ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Increasing demand factors in residential
by gfretwell - 03/28/24 12:43 AM
Portable generator question
by Steve Miller - 03/19/24 08:50 PM
Do we need grounding?
by NORCAL - 03/19/24 05:11 PM
240V only in a home and NEC?
by dsk - 03/19/24 06:33 AM
Cordless Tools: The Obvious Question
by renosteinke - 03/14/24 08:05 PM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
1 members (Scott35), 258 guests, and 16 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#130453 05/30/06 08:05 AM
Joined: Aug 2001
Posts: 7,520
P
pauluk Offline OP
Member
From Kenbo:

Quote
Help!

I set my students some home work the other day.

Image 1 is the question and his answer
Image 2 is his working out sheet
Image 3 is my solution

The answers are greatly different but I can not see where it has gone wrong.

I know the reason for the differences is staring me in the face but I just can not see it.

[Linked Image]

[Linked Image]

[Linked Image]

Stay up to Code with the Latest NEC:


>> 2023 NEC & Related Reference & Exam Prep
2023 NEC & Related Reference & Study Guides

Pass Your Exam the FIRST TIME with the Latest NEC & Exam Prep

>> 2020 NEC & Related Reference & Study Guides
 

Joined: Aug 2001
Posts: 7,520
P
pauluk Offline OP
Member
Hint: Look at the voltage he's used throughout the three tables. [Linked Image]

Joined: Apr 2004
Posts: 349
Member
Just working this out real quick, I come up with about 9.14 ohms total resistance and approx 6.3KW. Line current = 26.26 amps.

Radar


There are 10 types of people. Those who know binary, and those who don't.
Joined: Apr 2004
Posts: 349
Member
To answer the actual question, I think the problem is in the calculation of watts. Regardless of who's total resistance is most correct, they're all in the 9.something range. 9.something line amps operating in a 240V circuit will yield somewhere around 26 amps, and that will result in roughly 6,400 watts (ballpark answers).

Radar


There are 10 types of people. Those who know binary, and those who don't.
Joined: Apr 2006
Posts: 233
K
Member
Realise the problem now thanks Paul

Radar you are correct. The calculations in the 3rd sheet is the correct answer.

What I did not realise the student had done wrong is to treat the whole calculation as entirely parrallel. He correctly worked out the equivalent resistance for each of the three groups of resistors but then he should have treated the rest of the problem as a series circuit.
Then it would have worked out.

Some times you just can not see something obvious when you look to hard... [Linked Image]


der Großvater
Joined: Aug 2001
Posts: 7,520
P
pauluk Offline OP
Member
Quote
He correctly worked out the equivalent resistance for each of the three groups of resistors but then he should have treated the rest of the problem as a series circuit.

By my calculations, the left-hand group of four resistors will have 63.1V across them, the middle pair 49.7V, and the right-hand group 127.2 volts.

I'm a little puzzled as to why he decided to work out the total power the long way like this. He'd already worked out the overall resistance as the first part of the problem, so why not just use E^2/R, or do as you did and work out the current then get the power from I x E?


[This message has been edited by pauluk (edited 05-31-2006).]


Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5