I took my Washington Administrator test on Friday - and it looks like I have re-take the load calculations section. I want your opinions on a hypothetical duplex with a standard calculation: if each unit had only 3 appliances (per 220.53), would you apply the 75% demand factor? The duplex as a whole would have 6 total appliances, which qualifies it for the 75% demand factor; yet the way I was taught was to look at each side of the duplex individually. Therefore, only 3 appliances per unit would mean it doesn't qualify for the 75%. What is the correct way to calculate this?
Duplex - standard calculation
- Thread starter TealHeals
- Start date
- Location
- Illinois
- Occupation
- retired electrician
This is from one of Mike Holt's calculation work sheet for duplex service.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
The title of 2026 NEC 120.53 is "Appliance Load -- Dwelling Unit(s)," and the section is worded so that for a feeder or service supplying multiple dwelling units, the 75% factor applies whenever the aggregate number of applicable appliances is 4 or more. Not per dwelling unit.
Cheers, Wayne
Cheers, Wayne
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
If you are calculating the load on one unit, you have 3 appliances. If you are calculating the load on the service to the building, you have 6 appliances. So it depends on how the test question is worded (i.e., what it tells you and what it asks). Do you know, for example, whether the utility has one service to the building as a whole (which splits into separate "feeders" to each unit), or whether the utility gives a separate service to each unit?
This hypothetical situation was one service to the entire building. So in hindsight, I see now how the appliances would aggregate.