Duplex Load Calculations

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I need help with this one I believe I am doing it correctly but the answer is just not there. Can anyone assist me with this. I have checked all of the examples in the exam prep book but I is not helping. I will show you how I am calculating it. Can you tell me where I am going wrong? I am using 2008 NEC
1) Given a duplex fed with a single 120/240 1 phase 3 wire system. Each unit has 1600 sq ft and each of the following.
7 KW range
5 amp garbage disposal
11 amp dish washer
3KW Water heater
15KVA Heater
2 ? 23 amp 240 volt Air conditioning units

Using the optional method for load calculations the feeder load is???

1600 sq ft X 3 = 4800 VA (220.82 (B) (1))
2 - 1500 VA small appliance circuit = 3000 VA (220.82 (B) (2))
1 ? 1500 VA laundry circuit = 1500 VA (220.82 (B) (2))
Range 7000 VA (220.82 (B) (3))
Garbage Disposal 5 amp X 120 volt = 600 VA (220.82 (B) (3))
Dish Washer 11 amp X 120 volt = 1320 VA (220.82 (B) (3))
Water Heater 3000 VA (220.82 (B) (3))
Total General Loads =21220 VA
100% of the first 10 KVA 21220VA ? 10000VA = 11220VA
11220 X .40 = 4488 VA
10000 VA + 4488VA = 14488 VA
General Calculated Load for one of the two units = 14488 VA

Heater 15000 VA use this because it is larger (220.82 (C))
A/C 23 amp X 240 volt = 5520 VA X 2 AC units = 11040 VA OMIT (220.82 (C))

14488 VA General Calculated Load
15000 VA Heating Load
29488 VA X 2 units (duplex) = 58976 VA
58976 total VA for the duplex
58976/240 V = 245.73 Amps round up
ANSWER = 246 total calculated load
 

jaylectricity

Senior Member
Location
Massachusetts
Occupation
licensed journeyman electrician
He means install a 320 amp service with two disconnects (one for each side of the duplex) Then feed each house with a 200 amp panel.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I think his "320" refers to the rating on the meter socket. Some of the common duplex meters sockets have a 320 amp rating. As long as you have more than one service disconnect, your conductors only need to be sized for the calculated load.
 
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