# Help with simple calculation

Discussion in 'The Projects Forum' started by crossbone, Oct 8, 2007.

1. ### crossbone Thread Starter New Member

Oct 8, 2007
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I'm building my own studio lights at home and while Ohm's law is quite simple, I can't figure out how to model the voltage source from the a/c wall. I'm pretty sure if I draw too much current the circuit breaker will trip. so how do I calculate the maximum power or current output from each socket? and once I have the number, what's a safe and economical test?
thanks!

2. ### recca02 Senior Member

Apr 2, 2007
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the maximum power that u can draw from the ac outlet is terminal voltage multiplied by the current the circuit breaker would trip at.

3. ### crossbone Thread Starter New Member

Oct 8, 2007
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I suspected so. I checked the breaker but the fuses are not rated. instruction on the panel says 200A max, but didn't specify if that's for the entire house or each fuse. however, etched on the fuses are various unitless numbers like 15, 20, and 100. could these be the current ratings?
thanks.

4. ### beenthere Retired Moderator

Apr 20, 2004
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The devices in your power panel are circuit breakers, not fuses. They will open (trip) on an overcurrent condition, but may be reset to continue operation.

Each lamp should be rated in watts. That is a rating of how much power it taks to light them. Power is a product of the line voltage multiplied by the current. So, if you have a 60 watt bulb, you may determine its current draw by dividing 60 bt 120 (the line voltage). You see that each 60 watt lamp takes a current of 500 ma, or .5 amp, to operate.

After that, take the current draw of each lamp times the number of lamps to get the total current. Insure that the curcuit can safely supply that current, and you're in business. Keep in mind anything else that might also operate on that circuit, and that the mandated limit for actual current is less that the breaker's rating. The National Electrical Code says that 12 amps is tops in a 15 amp circuit, and 16 (I think) is the limit for a 20 amp circuit.

5. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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The 200A rating is the maximum current that the panel can serve under safety standards, independently of the fuses it may use.

6. ### dougp01 Member

Dec 6, 2005
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Keep in mind that a cold lamp can draw 2 to 3 times as much current as you would otherwise calculate. For example a 60W bulb would need about 0.5 Amps to run on a 120Vac circuit. But it can draw a couple of amps the first few cycles when heating up. To estimate inrush current to the bulb, do a cold measurement of the filament with your Ohm meter and divide this value into 120 V, the result is the cold current in amps.

If you are running close to the limit of your circuit breaker, just turn on the lamps one at a time or put them on a dimmer.

-doug

7. ### crossbone Thread Starter New Member

Oct 8, 2007
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Thanks for the tips, guys. another thing, I like to add my own fuse to the lights. I assume it should be at or just above the cold current then? what's the rule of thumb in determining the value? btw, the bulb I'm using is flourescent.
thanks!

8. ### recca02 Senior Member

Apr 2, 2007
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i think a good way to judge the value of current for fuse would be to keep it below the current value any device component can handle.

9. ### cumesoftware Senior Member

Apr 27, 2007
1,330
10
You are right, but in the case of a filament bulb, the inrush current only lasts a few microseconds, and it is not a serious problem.

The inrush current persists only for a few microseconds, and it is not enought to fuse a fast fuse. You may consider the next value above the expected current.

10. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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The etched numbers on the circuit breakers are indeed the Amperage ratings.

There is an easier way to do the math. Multiply voltage by breaker rating to get maximum watts for the branch circuit. (Example: 120Vac x 15A = 1800 watts.) Your lamps are rated in watts, yes?

The "80% rule" from the NEC which Beenthere cited is for your own safety and your own convenience. It is always a good idea to have "overhead." Fewer tripped breakers and cooler wires result.

11. ### crossbone Thread Starter New Member

Oct 8, 2007
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cool. thanks again. now I need to figure out how to estimate the value of the fuse.