Admin. exam test question

[rjack]

I am trying to take the WA. Admin. exam and am having difficulties with the steps for load calcs. I have done this question 3 times and have come up with 3 different answers. Can someone walk me through it?

Example: A duplex with a single 120/240 1 phase service
Each unit is 1900 sq. ft. with
9 kw range
5 amp garbage disposal
11 amp dishwasher
3 kw hot water heater
20 kw heater
3-21 amp 240 v ac units
NEC optional calc the service load is ___ amps

A. 225
B. 248
C. 263
D. 281

 

[bphgravity]

Add the following:

1. total square footage x 3
2. 2 small appliance x 1500 VA
3. 1 laundry x 1500 VA
4. 9000 VA range
5. 600 VA disposer
6. 1320 VA dishwasher
7. 3000 VA water heater

Take the 1st 10,000 VA at 100% and the remainder at 40%

Add this to 65% of the 20,000 VA heater load.

Divide total VA by 240 V = total service amperes.

 

[George Stolz]

...and then multiply it by two, for the other half of the identical duplex dwelling unit.

 

[charlie b]

...and then multiply it by two, for the other half of the identical duplex dwelling unit.

That will give you an answer that is too high. You're too quick with your fingers this morning. Have another cup of coffee.

You only need to count 65% of the stuff in the second unit. Your method would count 100% of the first 10KVA in the first unit plus 100% of the first 10KVA in the second unit.

 

[rjack]

Thanks for the help!

Thanks for the help!

I recalculated the Question and came up with 224.4 amps, which I am hoping means that 225 amps is the correct answer. I appreciate the help.

 

[bphgravity]

I calculate 239A figuring one dwelling unitthen multiplying by 2 and 214A doubling the loads first and then applying the demand factors.

I'm not sure which is the correct way or why I'm not getting an answer matching the OP?

 

[rjack]

Still confused

Still confused

My next question is where in the code book does it say that I can take the second unit at 65%? Also did anyone else come up with an answer of 224.4 amps,which rounds up to 225?

 

[charlie b]

My next question is where in the code book does it say that I can take the second unit at 65%?

Article 220 has you calculating one thing at a time. If you are sizing the panel to one unit, you add up that unit's loads, then apply the demand factors. If you are sizing the service to both units, then you add up all loads from both units, then apply the demand factors.

No matter which sub-part of 220 you are working with, this concept will apply.

 

[rjack]

confused

confused

Thinking I needed to recheck my work, I reworked the problem again and guess what, I came up with another answer of 196.94 amps???? I am signed up to take the retest tomorrow, its going to be a long day.... Oh by the way thanks everyone, for the help.

 

[M. D.]

Does 220.82.(c) apply in this case?

if so wouldn't the A.C. @ 3x21= 63 X 240 = 15,120va (item 1 220.82(c) )
be a larger load than the heater @ 20,000 x .65 = 13,000 ??(item 5 220.82(c) )

11400 = gen light

6000 = small app

3000 = laundry

18000 = range

1200 = pig

2640 = dishwasher

6000 = hot water

48,240 = total general load

first 10,000@100%

remainder @ 40% 38,240x .4 = 15,296

220.82(c) 100% a.c. 30,240

10,000
15,296
30,240
----------
55,536 / 240 = 231amps ???

I'm not sure we got the whole question????

 

[Smart $]

I get the same as you, MD, when I include laundry outlets. Laundry outlets are not specified and are not required: 220.82(B)(2) to 220.52(B) to 210.11(C)(2) to 210.52(F) Exception No. 2... but that would reduce the calculated load by 5 to 226 A. Still not one of the choices!

 

[mscada]

Different Opinion

Different Opinion

Per NEC 220.85 you need to treat it as Multyfamily Dwelling with 3 units. Calculate load for 1 Unit then multiply by 3(!) and multiply by 45%, The Answer is 248A.