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Simple Question for Electrician


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#1 Stephanie Zimmer

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Posted 09 December 2005 - 09:09 PM

Hi,
Just a simple question. I'm not sure how to effectively read a breaker box to decide how much power I can draw from one location. Can someone explain how to read a breaker box for maximum voltage and amps per circuit? Thanks.
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#2 Michael Collier

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Posted 09 December 2005 - 10:27 PM

I've never read a breaker, so I cant tell you exactly where it would be, but somewhere it should say max amps before it will trip. look for a number like 20A, 25A, 10A etc. I believe it will be on the face of each breaker, so that doesnt give it a lot of room to hide.

To find the watts multiply your supply voltage by the max amps. So if its 20 amps, then times that by 120Vac (US standard) and you get 2400watts max power. voltage is a constant factor, it will not fluctuate (much)

Standard household cuircuts are usually 20Amps. In an industrial environment it sometimes reaches 50A

You have to account for everything on the circut though. If there are 3 computer monitors that have to be on in the scene you have to figure 150w a screen (aprox, each monitor will list its power requirements, as will any electronic device in the US) and factor that into your max power requirements.

Also make sure you dont reach the limit. Give about 10% headroom (if the breaker is 20amps, only use 18 amps) when you switch a light on it takes more power than the rating because of induction.

A good rule of thumb is to put no more than 2000 watts on any given circut.
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#3 Chris Keth

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Posted 09 December 2005 - 11:42 PM

I've never read a breaker, so I cant tell you exactly where it would be, but somewhere it should say max amps before it will trip. look for a number like 20A, 25A, 10A etc. I believe it will be on the face of each breaker, so that doesnt give it a lot of room to hide.

To find the watts multiply your supply voltage by the max amps. So if its 20 amps, then times that by 120Vac (US standard) and you get 2400watts max power. voltage is a constant factor, it will not fluctuate (much)

Standard household cuircuts are usually 20Amps. In an industrial environment it sometimes reaches 50A

You have to account for everything on the circut though. If there are 3 computer monitors that have to be on in the scene you have to figure 150w a screen (aprox, each monitor will list its power requirements, as will any electronic device in the US) and factor that into your max power requirements.

Also make sure you dont reach the limit. Give about 10% headroom (if the breaker is 20amps, only use 18 amps) when you switch a light on it takes more power than the rating because of induction.

A good rule of thumb is to put no more than 2000 watts on any given circut.



All good stuff to know. Usually when I'm lighting, I'll do my math with 100V rather than the US standard of 120. In addition to being easier to do the math quickly in your head, this gives me a good cushion for safety. If it's getting close on a circuit and I need more lights on, I'll figure it out exactly using 120V and come closer to the breaker value.

Also, even though this is common sense, NEVER bypass a breaker or bridge a fuse with a solid slug. This is a really good way to burn up the wiring in the walls and set a building on fire.
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#4 Stephanie Zimmer

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Posted 10 December 2005 - 03:43 PM

Thanks guys.

Michael, thanks for taking the time to explain.
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#5 david west

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Posted 10 December 2005 - 04:18 PM

on the newer breakers the cute little color coded handle will have the number stamped into the handle....
otherwise, as stated above, there will be a number on the face of the breaker...


older houses are more dangerous as you are reading a value in a fuse, but sometimes people put bigger fuses in and the wiring has already lost some of its ability to insulate --- so be extra careful in older homes...
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#6 Paul Bruening

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Posted 10 December 2005 - 04:37 PM

Hello Stephanie,

This is a touchy topic on this forum due to the dangers of electricity. However, The breakers will tell you the capacity of its circuits.

The largest number will be the main breaker. It will be a double unit with the throw levers locked together. In most homes it can be as high as 2- 100 amp breakers. Smaller apartments may have only a 2-50 amp main. Older homes may have anything including the rather hazardous blow-out fuse system.

You'll see one or two smaller rating double breakers. Commonly, they will be a 2- 50 amp and a 2- 30 amp. These are for the electric range (2- 50) and the electric clothes dryer (2-30). Sometimes there will be a third or fourth for electric heating and/or heavy AC units with appropriate ratings. You'll want to stay away from them for the most part since temp control is useful during shoots.

The rest will be single breakers rated at 20 amp or less. These cover all of the other light and wall plug circuits through out the house (keep in mind that most wall sockets can handle only 15 amps even though they are on a 20 amp breaker).

I can't tell you on this forum how to tap the box. Yet, a preferred trick for DPs on low budget productions is to have a line feed box made that can use the range, wall plug and dryer, wall plug as a safe source for juice. A licensed electrician can make them up for you. That will give you 2- 50 amp sources and 2- 30 amp sources. That gives you a grand total of 160 amps available at each source's limits.

You still have to run as few devices through out the rest of the house to avoid loading the mains beyond their safe limit. If you have to run the AC or electric heat then cut the lights until the house gets to temp.

I hope this is of use to you.
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