ECN Forum Forums General Topics of the Trade General Discussion Area Track 12

Mark,

The wattage of incandescent lamps changes with the amount of voltage applied. If you reduce the voltage the current does not increase. Sometime look at a package for 130 volt lamps which will list one wattage for 130 volts and a lower wattage if used on 120 volts. The same applies to any resistive load. A typical household range will have a higher wattage rating when connected at 240 volts than when connected to 208 volts. With inductive loads such as motors the current does increase as the voltage decreases.

Curt


[This message has been edited by caselec (edited 12-05-2004).]

Mark you are correct it is basic ohms law lets take a look.

Lets say the circuit is 300 watts at 12 volts

300 watts / 12 volts = 25 amps

12 volts / 25 amps = 0.48 ohms

Now we know that the collection of lamps has a total resistance of 0.48 ohms

Now drop the voltage to 10 volts

10 volts / 0.48 ohms = 20.8 amps

Drop the voltage to 3 volts

3 volts / 0.48 ohms = 6.25 amps.

Drop the voltage all the way down to 1 volt

1 volt / 0.48 ohms = 2.08 amps

As far as the dimmers, I understand how a modern dimmer works, it does not matter the effect is the same.

Bob

Bob, its apples and oranges, both are fruit.

Your describtion of the apple is fine. (What happens at the lamp)

I'm talking about the lemon. (The length of circuit between the source and load, which includes the track itself.)

I guess we will agree to dis-agree.....

[This message has been edited by e57 (edited 12-06-2004).]

Mark It is not apples and oranges (well our thoughts are )

You said


Not true for this load.

You also said the problem was melted fixtures

I have shown that the current and heat would be less at the fixture.

Now you say the problem is in the circuit feeding the fixture.

Yes 14 AWG NM is to small from a safety and design stand point for a 25 amp load of any voltage.

Change it to 10 AWG NM and the safety issue is gone even though the design issue is still there.

If we used 10 AWG for the circuit in question we would be NEC compliant even though we would have 6 volts or 50% voltage drop.

The fixtures would be very dim but there would be no safety issue, the tracks, fixtures and power supplies would be fine.

The lamps would last many years at this reduced voltage.

If you want to leave it as a disagreement that is fine. Perhaps some other members can chime in and explain it in a different way.

Bob

Bob's explanation is right on the money (and he and I have agreed to disagree from way back.)
There's nothing I could add that would make it any easier to understand.
btw-AC impedance DC resistance

You don't switch to DC with a transformer. All non-battery powered 12V lighting I've worked on has been AC.