ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

Topic Options
Rate This Topic
#181853 - 11/03/08 07:23 PM need some help
Erick Offline
New Member
Registered: 07/31/08
Posts: 5
Loc: Ut
Guy's I am a crazy with embarrassment but here goes: An electric heater is rated for 2400 watts at 240 volts. What power is consumed when the heater is operated at 120 volts? I=P/E, R=E/I, I=E/R, P=EI, I know the steps but with unequal voltages?? man I am so stumped, I know the answer it's (600) but don't know the formula Ohms law out the window to a point what am I missing?
Top
Work Gear for Electricians and the Trades
#181854 - 11/03/08 08:17 PM Re: need some help [Re: Erick]
renosteinke Offline
Cat Servant
Member
Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
Watts = Amps times Volts
Volts = Amps times Ohms
therefore .. Watts = Amps times amps times Ohms
so ... amps times amps is a 'squared' relationship
that is, reducing the volts by half, and you cut the amps by four (2x2). Thus, 1/4th the heat.
Top
#181855 - 11/03/08 08:22 PM Re: need some help [Re: Erick]
canuck Offline
Member
Registered: 07/15/06
Posts: 27
Loc: Canada
First, use this formula to calculate the resistance @ 240 Volts

p = e^2.......p = watts e = volts r = ohms(resistance)
.....---
.....R


2400 = (240)^2
.........------
...........R

R = ((240) x (240))
.....-------------
.....2400

R = 24 ohms

Then use the same formula to calculate the power at 120 volts:

p = (120)^2
.....------
.....24

p = 600 watts

Here are some more formulas that are helpful:
http://www.the12volt.com/ohm/ohmslaw.asp#pie




Edited by canuck (11/03/08 08:26 PM)
Top
#181856 - 11/03/08 09:32 PM Re: need some help [Re: canuck]
Erick Offline
New Member
Registered: 07/31/08
Posts: 5
Loc: Ut
Thanks very much you guy's are great! Erick
Top
#181858 - 11/03/08 10:13 PM Re: need some help [Re: Erick]
SolarPowered Offline
Member
Registered: 07/05/04
Posts: 625
Loc: Palo Alto, CA, USA
However, remember that the resistance of the heating element is likely to change with temperature, so 600W is only an approximation.
Top

Member Spotlight
Member Since: 04/03/02
Posts: 6775
New in the Gallery:
SE cable question
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box


Who's Online
0 registered (), 87 Guests and 9 Spiders online.
Key: Admin, Global Mod, Mod
 
New in the Gallery:
SE cable question
 
Top Posters (30 Days)
Admin 51
HotLine1 43
gfretwell 17
Trumpy 16
Ruben Rocha 13
 
Newest Members
clee512, Jdscott2005, FAIZAN, Regitest2, sureshazhagai

ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals