VD help

[patsfan51]

Hello,

I have two questions that I cant seem to figure out. Could someone point me in the right direction?

A 240v single phase circuit has a connected load of 10A. The total conductor length is 400' and the resistance is 1.75 ohms per 1000'. What is the net computed loss of this circuit in watts?
a) 70
b) 123
c) 175
d) 350
correct answer is a) 70.
Where do i begin if I dont have a wire size? I tried this for about a half hour and major brain cramp. any help would be appreciated. what am i missing/overlooking?

The other question is motor efficiency.
A 120v, 1/2HP, 80% efficient ventilation fans is replace with a new 120v, 1/2HP, 90% efficient ventilation fan. The new fan will draw what percent less current?
a) 10
b) 11
c) 20
d) 21
The answer is 11%. I came up with 7.84a on 80% and 8.82a on 90%. Why not 10%. the difference is .98 amps. the only way i got to 11% is 8.82x.11=.9702. does anyone know the proper method for this situation?
any help would be appreciated. just looking for a bump in the right direction on how to figure these out. thanks

 

[rgiraldo]

Number 1

Ohms Law 10a x 10a /.07 ohms = 70 watts

They said that 1000' of wire is 1.75 ohms. 1.75 ohms/1000' = 0.00175 ohms per ft

0.00175 x 400' = 0.7 ohms

 

[rgiraldo]

oops 10a x 10a x 0.7 = 70 watts . sorry about that

 

[nakulak]

the percentage ratio of current draw of the 90 fan versus the 80 is .90/.80

if you had divided the tho numbers, you get the same result. percentage is a ratio, not a subtraction or addition operation. versus usually means a division operation.

 

[Besoeker]

the percentage ratio of current draw of the 90 fan versus the 80 is .90/.80

You'd expect the 90% efficient motor to draw less current so it would be 0.8/0.9 and I suppose that's what the examiner would be expecting but it's another of those badly composed questions.
You have to assume that both motors have the same power factor.

 

[patsfan51]

thank you.

i understand the fan eff. one now.

but the voltage drop... why 10ax10a? i cant figure out whay the ten amps twice?

 

[Besoeker]

thank you.

i understand the fan eff. one now.

but the voltage drop... why 10ax10a? i cant figure out whay the ten amps twice?

The question is asking for losses, not voltage drop.

Power loss is I^2 *R or I*I*R
R is (400/1000)*1.75 or 0.7Ω
So power loss is 10*10*0.7 = 70W