Replies

1. 
   MarkH128 | Jan 02, 2003 08:59pm | #1

   What kind of warning do you want?

   Your calculations are for amperage, not power. 20 watt bulbs are drawing 20 watts on 12 volts or 120 volts.

2. 
   UncleDunc | Jan 02, 2003 08:59pm | #2

   Power is measured in watts. Current is measured in amps.

   Watts = volts x amps.

   At the bulb, 4 bulbs at 20 watts each, dissipate 80 watts, and draw 6.7 amps of current, but at the transformer, measured on the 120 volt side, the current is only 0.67 amps and the power is still 80 watts, plus a little extra for the transformer losses.

1. 
   LynchPin | Jan 03, 2003 01:23am | #6

   Sorry -- right, power is watts. Misused that word. Meant

   current (when I was talking about "more than 2.5 times").

   So my real question is how do electricians determine the

   draw of these low-voltage fixtures when they are calculating

   the load on a given circuit? i.e., a 15 amp circuit can have

   1440 volt-amperes (80% of the capacity of the circuit). How

   do you determine the wattage of my example of the 4 20W bulbs

   on 12V? Is it just 80W, or do you have to account for the

   transformer or what?

   Thanks,

   John

1. 
   UncleDunc | Jan 03, 2003 02:11am | #7

   I don't know how the pros do it. Perhaps one of them will chime in.

   If this is a fixture, and not a system assembled from components, there should be a wattage number on a nameplate somewhere. If not, you could just round the current draw (on the 120V side) from .67 up to 1 amp. That would be a very generous allowance for the transformer losses, and make your arithmetic easier besides. I would probably just ignore the transformer losses, call it 80 watts, and figure that the current de-rating for the whole circuit would cover it.

1. 
   Haole27 | Jan 14, 2003 07:13pm | #8

   look at the transformer specs, that is the AC current draw

3. 
   booch | Jan 02, 2003 10:39pm | #3

   VA= Watts.

   That is the power measurement.

   Or as a friend calls it "West Virginia Law" w=v*a

   "The time has come," the Walrus said, "To talk of many things: Of shoes--and ships-- and sealing wax--Of cabbages-- and kings, and why the sea is boiling hot-- and whether pigs have wings"
1. 
   luvmuskoka | Jan 02, 2003 11:14pm | #4

   Hi Booch,

   So...my sander requires 220v and pulls 13.1 amps at start up. 220X13.1=2882 Watts.  How does that compare to say...an electric range or dryer?Ditch

1. 
   booch | Jan 03, 2003 01:14am | #5

   This better be a walk behind floor model sander or you are talking about the inrush... but they don't put inrush on the nameplate ... so this is probably a bigger model.

   A range is commonly 30A (+10 -5A) @ 220 Volts. So 6600VA or watts. In industry they go by Kva at that point) On a stove it is all resistive so the inrush is nominal, A dryer would have probably 2/3 of the load as resistive (heating elements) and the balance on the motor to turn the drum and fan.

   If anyone cares, Incandescent Light bulbs do have a peculiarity. It is different from motor inrush but similar.

   Motors pull 6x the nameplate amperage for the first fractional seconds until they come to speed. Then they level out to something below the nameplate amperage. That inrush is why non dual element fuses tend to blow on motor loads

   Incandescent lightbulbs are a resistive load so the inrush is small so it doesn't get up to the 6x rating. HOWEVER The first millasecond they are almost a dead short until the resistive heat builds up in the filament. The hotter the filament the greater the resistance the less the current. The only reason this would matter is if you are running a solid state output (proximity switches, "hockey puck" relays, or solid state timers. Even though the output may be rated for 1 amp at 120 volts you can't put a 100 watt incandescent lightbulb on the output. The reason is the current is way over the internal limit of the solid state switch for that millasecond. It might not die first cycle but you "use up" the circuitry with the heat generated internally in the solid state device.

   I had a customer toast 6 - 250 dollar proximity switches with a test just like above. He wanted to make sure he was getting good product so he tested them to make sure they worked. The normal load he ran was to a programmable controller that only pulled 5ma. I guess that 100 watt incandescent must have been the smallest bulb he had in the factory.

   Friend with a "Lighthouse" shop has a beacon that he flashed on and off with a solid state timer. The ss timers kept on going out. Same story. I gave him a repeat cycle relay (mechanical) and it has worked for a year now.

   But... The floor sander hopefully has its own thermal overload? Larger motors tend to have that. It is generally the first thing to frap out on a motor.