test question

[treyp3]

"what size overcurrent protection device is required for a 120/240v service for sizing the overcurrent protection device based on loads where supplying the following loads?"

50a lighting load (continous operation)
42.a receptacle load (noncontinous) demand factors have been applied
35a appliance load (continous)
25 kva heat
motor load 230v 1 phase 10HP
motor load 120v 1 phase 5 HP
motor load 120v 1 phase 2HP

I say 400, the book says 500. The difference came at motor load 5HP. The book thakes the 56a I used and multiplies by 250% = 150a, thus the 100a difference. Please help.

 

[kingpb]

How are the loads split between phases?

 

[treyp3]

How are the loads split between phases?

Its a test question. It assumes the load is ballanced.

 

[Dennis Alwon]

125% of the largest motor not 250%

 

[augie47]

continuous appliance is most unusual.
without voltages on the appliances you must assume.
but, I'm in a different ballpark, I came up with 303 amps.

 

[treyp3]

125% of the largest motor not 250%

Cool, thats what I did to get 400a breaker.

 

[kingpb]

I came up with 500A by rounding up to the next standard size, but not using 250%. Just 125% of the 5Hp motor since it draws 6A more than the 10Hp 230V motor.

Frankly, IMO it's a lame question because your not supposed to calculate service size based on ampacity, you should always use VA, KVA, etc.

 

[treyp3]

continuous appliance is most unusual.
without voltages on the appliances you must assume.
but, I'm in a different ballpark, I came up with 303 amps.

Well, they are trying to mix it up I guess. This came from Stallcup master study guide. Continous would mean to ad another 25%. So, 35a appliance continouse load would be taken as 43.75 amps. Pretty sure its 396.75 amps total for a 400 amp breaker. However, Ive been wrong before!

 

[Dennis Alwon]

continuous appliance is most unusual.
without voltages on the appliances you must assume.
but, I'm in a different ballpark, I came up with 303 amps.

Gus did you use 25kva as 25 amps? instead of 104 amps

 

[Dennis Alwon]

Trey I got exactly what you got.

 

[Dennis Alwon]

Actually I think that the continuous appliance is calculated at 100% and not 125% so answer would be... 387.5 amps

 

[augie47]

Gus did you use 25kva as 25 amps? instead of 104 amps

I made some assumptions...
lighting and recepts are 120
appliances are 240...
loads are evenly divided...


and perhaps some mistakes :grin:
but...............

50 x 120v= 6000 va x 1.25 continuous = 7500 va
42 x 120v= 5040 va 5040
35 x 240v (???) = 8400 x 1.25 10500
25kva 25000
10HP @ 240 = 50 amps = 12000 x 1.25 15000
5 HP @ 120 = 56 amps = 6702 6720
2 HP @ 120 = 24 amps = 2880 **
72640
72640/240 = 302.6 amps
** We could delete this if the 5HP & 2 HP were on opposing phases leaving us with 291 amps

 

[treyp3]

I made some assumptions...
lighting and recepts are 120
appliances are 240...
loads are evenly divided...


and perhaps some mistakes :grin:
but...............

50 x 120v= 6000 va x 1.25 continuous = 7500 va
42 x 120v= 5040 va 5040
35 x 240v (???) = 8400 x 1.25 10500
25kva 25000
10HP @ 240 = 50 amps = 12000 x 1.25 15000
5 HP @ 120 = 56 amps = 6702 6720
2 HP @ 120 = 24 amps = 2880 **
72640
72640/240 = 302.6 amps
** We could delete this if the 5HP & 2 HP were on opposing phases leaving us with 291 amps

No. Lighting load = 50A, recepts = 42.5a ect...all dereating has been done

 

[treyp3]

Actually I think that the continuous appliance is calculated at 100% and not 125% so answer would be... 387.5 amps

Its a goofy question for sure, but Im guessing that the continouse vs non continous examples mean to make the question harder thus testing your knowledge of continous application. I hope my test is more straigh forward.

 

[Dennis Alwon]

Gus I don't think you can calculated the lighting load at 120v then divide by 240 in the end.

Also the 10hp is not the largest motor-- is it-- I thought it was the largest amp motor.

Lighting is 50*1.25= 62.5 amps
Recep......................42 amps
25000Kva.................104 amps
35 amp appliance.......35 amps
50 amp motor............50 amps
55 amp motor.............70 amps
24 amp motor.............24 amps

add them up and 387.5

 

[treyp3]

I came up with 500A by rounding up to the next standard size, but not using 250%. Just 125% of the 5Hp motor since it draws 6A more than the 10Hp 230V motor.

Frankly, IMO it's a lame question because your not supposed to calculate service size based on ampacity, you should always use VA, KVA, etc.

Could you please show the math. I cannot seem to get the 500a that the book says is the answer.

 

[treyp3]

Gus I don't think you can calculated the lighting load at 120v then divide by 240 in the end.

Also the 10hp is not the largest motor-- is it-- I thought it was the largest amp motor.

Lighting is 50*1.25= 62.5 amps
Recep......................42 amps
25000Kva.................104 amps
35 amp appliance.......35 amps
50 amp motor............50 amps
55 amp motor.............70 amps
24 amp motor.............24 amps

add them up and 387.5

35a continous may mean 125% has been taken into account I guess. I think this may be a better study book than I thought. It really drives me to reach out and come up with a solid answer and why. Thanks again Dennis and everyone else on the forum!

 

[Dennis Alwon]

Actually I think in reality Gus' answer is more like it. The lights at 62.5 amps will be distributed between the phases and have about half on each phase. Now there is really no way to accurately calculate this problem given what we have.

 

[CONTROL FREQ]

Just my guess... (sorry, edited that post cause I'm a slow typer)
Mike

 

[treyp3]

Just my guess... (sorry, edited that post cause I'm a slow typer)
Mike

recepts, heat and motors are not continous, ie no 125%. Also, you left out largest motor.