Dwelling Load Calculations for the C-10 Exam
Load calcs scare people on the C-10 more than they should. It's arithmetic with rules. Once you know which numbers are fixed by code and which demand factors apply, the whole thing turns into a checklist you can run in your sleep. The exam pulls almost all of it from Article 220, and it loves to test whether you know there are two legal ways to size a dwelling service — and that they land on different answers.
Let me walk it the way I'd walk a first-year apprentice through it on a real service upgrade.
Two methods, two answers
There's the standard method (Article 220, Part III) and the optional method for dwellings (220.82). Both are legal. The standard method builds the load piece by piece and applies demand factors to each category. The optional method lumps most of it together and hits it with one big demand factor — 100% of the first 10 kVA, then 40% of the remainder — after adding in the fixed appliances.
Here's the thing to burn into memory for exam day: the optional method almost always produces a smaller calculated load, which means a smaller service. So if a question hands you a house and asks for the minimum service, check whether it's steering you toward 220.82. If the problem says "using the optional method," don't build the standard tree — you'll burn four minutes and get a number that doesn't match any answer choice.
Read the stem twice. The method is usually named or strongly implied.
Building the standard-method load
Under Part III you assemble these categories, then apply demand factors.
General lighting and receptacles. Dwellings are 3 VA per square foot of habitable floor area (220.42). Use outside dimensions, and leave out open porches, garages, and unfinished space unless it's adaptable for future use. A 2,000 sq ft house is 6,000 VA of general lighting before any demand factor.
Small-appliance branch circuits. You're required to have at least two 20 A small-appliance circuits for the kitchen/dining area (210.11(C)(1)). Each counts as 1,500 VA in the calc, so two circuits is 3,000 VA minimum.
Laundry circuit. One 20 A laundry circuit (210.11(C)(2)) at 1,500 VA.
Add those three together, then apply the demand factors from Table 220.45: the first 3,000 VA at 100%, the remainder at 35% (up to 120,000 VA, then 25%). This is the single most-tested demand factor on the whole exam. Miss it and your service size is wrong.
Fixed appliances. Dishwasher, disposal, water heater, and similar fastened-in-place appliances at their nameplate rating. If there are four or more fixed appliances (not counting the range, dryer, space heating, or A/C), you may apply a 75% demand factor (220.53).
Ranges and dryers — the two tables people blank on
Cooking equipment and clothes dryers get their own tables because you never run every burner and the oven wide open at once.
Electric ranges use Table 220.55. For a single household range rated not over 12 kW, Column C gives a demand load of 8 kW — not 12. That's the classic trick. If the range is over 12 kW, you bump the 8 kW up 5% for each additional kW above 12. Multiple ranges have their own column values.
Electric dryers use Table 220.54. The calculated load is the nameplate or 5,000 VA, whichever is larger, for one to four dryers. So a 4.5 kW dryer still counts as 5,000 VA in a single dwelling.
| Item | Nameplate | Calculated load | Reference |
|---|---|---|---|
| General lighting (2,000 sq ft) | 6,000 VA | 3 VA/sq ft | 220.42 |
| Two small-appliance circuits | — | 3,000 VA | 210.11(C)(1) |
| Laundry circuit | — | 1,500 VA | 210.11(C)(2) |
| First 3,000 VA of the above three | — | 100% | Table 220.45 |
| Remainder of the above three | — | 35% | Table 220.45 |
| One 12 kW range | 12 kW | 8,000 VA | Table 220.55, Col. C |
| One 4.5 kW dryer | 4,500 VA | 5,000 VA | Table 220.54 |
Heating vs. air conditioning — take the larger
You don't add space heating and air conditioning together. Per 220.60, when two loads won't run at the same time, you use the larger of the two and drop the smaller. Electric heat and central A/C rarely run together, so you compare them and keep whichever is bigger. On the optional method, heating gets its own set of percentages in 220.82(C) depending on the system type — read that subsection carefully, because it treats heat pumps, electric furnaces, and supplemental heat differently.
Turning VA into a service size
Once you've got total VA, the rest is quick:
- Divide total VA by 240 volts for a standard single-phase 120/240 V dwelling service. That's your calculated amps.
- Round up to the next standard overcurrent device size from 240.6 — 100, 125, 150, 200, and so on.
- A one-family dwelling service can't be smaller than 100 A, 3-wire (230.79(C)).
Say you land at 190 A calculated. You'd go to a 200 A service. The conductor that feeds it comes from the ampacity tables, and for dwellings there's a break you should know cold: 310.12 lets service and feeder conductors for a single dwelling be sized at 83% of the service rating, so a 200 A service can often be fed with conductors rated for 166 A. That ties straight into conductor ampacity and wire sizing, and the overcurrent protection rules settle the breaker that finishes the job. If the run is long, voltage drop may push you to a bigger conductor than ampacity alone requires.
How the exam actually asks it
The questions come in a few reliable shapes: "What is the calculated general lighting load for a ___ sq ft dwelling?" (multiply by 3), "What is the demand load for a single 11 kW range?" (Table 220.55, still 8 kW), and full "size the service" problems that make you run the whole tree. The full problems are worth the most and take the longest, so know both methods and know which demand factor lands where.
Work these with a calculator and the code book tabbed to 220.42, Table 220.45, Table 220.55, and Table 220.54. Speed comes from repetition — running enough practice problems that the sequence is automatic before you sit down. For how load calcs fit alongside the rest of the trade material, the C-10 electrical exam study guide maps out every topic worth your time.
These concepts are here to help you study — always work from and defer to the current adopted California Electrical Code and confirm any specific requirement against the code book you'll use on exam day. If you can run a full standard-method calc and a full optional-method calc for the same house and explain why the numbers differ, you're ready for whatever the C-10 puts in front of you.