Liscencing test question

Status
Not open for further replies.

dave81

Member
I took a test yesterday that had a question that kind of stumped me need a little help here. It went like An electric furnace that was 240v and rated at 10000va output that was going to be fed by a 208v system what would be the va output of the furnace? it was multiple choice some of the answers were something like 7500, 7600, 6900, or 5400. Anybody have an idea and how do you get that? :confused:
 
Re: Liscencing test question

The key to the solution - the resistance of the device is determined when it is manufactured, and is the only value that does not change with a change in voltage.

Ed

[ March 03, 2004, 07:53 PM: Message edited by: Ed MacLaren ]
 
Re: Liscencing test question

Power = V^2 / R, so therefore R = V^2 / P
Let us then call V1 = 240, V2 = 208, P1 = 10000, and P2 is the unknown value we seek.

R1 = V2^2 / P1
R2 = V2^2 / P2
But as Ed said, R does not change.

So (240)^2 / 10000 has to equal (208)^2 / P2

Solving, P2 = 10000 times (208)^2 / (240)^2
P2 = 7511

I?d go with the 7500 answer.
 
Re: Liscencing test question

Well, here's my way (same as Charlie B, but different):

R=ExE/P
R=240x240/10000
R=5.76

I=E/R
I=208/5.76
I=36.11

208x36.11= 7511.11
A. 7500
 
Re: Liscencing test question

Well thanks guys I appreciate the help. That all seems easy now I should have gotten that I just brain f****d.

(Colorful language edited out by CEB)

[ March 04, 2004, 12:26 PM: Message edited by: charlie b ]
 
Status
Not open for further replies.
Top