ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Violation?
by renosteinke - 01/27/23 09:52 PM
Does NEC 551.71 (F) apply to dwellings?
by BigB - 01/20/23 10:46 AM
Power submeter connections
by HotLine1 - 01/19/23 09:09 AM
AFDD's coming to the UK
by Texas_Ranger - 01/17/23 07:22 PM
New in the Gallery:
Burger King crown sillyness
Burger King crown sillyness
by wa2ise, December 11
240/208 to a house
240/208 to a house
by wa2ise, October 9
Who's Online Now
1 members (Scott35), 17 guests, and 22 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#129897 11/09/05 11:32 AM
Joined: Oct 2004
Posts: 132
M
Member
I have an instructor telling me that to do a VD calc, you use the following formula:
VD=(Amps x 1 way Length x R)/1000'

Where R = ohm/kFt from NEC Ch 9, Table 8

It seems to me like you would have to double the length since we are only counting the resistance in one direction...is this correct? I have always used the online calculators, and I'm trying to learn how to do it on paper.

Thanks, Mike

Joined: Sep 2003
Posts: 650
W
Member
Yup. One way or the other you need to incorporate the round trip distance.

Some equations do this by fudging the resistance values (essentially using the resistance of 2000 feet of conductor, and reporting it as the resistance for 1000 feet 'there and back again')

Others look at the voltage drop on one side only because there is assumed to be no voltage drop in the neutral.

But in all cases there needs to be some consideration of the full circuit.

-Jon

Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
VD=(Amps x 1 way Length x R)/1000'
This would be the correct formula if you are
using a 240 volt 2 wire circuit. There is no return distance required.
If you have a 2 wire 120 volt circuit then you need to count the return distance in the caculation.

Joined: Mar 2005
Posts: 38
D
Member
Also, you need to use a return distance of the square of three (1.73) for a three phase circuit.


Dolphins Software

Link Copied to Clipboard
Featured:

Tools for Electricians
Tools for Electricians
 

* * * * * * *
2023 National Electrical Code (NEC)
2023 NEC Now Available!
 
* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
Tom
Tom
Shinnston, WV USA
Posts: 1,044
Joined: January 2001
Top Posters(30 Days)
BigB 9
triple 3
Popular Topics(Views)
302,542 Are you busy
231,900 Re: Forum
216,535 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5