Journeyman refresher Refresher 9 of 10 · about 9 min
Rusty on load calculations?
What you’ll learn
- The load-calculation method as ten steps you can follow with the book open.
- Connected load versus demand load, and where each factor comes from.
- Turning total VA into amps for single-phase and three-phase.
- The traps that cost marks: demand twice, wrong voltage, a load counted in two places.
Before this: Services and the load calculation method
The method in ten steps
- Identify the occupancy. Dwelling, apartment, commercial, industrial — the book has a different method for each.
- Find the right rule in the book. Tab it. Read the whole rule and its subrules before you start adding.
- Measure the area the way the book defines it. Which floors count, inside or outside dimensions.
- Basic load. Area × a load per unit area, or blocks of area — whichever your book uses.
- Demand on the basic load, if the book steps it down.
- Heating and cooling. Apply the book’s rule; non-coincident loads count once.
- Named appliances. Ranges, water heaters, dryers, EV chargers, each at the factor the book gives it.
- Everything else the book says to add, at its own factor.
- Add the demand figures — once. Write the unit on every line.
- Divide by the voltage (× √3 for three-phase) and hand the current to the book’s sizing rules.
- I = VA ÷ Vsingle-phase (use 240 on a 120/240 service)
- I = VA ÷ (√3 × V)three-phase, line-to-line V
- Demand = load × factorfactor from your book
Worked: invented practice numbers (not code values)
- Practice rule: basic load 5,000 VA for the first 100 m², plus 1,000 VA for each additional 100 m² or part. House: 180 m². Basic = 5,000 + 1,000 = 6,000 VA.
- Practice rule: range 12,000 VA counted at 50% → 6,000 VA. Heat 10,000 VA at 100%; A/C 4,000 VA non-coincident, so heat governs → 10,000 VA.
- Water heater 4,500 VA at 100%. Total demand = 6,000 + 6,000 + 10,000 + 4,500 = 26,500 VA.
- Single-phase 120/240 V: 26,500 ÷ 240 = 110.4 A. That current now goes to your book’s rules for the service size — which is not this page’s call.
On the job
Tips
- Lay the calculation out as a table: load, connected VA, factor, demand VA. Mistakes show up in a table.
- Check the total by adding the column a second time from the bottom up.
- Try the load calculation helper with your own book’s values to check a hand calc.
Common mistakes
- Applying a demand factor twice — once to the load, again to the total.
- Dividing a 120/240 V total by 120 V.
- Counting heat and air conditioning both at 100% when the book says otherwise.
- Measuring the wrong area — a garage or unfinished basement the book treats differently.
Check yourself
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1.What is the difference between connected load and calculated (demand) load?
The service is sized from the demand figure, which is the connected load reduced by the factors your code book permits for that occupancy and those loads.
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2.A total calculated load of 36,000 VA on a 120/240 V single-phase service. What current does that represent?
Single-phase: I = VA ÷ V = 36,000 ÷ 240 = 150 A. Use the line-to-line 240 V for a 120/240 service, not 120 V.
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3.The same 36,000 VA load on a 208 V three-phase supply?
Three-phase: I = VA ÷ (√3 × V) = 36,000 ÷ (1.732 × 208) = 36,000 ÷ 360.3 = 99.9 A.
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4.Your book lets two loads that cannot run at the same time (say, space heating and air conditioning) be treated specially. What is the usual idea?
Non-coincident loads are the reason demand exists. The exact rule, and what counts as non-coincident, are in your code book — the method is to count the one that governs, not both.
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5.Which is a common load-calculation error?
Once a load has been reduced by its demand factor it goes into the total as-is. Reducing the total again (or putting a load under two headings) undersizes the service on paper.
Questions people ask
Why does this page not give the code values?
Because they belong to your code book, differ between CSA C22.1 and the NEC, differ between occupancies, and change with each edition. The method — area, basic load, demand, other loads, total, amps — is the same everywhere, and it is what the exam tests you on with the book open.
Do I use 120 V or 240 V for a 120/240 V service?
Divide the total VA by 240 V. The service is a two-line 240 V supply; the 120 V loads are split between the two lines, so the total divided by 240 is the current in each line on a balanced service.
Is the calculated current the service size?
No. It is the number your code book’s rules turn into a service or feeder size — with minimums, rounding rules and conductor and overcurrent requirements of their own. The calculation is the input to that step, not the answer.
Related
- Services and the load calculation method — the full lesson
- Load calculation helper — the tool — every factor yours
- Rusty on three-phase math?
- Finding it in the code book
- Reading a panel schedule
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