Power draw formula

S

sandisfield

Active member
I recall there is a formula for determining how many amps a fixture will draw. Can someone remind me?

I'm shooting in a home and don't wish to blow anything (power-wise).

I also recall that there are typically 15 and 20 amp circuits in household wiring, with the latter being identified by the little notch visible in the outlet. Am I recalling correctly?

S.
 
Ohm's law : P=VI , wher P is power in watts, V is Voltage and I is current or amperage. Or to determine amperage is I = P/V, or determine volts is V = P/I . So

If you have for example a 350 watt light bulb from a 120 volt source , then the amperage according to the formula is: I (amperage)= P (power in watts) divided by V ( voltage )

which translates to 350 divided by 120 = 2.91 amps.

As long as 2 values are known in the formula, the unknown can easily be determined. Don't forget that 1000ma or milliamps is equal to 1 Amp.

Typically all fuses should be marked as to their Amp capacity.. Be careful when dealing with electricity and fuse boxes, never substitute a higher rated fuse than the one than specified.
 
Also keep in mind "start-up spikes". A florescent Kino will have a certain level while running, but on striking the lamp, the amperage requirement will be over that for a short period.

I also recall that there are typically 15 and 20 amp circuits in household wiring, with the latter being identified by the little notch visible in the outlet. Am I recalling correctly?
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Not necessarily. Most houses will have identical plugs throughout the house. The only way to tell the difference and be sure is by reading the breaker/fuse in the box.
 
Good answers above.

If you want the industry rule of thumb... nothing scientific or etc.... If discussing tungsten units, it is generalized as a 1k= 10amps (it's actually around 8.4amps but they round up for caution, since you don't want to blow a circuit and you never know what else is on that circuit.)

Typically, the largest light you can have on one modern household circuit is a 2k, or a 1.8k HMI. Very old buildings may not have an 18 or 20 amp circuit. They might be 15. Why technical scouts are very important!

As mentioned, check the fuse box and it will tell you.


Regarding initial power draw, Doc is right. However, it's HMI's you need to watch for. Kino's are tiny, you'll likely never trip a breaker with a kino. You can put 8 Kino 4-bank units (32 globes) on a circuit... so i suppose if you turned them all on at once, you might pop a circuit.

For HMI's, if you have a 20amp circuit that is empty, you can likely get away with (albeit very close to the limit) use a 1.2k HMI and a 1k tungsten light on the same circuit... however the initial power draw of the HMI will likely trip the circuit. The secret to avoiding this is to plug in the HMI first, let it power-up and settle, and then add the 1k on a dimmer, and slowly bring it up, as sometimes when hot striking a 1k, an arc or surge can happen and also trip a circuit.
 
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Also to be aware of is if there are other appliances on the same circuit. Just because a fuse is rated say for 15 amps may only hold for another 4 amps if the other connected appliances total 11 amps on that same circuit. Check especially for fridges and/or AC's. Hair dryers are another power hog items.
 
LOL, I couldn't remember the name of HMI's. I knew Kino's were florescent and use a ballast.

"Dammit Jim, I'm an armorer, not a DP" LMAO!
 
Outstanding, many thanks.

Now, how do I differentiate between circuits. That is, how do I tell if one outlet is on the same circuit as another?

S.
 
Without a schematic or a good legend in the breakerbox...........flip the breaker and see what turns off. That's an electrician trick without using a hot tracer. Once you find a circuit off, take a small lamp and plug it in in any outlet in the same room and adjacent rooms. Check to see if it turns on. Check to see if the house lights and the outlets are tied together (very common) in each room.

Otherwise hire an electrician with a hot tracer. He can hook it up without disconnecting and disrupting the electricity to the circuits. It's a lot faster, but not as inexpensive.

Once that is done, you will know what outlets are on what circuit, and you will know what their amp ratings are.
 
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With pedant head on - with kit with ballasts, it's worth remembering that AC has power factor to take into account. This is because Volts x Amperes only gives you the real power when the load has a power factor of 1. When the power factor is less than 1, you are drawing more current than the real power consumed would imply. You can think of this as 'imaginary' power.
However, it's still real amps and so will still trip a breaker and heat up the supply wiring, so many electricity suppliers will charge you extra if you have a 'bad' power factor - hence the fitting of power factor correction to devices and buildings. Power factor compensation is a feature of better designs to restore the PF to close to 1.

We always use A x V = W because it works, and is usually close for practical purposes, but add transformers and it can go adrift. It's entirely possible to draw many hundreds of amps while consuming almost no real power! (This is also why AC generators are rated in "VA" instead of Watts)
 
for tungsten lights divide the lamp wattage by the voltage = amps
eg: (in australia) 2000w blondie divided by 240volts = 8.3 amp 800watt redhead divided by 240volts = 3.3 amps
 
sandisfield said:
Now, how do I differentiate between circuits. That is, how do I tell if one outlet is on the same circuit as another?
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http://www.harborfreight.com/circuit-breaker-detective-96934.html

I never leave home without it.

Whenever I'm gaffing a set, I always demand to see the locations ahead of time. Plug it in, wave it over the breakers, and you have your answer. It's ideal to have two people do it. Make sure you label each breaker, if they are not labeled, and each outlet - if the location authority will allow it - with paper tape, marking the ampacity and what breaker it's on. I usually do "3 - 15," for example.

It's going to save you a ton of time when you're in production, which is when every second matters.
 
That's a hot tracer. Didn't know HF sold them.

Just a note: be careful with electricity. It has a tendency to bite, if you are not careful. And if you have any questions or concerns, find an electrician.
 
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Jordan Scott Price said:
http://www.harborfreight.com/circuit-breaker-detective-96934.html

I never leave home without it.

Whenever I'm gaffing a set, I always demand to see the locations ahead of time. Plug it in, wave it over the breakers, and you have your answer. It's ideal to have two people do it. Make sure you label each breaker, if they are not labeled, and each outlet - if the location authority will allow it - with paper tape, marking the ampacity and what breaker it's on. I usually do "3 - 15," for example.

It's going to save you a ton of time when you're in production, which is when every second matters.
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I was going to post this link. After seeing it had been posted I wanted to rate it. However I am lowly in the DVX universe and have not enough points to rate. So +1! It can be done the old fashioned way but a hot tracer helps a ton.
 
Electricity does bite the body and the pocket book. I hooked up a 4k ballast to a 240 volt line by accident last night. Now that will cost me about $1500 to repair. Stupid mistake by a tired guy. Ouch!
 
Paul Hudson said:
Electricity does bite the body and the pocket book. I hooked up a 4k ballast to a 240 volt line by accident last night. Now that will cost me about $1500 to repair. Stupid mistake by a tired guy. Ouch!
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Oh, bugger! I haven't done that one yet. But a producer backed over my Honda inverter generator once until it bounced out the front of the car. So for good measure she pulled forward, ran over it again until it bounced back out the rear. Ground it to bits. Totaled. Thank god for L&D insurance.
 
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