0 members (),
37
guests, and
14
robots. 
Key:
Admin,
Global Mod,
Mod



Joined: Oct 2002
Posts: 830
OP
Member

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 ..




Joined: Jul 2002
Posts: 680
Member

Ohms law applies at low voltage.




Joined: Oct 2002
Posts: 830
OP
Member





Joined: Jul 2004
Posts: 625
Member

That's not Ohm's Law, however...




Joined: Oct 2003
Posts: 156
Member

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.




Joined: Aug 2001
Posts: 329
Member

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.




Joined: Nov 2002
Posts: 792 Likes: 2
Member

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.




Joined: Jul 2002
Posts: 680
Member

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?




Joined: Jul 2004
Posts: 625
Member

Ohm's Law is E = IR.
Watt's Law is P = IV.




Joined: Oct 2002
Posts: 830
OP
Member

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



Posts: 264
Joined: February 2013



