Let's talk about load calculations.
They're not sexy. They're not exciting. But they're the foundation of every service upgrade, every new build, and every "will this circuit handle my new hot tub?" conversation.
And here's the thing: most of the time, they're pretty straightforward. You plug in the numbers, apply the demand factors, and you're done.
But sometimes—usually when you're in a hurry, or when the scope changes mid-job—something gets missed. And that something can be the difference between a service that's sized right and one that's undersized (or way oversized, which costs the client money they didn't need to spend).
Why a Checklist Beats "Doing It from Memory"
Here's the problem with doing load calcs in your head (or just "winging it"): your brain is really good at filling in blanks with assumptions.
You assume the house has two small appliance circuits. You assume the laundry is 1500W. You assume the HVAC numbers you used last time are close enough.
And most of the time? You're probably fine. But occasionally, those assumptions bite you. The inspector asks where you got a number. The service undersizes when the client adds an EV charger. The panel can't handle the load you promised it would.
A checklist doesn't make you smarter—it just makes sure you don't skip stuff when you're thinking about three other jobs.
If you want to see how this looks in a real tool, try the load calculator demo.
The Checklist: Inputs That Commonly Get Missed
Alright, let's walk through what actually matters when you're doing residential load calcs under NEC Article 220.
1. Job Context (The Stuff You Need Before You Start)
Before you start plugging in numbers, make sure you know:
What type of dwelling is this? Single-family? Duplex? Multi-family? The rules change depending on the answer.
What's the service voltage? 120/240V single-phase is standard for resi, but confirm. If you assume wrong here, everything downstream is wrong.
What's the existing service size (if applicable)? If this is an upgrade, you need to know what they currently have. A lot of "service upgrades" turn into "oh, we can actually keep the 100A panel" once you run the numbers.
What are we calculating? Are we sizing a new service? Adding a subpanel? Just checking if a circuit can handle a new load? Be clear on what you're actually solving for.
2. General Loads (The Foundation)
This is where most of the square footage math lives. Don't skip these:
General lighting load NEC says 3 VA per square foot for dwellings. Multiply by the finished square footage (not counting garages, unfinished basements, etc.). Easy to mess up if you use the wrong square footage number.
Small appliance circuits Two required minimum, 1500 VA each. So that's 3000 VA right there. Did you include them? (If the answer is "I think so," you didn't.)
Laundry circuit 1500 VA. Required in dwellings. Don't forget it.
Apply demand factors After you total up lighting + small appliance + laundry, you get to apply demand factors per Table 220.42. This is where the math gets friendlier—first 3000 VA at 100%, next 117,000 VA at 35%, everything else at 25%. But you have to actually apply it.
3. Fixed Appliances and Equipment
This is where people start guessing. Stop guessing. Use nameplate ratings when you can, and use standard values when you can't.
Common fixed appliances to account for:
- Dishwasher
- Garbage disposal
- Microwave (if built-in)
- Electric range or cooktop
- Wall ovens
- Water heater (big one—don't miss this)
- Dryer
Apply demand factors for fastened-in-place appliances If you have four or more appliances, you can apply a 75% demand factor (NEC 220.53). This is free capacity you're leaving on the table if you forget it.
Electric range/cooktop demand Use Table 220.55. A 12 kW range doesn't pull 12 kW—it pulls 8 kW under normal use. The table accounts for this. Use it.
4. HVAC (The "Largest of Heating or Cooling" Rule)
Here's a gotcha that trips people up constantly:
You don't add heating AND cooling to the load calc. You take the larger of the two (NEC 220.60), because they're never running at the same time.
But you have to actually check which one is larger:
Air conditioning → Use the nameplate rating on the compressor + air handler.
Electric heat → If it's a heat pump, use the heat pump rating. If it's resistance heat (baseboards, furnace), add it all up.
Gas/oil heat → Only include the blower motor (usually pretty small).
Which one governs? Cooling usually wins in most climates, but not always. Write down which one you picked and why. Inspectors love asking about this.
5. EV Charging and Other High-Demand Modern Loads
This is the new one that's catching people off guard.
EV chargers are real loads. A Level 2 home charger is usually 40-50 amps, 240V. That's a significant chunk of capacity—often more than the entire HVAC system.
If the client has an EV (or is planning to get one), you have to include it. Otherwise, you're sizing a service that'll be maxed out the day they plug in.
And here's the kicker: a lot of service upgrades that seemed unnecessary suddenly become very necessary once you add an EV charger. A 100A service that was "fine" is now undersized. That's not upselling—that's math.
Other modern loads to watch for:
- Pool pumps
- Hot tubs / spas
- Home battery systems / solar inverters
- Workshop equipment (welder, compressor, etc.)
Don't assume. Ask. Then include them.
What to Save So You Don't Redo This Later
Okay, you've done the calculation. Now what?
Save the breakdown, not just the total. If you just write down "calculated service load: 18,500 VA," that's useless in six months. Save the actual breakdown: lighting, appliances, HVAC, whatever. So when the scope changes (and it will), you can adjust instead of starting over.
Export a PDF and attach it to the project. Inspectors want documentation. Clients want documentation. Future-you wants documentation. One click, done.
Tie it to the job. If your load calc is just a random file on your desktop, you'll lose it. Save it with the project so when you're quoting a panel upgrade or sizing a subpanel, the numbers are right there.
Build a Workflow, Not a File Pile
If you're doing load calcs in a spreadsheet, or on scratch paper, or in your head—stop. Get a system that:
- Walks you through the inputs (so you don't skip stuff)
- Applies demand factors automatically (so you don't have to look up tables every time)
- Saves the full breakdown (so you can reference it later)
- Generates a clean PDF (so you can hand it to inspectors or clients without embarrassment)
- Ties to the project (so it's not just floating in a void somewhere)
That's what a load calculator should do. Not replace your knowledge—just make sure your knowledge gets applied consistently every time.
How The Electrician Suite Handles Load Calculations
I built the load calculator in The Electrician Suite to follow NEC Article 220 step-by-step. You fill in the fields, it applies the right demand factors, and it shows you the breakdown in real-time.
Here's how it fits into the workflow:
1. Calculate the load → Enter square footage, appliances, HVAC, etc. Calculator applies NEC demand factors automatically.
2. Save it to the project → No more lost spreadsheets. The calc lives with the project.
3. Export a PDF → One click, clean PDF with the full breakdown. Hand it to the inspector, email it to the client, whatever.
4. Reference it later → When the client adds an EV charger six months later, pull up the original calc, adjust the inputs, and you're done. No starting from scratch.
It's not magic. It's just structure. And structure is what keeps you from missing stuff when you're thinking about ten other things.
Stop Guessing on Service Sizing
If you've ever had an inspector question your service size, or if you've ever undersized a panel because you forgot to include something, it's time to get systematic.
Not because load calcs are hard—they're not. But because consistency matters, especially when things get busy.
Check out the live load calculator demo to see how the step-by-step process works, or read about how this connects to the quote-to-invoice workflow.
And if you're ready to stop doing load calcs on scratch paper, try the free plan—no credit card, just a cleaner way to size services and keep records.