setup, and it's intended to switch loads on and off, rather than smoothly adjust their current level. https://pspproducts.wpengine.com/?p=222 It can be interfaced with control wires on EVSEs and water heaters that are set up to be controlled that way, or can be interfaced with contactors to cut off power to whatever loads are deemed non-essential. But it requires an engineer or electrician willing to figure out those details.

    In addition to the cost savings one gets from avoiding a service upgrade, another advantage of the load management approach is that it reduces the peak load that you are putting on the grid. As more people electrify everything, we will have increasing issues with peak load, both in distribution networks and in generation. How that will be resolved is somewhat of an open question, but it's possible that we will see residential demand charges start to be applied, or perhaps discounts for customers who control their peak load. So taking the load management approach could turn out to be even more economically advantageous if a rate structure along those lines kicks in, but even now, it can be a less expensive and more civic-minded approach to take.

    1. Jon Harrod | | #5

      Thanks, Charlie. I really like the idea of using measured demand, like you're describing, rather than a prescriptive calculation when it's possible to do so. I've often thought of that when sizing heating equipment, but the same applies to electrical service. And I agree, demand management and load shaping are going to be critical to making electrification go well.

    2. Charlie Sullivan | | #6

      Update: It turns out the 15+ year old energy monitor I wasn't playing well with our new EV. It uses powerline communications that were disrupted when the EV was charging. So my data saying we were up against the limit of our service was flawed. I replaced it with an IotaWatt monitor and so far our peaks are below 40 A. I'll see what happens over the next month or two.

      So while everything I said about shedding controllers is true, and some people might need them, we probably won't.

      I recommend IotaWatt as an economical choice for monitoring, including logging individual phase currents, and the ability to monitor many branch circuits by adding inexpensive sensors. It would make sense to install one if you are even thinking about electrification, so you have the data to make decisions when the time comes. Also, if you put a sensor on the branch circuit for the heating system, you can also use that data to find the heating system run time, valuable data for sizing a new heat pump when you take that electrification step.

      1. Steve Knapp CZ 3A Georgia | | #7

        Charlie,

        Thanks for posting the info on IotaWatt. I like the idea of using an open platform rather than committing to a proprietary technology (smart breakers) that may not make it over the long term.

  2. Paul Wiedefeld | | #2

    Jon, such a helpful article for me as I hope to electrify my house. Something I've seen on other online resources, but not in this article, is a buffer of 20% of the panel rating (so a 100 amp panel should have a max of 80 amps calculated as you show). Is this a best practice or being too conservative? If too conservative, is there a more appropriate buffer?

    1. Jon Harrod | | #4

      Thanks, Paul. My understanding of the 80% rule is that it applies to sizing conductors for continuous loads (e.g. lighting or motors that are on continuously for 3 hours or more, including typical HVAC circuits). The conductors need to be sized at 125% of the rated loads, hence the 80%. Electrical service can be sized to meet the calculated load per the NEC 220.83 method without additional safety factors. However, it should be noted that the adjustments to general loads are based certain assumptions about behavior which work for most homes but don't cover every possible scenario.

      1. Expert Member
        Zephyr7 | | #9

        I'm a little late to this article, but seeing as how I work with the issue of "continuous or not" loading commerically all the time I thought I'd comment.

        I'll start by saying this doesn't come up nearly as often in residential projects as it does in commerical projects. There are a few key things to think about though. It's the BIG loads that you need to pay particular attention to. Typical feeders that run things like receptacles in a room aren't usually an issue. Things to watch for:
        Water heaters
        ELECTRIC CAR CHARGERS
        Certain large tools that might run for long periods (think dust collectors in a large shop)

        Loads that are expected to run for 3 hours or more are considered "continous", and the 80% rule applies. This means you aren't supposed to load a circuit to more than 80% of rated capacity, and this includes most fuses and circuit breakers, and wiring. An example is a 20A circuit, run with 12 gauge wire and a 20A breaker, is only good for 16A of continous load. Can you upgrade the wire to 10 gauge to get a 20A continous capacity? Nope, you can't -- because the breaker is only 80% rated too. Specialty 100% rated breakers are available, but they're expensive, nearly always special order, and I've never seen one for residential applications. Even commerically, I find it's often cheaper to upgrade to a 600A switch than to get a 400A switch with a 100% rating, for example.

        Electric car chargers are going to be a big new thing, and they will typically be running for 6-8 hours or more. My own car charger typically runs for around 8 hours. These should be served by suitable circuits. The car charger manual will talk about this -- don't just oversize the supply circuit because the charger may specify a maximum current rating for the OCPD (OverCurrent Protection Device), the circuit breaker, and you can't exceed that. You can oversize the wire if you want a little extra insurance, since most of the heating issues tend to be with the wiring. Chances are the car charger instructions have allowed for this, and may specify a 40A circuit with a 40A breaker, but will only charge at 32A -- 80% of the supply circuit rating.

        The need for continous ratings doesn't apply to service entrances, since it's already expected that not everything will be on all the time at the same time. You would only have to worry about the 80% rule here if you had enough portion of the total load classed as continuous to exceed 80% of the rating of the service entrance, and I don't see that happening in a residential system unless you have a relatively small 100A service with several car chargers connected along with a typical home load. Even commerically this rarely comes up in normal systems.

        If you want some extra insurance on the service entrance conductors, don't size them from the special table in the code book for residential service conductors -- size them from the "real" ampactity chart 310.15. If you compare the two, you'll find that residential service entrances are allowed to be one size smaller than the "regular" ampacity chart allows, for example a 200A service can use a 2/0 copper conductor, which would be rated for only 175A in the 75*C column (where conductors need to be sized from for most applications). A 3/0 copper conductor would otherwise be required. The smaller conductor is allowed since the code understands that a typical residential service will normally run far below rated capacity, even over extended periods.

        It's worth mentioning that if a home is built as an all-electric home, with electric water heating (especially tankless electric water heaters), and multiple car chargers (which can be expected to run at the same time at night), it's probably a good idea to NOT follow the residential service conductor ampacity table and go with the normal conductor ampacity rating instead. Larger wire will run cooler for the same electric load.

        Bill

        1. Charlie Sullivan | | #10

          When you are considering conductor sizing for a continuous, frequently used load, it's also worth thinking about the fact that oversizing to reduce heat also improves efficiency. For example, a 50 foot run of 8 AWG wire has 2 V drop at 32 A. That's a small percentage of of the 240 V nominal voltage, but it's 64 W of loss, so it's the equivalent of running an incandescent light bulb whenever the charger is running, and would be nearly 100 kWh/year if it was charging at that rate 4 hours every night. Upsizing to 6 AWG could pay back in a year or two. Standards allow as much as a 5% voltage drop, but designing or much less than that is a good idea for loads that are on for enough time that the cost of the loss adds up.

          Keeping wire runs short is a less expensive way to reduce voltage drop and loss.

  3. Steve Knapp CZ 3A Georgia | | #3

    When updating our newly purchased townhouse, we decided to eliminate the gas appliances. Before we could move forward, we had to update the 10-year-old, 150-amp service panel. Getting the panel reorganized, adding a subpanel, and running a few new circuits was relatively inexpensive. But as Jon suggests, you do have to have a plan.

  4. EudoraAquino | | #8

    I don’t have time to read it all at the minute but I have bookmarked it
    and also added your RSS feeds, so when I have time I will be
    back to read a great deal more.

    Regards,
    Eudora Aquino @ Homeadvisor

Log in or create an account to post a comment.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |