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

Page 1 of 2 1 2 >
Topic Options
Rate This Topic
#45973 - 12/09/04 04:16 PM ohm's law , low voltage also?
sparkync Offline
Member
Registered: 10/17/02
Posts: 812
Loc: NC
I'm doing a low voltage project, and not use to the amperage etc. the smaller wires. When calculating the amperage, I'm assuming the math technique is the same. I have 1 alarm that is 15 watts. the voltage is 12 volts. Therefore 15 watts divided by 12 volts gives me 1.25 amps. This is the right formula no matter what the voltage etc. is, isn't it? I need a little assurance here:; Thanks ..
Top
Work Gear for Electricians and the Trades
#45974 - 12/09/04 04:27 PM Re: ohm's law , low voltage also?
walrus Offline
Member
Registered: 07/25/02
Posts: 680
Loc: Bangor Me. USA
Ohms law applies at low voltage.
Top
#45975 - 12/09/04 04:46 PM Re: ohm's law , low voltage also?
sparkync Offline
Member
Registered: 10/17/02
Posts: 812
Loc: NC
Thought so; Thanks ....
Top
#45976 - 12/09/04 07:29 PM Re: ohm's law , low voltage also?
SolarPowered Offline
Member
Registered: 07/05/04
Posts: 625
Loc: Palo Alto, CA, USA
That's not Ohm's Law, however...
Top
#45977 - 12/09/04 07:43 PM Re: ohm's law , low voltage also?
dereckbc Offline
Member
Registered: 10/08/03
Posts: 156
Loc: Tulsa, OK
Ohm's law applies, but when working with LV, wire and connection resistance comes into play. I assume your device consumes 15 watts over a range of voltage say from 11 to 14 volts applied at the device.
Top
#45978 - 12/10/04 03:31 AM Re: ohm's law , low voltage also?
Steve Miller Offline
Member
Registered: 08/30/01
Posts: 325
Loc: Loudoun Cty, VA
Just as an FYI: if your project is security alarm or fire alarm there are other requirements for the load calc.

Also be a bit careful about the length & size of the wires. #18/20/22 alarm wiring is often very long runs and can add a lot more resistance than the 14/12/10 we normally use.
Top
#45979 - 12/10/04 02:07 PM Re: ohm's law , low voltage also?
wa2ise Offline
Member
Registered: 11/29/02
Posts: 782
Loc: Oradell NJ USA
A certian length of say 14 gauge wire will have the same voltage drop at the same amps no matter what the supply voltage is. However, that voltage drop will be a bigger percentage of the supply voltage when the supply voltage is lower. Thus, what is an acceptable drop at 120V would be bad when the supply is 12V. 12 volt light bulbs will look dimmer than 120V bulbs seeing the same voltage drop. This is a big reason why POCOs use high voltages for long distance transmission.
Top
#45980 - 12/10/04 05:15 PM Re: ohm's law , low voltage also?
walrus Offline
Member
Registered: 07/25/02
Posts: 680
Loc: Bangor Me. USA
Quote:
That's not Ohm's Law, however...

Isn't it derived from ohms law though??. Seem to remember someone proving that one time or another?
Top
#45981 - 12/10/04 05:58 PM Re: ohm's law , low voltage also?
SolarPowered Offline
Member
Registered: 07/05/04
Posts: 625
Loc: Palo Alto, CA, USA
Ohm's Law is E = IR.

Watt's Law is P = IV.
Top
#45982 - 12/10/04 06:57 PM Re: ohm's law , low voltage also?
sparkync Offline
Member
Registered: 10/17/02
Posts: 812
Loc: NC
My "Ugly's book" says that the three basic Ohm's law formulas are: I=E/R, R=E/I, and
E= IxR

I = Amperes
E = Volts
R = Ohms
P = Watts

Maybe Ugly's wrong Thanks anyway for the input... Steve
Top
Page 1 of 2 1 2 >

Member Spotlight
Member Since: 02/23/13
Posts: 204
New in the Gallery:
SE cable question
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box


Who's Online
0 registered (), 48 Guests and 12 Spiders online.
Key: Admin, Global Mod, Mod
 
New in the Gallery:
SE cable question
 
Top Posters (30 Days)
Admin 47
HotLine1 43
gfretwell 19
Ruben Rocha 12
Trumpy 9
 
Newest Members
KrmtFrg, Stack, Scotto, Freecrowder, clee512

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