Be sure to read the question carefully, and make sure you know if they are asking about a single unit or the service to the two units.
If you are calculating the load for either of the units, you only account for what is in the unit. But if you are calculating the service to the duplex building as a whole, you count everything. But you don't get there by doubling the value for a single unit. The demand factors don't work that way. For example, consider the general lighting load and the demand factor that applies to it. You take 100% of the first 3000 VA, then you take 35% of the next chunk. If you look at each of the two units separately, you will be taking 100% of the first 3000 VA for the left hand unit, and doing your calculation, and you will be taking 100% of the first 3000 VA for the right hand unit, and doing your calculation. If in the process of calculation the overall service you simply double the result of one unit's calculation, then you will account for 100% of the first 6000 VA. That will give you an answer that is too high by 1950 VA.
Also, a single unit might not have four or more "appliances fixed in place." So when calculating one unit, you won't be able to use the 75% demand factor of 220.53. But when calculating the service to the duplex as a whole, you might have a total of four or more "appliances fixed in place," when you count the total appliances in both units together. So here again, if you just double the answer for a single unit, your answer is likely to be higher than the answer that the test question considers to be the right answer.
Good luck on your exam. Let us know how you do.
