Dryer Load Question

[patsfan51]

Hi all,

I am studying for my jmans and came across a dryer calc. question that has me confused.
Q: An apt building has 25 units. Each unit has a 4000w electric clothes dryer.The total service demand for these dryers is ____ kw?
1) 100
2) 100000
3) 125000
4) 40.625

My calculations come to:
25x4000=100000
100000x.34 demand(t.220.54 35%minus .5% for each over 23)=34000w
This is not an option for an answer so am I missing something?
Does the "Where two or more single phase dryers supplied by a 3p 4wire service load shall be calc. on basis of twice the maximum number connected to any two phases." come into play?

Would anyone be able to point me in the right direction figure this out?
thanks

 

[jumper]

Hi all,

I am studying for my jmans and came across a dryer calc. question that has me confused.
Q: An apt building has 25 units. Each unit has a 4000w electric clothes dryer.The total service demand for these dryers is ____ kw?
1) 100
2) 100000
3) 125000
4) 40.625

My calculations come to:
25x4000=100000
100000x.34 demand(t.220.54 35%minus .5% for each over 23)=34000w
This is not an option for an answer so am I missing something?
Does the "Where two or more single phase dryers supplied by a 3p 4wire service load shall be calc. on basis of twice the maximum number connected to any two phases." come into play?

Would anyone be able to point me in the right direction figure this out?
thanks

I think it should be 25 x 5000, 220.54. Nameplate or 5000, whichever is higher.

 

[jumper]

I came up with 42,500? What exam book are you using?

 

[patsfan51]

using mike holts understanding the NEC but this question is from a code review class that I took last year...

 

[wbw1989]

25 units x 5000w= 125,000w x 35%= 43,750w
minus 0.5% for each dryer exceeding 23units
25 dyers-23= 2 would be 10%
43,750w x 10%=4375
43,750-4,375= 39,375w
thats what i came up with

 

[patsfan51]

Hmmm.... The .5% for each additional over 23..... I thought that was deducted from the 35% to make 34%?

 

[wbw1989]

my fault, i was doing 5% instead of .5%. Now im getting 42,500w and i was wrong it is deducted from the 35%

 

[raider1]

I came up with 42,500? What exam book are you using?

I concur, this is what I have come up with.

5000 watts times 25 dryers = 125,000.

demand factors in Table 220.54 would be 35% minus 1% for 2 dryers over 23.

So we would apply a 34% demand factor to 125,000 VA and have an adjusted calcualted load of 42,500.

Chris

 

[JFletcher]

What is the definition of "total service demand"? Isn't 220.54 and its table permissive (90.5(B)) by 220.14(B)? - "shall be permitted..." vs the rest of 220.14(A, C-L) which say "shall be calculated...". If so, I'm going with answer A, 100kw. Or am I wayyyy wrong?

I completely agree with Chris and everyone else that came up with 42.5kw is a much better answer.

 

[raider1]

What is the definition of "total service demand"? Isn't 220.54 and its table permissive (90.5(B)) by 220.14(B)? - "shall be permitted..." vs the rest of 220.14(A, C-L) which say "shall be calculated...". If so, I'm going with answer A, 100kw. Or am I wayyyy wrong?

I completely agree with Chris and everyone else that came up with 42.5kw is a much better answer.

You are correct, the code does not require that we use the demand factors in Table 220.54 but instead permit the demand factors to be used.

In my opinion this question does not contain the correct answer if a person uses the code permission for the demand factors.

I have found from being an instructor that many exams don't have correct answers.

Chris

 

[joebell]

25 units X 5000va= 125000 va

220.54 states that the demand factors "Shall be permitted". My thought is the answer is simply 125,000 va and the the demand factors do not need to be applied.

 

[jumper]

You are correct, the code does not require that we use the demand factors in Table 220.54 but instead permit the demand factors to be used.

In my opinion this question does not contain the correct answer if a person uses the code permission for the demand factors.

I have found from being an instructor that many exams don't have correct answers.

Chris

I believe that Chris is right and none of the answers is correct. A resi exam prep question that does not fully utilize the demand factors is worthless.


original question:
Q: An apt building has 25 units. Each unit has a 4000w electric clothes dryer.The total service demand for these dryers is ____ kw?
1) 100
2) 100000
3) 125000
4) 40.625



1 is wrong because it is mulltipling by 4000.
2 is wrong because it is multipling by 4000 and is va and not kw.
3 is wrong because it is va and not kw.
4 is wrong, but was probably supposed to be the answer. I believe I even figured out why. The guy did the math wrong.
25-23=2, 2 x.5=1, it seems that he added .5 to 2 for 2.5, 35-2.5=32.5
125000 x .325 = 40625.

 

[joebell]

Kva=kw

Kva=kw

I believe that Chris is right and none of the answers is correct. A resi exam prep question that does not fully utilize the demand factors is worthless.


original question:
Q: An apt building has 25 units. Each unit has a 4000w electric clothes dryer.The total service demand for these dryers is ____ kw?
1) 100
2) 100000
3) 125000
4) 40.625



1 is wrong because it is mulltipling by 4000.
2 is wrong because it is multipling by 4000 and is va and not kw.
3 is wrong because it is va and not kw.
4 is wrong, but was probably supposed to be the answer. I believe I even figured out why. The guy did the math wrong.
25-23=2, 2 x.5=1, it seems that he added .5 to 2 for 2.5, 35-2.5=32.5
125000 x .325 = 40625.

The last sentence in the 08 code in 220.54 states that "Kilovolt-amperes (KVA) shall be considered equivalent to Kilowatts (KW) for load calculations in this section"

That being said the only remotely possible answer would 3. 5000 KW X 25 units =125,000

 

[jumper]

The last sentence in the 08 code in 220.54 states that "Kilovolt-amperes (KVA) shall be considered equivalent to Kilowatts (KW) for load calculations in this section"

That being said the only remotely possible answer would 3. 5000 KW X 25 units =125,000

It is the units that are asked in the answer that count. 125000va or 125000w, yes.
125000kw,no. 125,000kw=125,500,000va/w.

 

[JFletcher]

5000 KW X 25 units =125,000

5000KW is off by a factor of 1000.

~~~~~~~~~~~~~~~~~

I still can't find a definition anywhere of "total service demand". I took it to mean all dryers operating at the sametime at their nameplate rating, which is 100kw.

I also picked #1 not because it was right, but because if I prove the other answers wrong (which I can), then what remains must be correct.

I'm curious how an instructor would grade the answers to this question.

 

[joebell]

It is the units that are asked in the answer that count. 125000va or 125000w, yes.
125000kw,no. 125,000kw=125,500,000va/w.

In therory I agree with you VA and Watts are 2 different animals as far as load calculations the code is saying they are interchangeable.

 

[joebell]

5000KW is off by a factor of 1000.

~~~~~~~~~~~~~~~~~

I still can't find a definition anywhere of "total service demand". I took it to mean all dryers operating at the sametime at their nameplate rating, which is 100kw.

I also picked #1 not because it was right, but because if I prove the other answers wrong (which I can), then what remains must be correct.

I'm curious how an instructor would grade the answers to this question.

Section 220.54 requires the load calculation for electric clothes dryers be 5000 watts or the nameplate rating which ever is higher, that is how I am coming up with 5000

 

[jumper]

5000KW is off by a factor of 1000.

~~~~~~~~~~~~~~~~~

I still can't find a definition anywhere of "total service demand". I took it to mean all dryers operating at the sametime at their nameplate rating, which is 100kw.

I also picked #1 not because it was right, but because if I prove the other answers wrong (which I can), then what remains must be correct.

I'm curious how an instructor would grade the answers to this question.

Raider1 is an instructor and like he says, many exams have wrong answers. I follow your logic as 100kw as the right answer out of what is presented, however, I still think the person messed up the math as I showed earlier.

 

[jumper]

In therory I agree with you VA and Watts are 2 different animals as far as load calculations the code is saying they are interchangeable.

VA=W ,yes. The question asks for KW.

Q: An apt building has 25 units. Each unit has a 4000w electric clothes dryer.The total service demand for these dryers is ____ kw?
1) 100
2) 100000
3) 125000
4) 40.625

 

[joebell]

VA=W ,yes. The question asks for KW.

Q: An apt building has 25 units. Each unit has a 4000w electric clothes dryer.The total service demand for these dryers is ____ kw?
1) 100
2) 100000
3) 125000
4) 40.625

Is this not a load calculation question? Would the text in 220.54 not apply. The last sentence says what it says. The only issue one could have other than that is what code cycle is the question based? 2008 then I believe the 125,000 answer to be correct, if it earlier than that say 2005 then the sentence did not appear in 220.54