Duplex - standard calculation

T

TealHeals

Member
Location
Oregon
Occupation
journeyman electrician & licensed signing supervisor
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?
 
This is from one of Mike Holt's calculation work sheet for duplex service.
Determine the appliances, 220.53.

Dishwasher rated 11A at 120V 1,320 VA

Disposal 8A at 120V 960 VA

Water Heater +3,500 W

Unit Connected Load 5,780 VA

Total Appliance Load Demand = 5,780 VA × 2 units × 75%

Total Appliance Load Demand = 8,670 VA
Click to expand...
 
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
 
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?
 
charlie b said:
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?
Click to expand...
This hypothetical situation was one service to the entire building. So in hindsight, I see now how the appliances would aggregate.
 
TealHeals said:
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?
Click to expand...
That duplex question on the administrator exam is notorious. I remember not being able to come up with a matching answer, and people frequently ask about it on this forum.
 
electrofelon said:
That duplex question on the administrator exam is notorious. I remember not being able to come up with a matching answer, and people frequently ask about it on this forum.
Click to expand...
I thought I had a good handle on the calculations. I passed my Oregon General Supervising Electrician test on the first try. Yet with Washington, I had a couple calculation answers that didn't match any of the choices - so I went with the closest. If it was ampacity, I rounded up to match their closest answer. I'm a little sad I have to re-take this portion.
 
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