Basic Gaffing Question

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#1 Craig Knowles

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Posted 08 January 2007 - 05:01 PM

I've got a really basic question: Say you have a regular old North American household/office outlet. How do you determine how many lights you can plug into it without overloading it?

Specifically, I've got an interior location shoot coming up where we will be plugging into on-location outlets in a conference room area of an office building. We're bringing 3 1Ks. How do I determine how many I can hook up to a particular outlet without risk of blowing a fuse?

Thanks.

Edited by Craig Knowles, 08 January 2007 - 05:01 PM.

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#2 Michael Collier

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Posted 08 January 2007 - 05:09 PM

Check the breaker box. Most US households support 20 amps, though some are ony 15. Things like the washing machine might be on a 50amp/220v outlet. Take the amps times volts and you got total watage ability. (120x20 = 2400 watts) that is acedemic figure, mostly, because in practicality there are other things that affect actual current usage. The bulb might be rated for 110v not 120, the longer cable runs might draw more than rated power (especially if your on a circut far from the box) I would recomend only on 1K per circut, and maybe a 500 or 650. 2 Kw is pushing it, especially if its all on one outlet.
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#3 Hal Smith

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Posted 08 January 2007 - 05:24 PM

I've got a really basic question: Say you have a regular old North American household/office outlet. How do you determine how many lights you can plug into it without overloading it?
Thanks.

The forever problem!

In commercial structures one can reasonably expect outlets to have 20 amp circuit breakers upsteam. If there's absolutely nothing else powered up on that circuit you can expect around 2400 watts available per circuit. Some office appliance outlets are often the best to use, for instance there's a good chance an outlet running a copier in a copy room was wired with a straight run to the breaker panel. Kitchen outlets are often wired similarly due to the heavy loads of microwave ovens, coffee makers, etc.

It's probably better to expect 15 amps/1800 watts as being available in residential circuitry. There are places in the US where the residential codes require #12 wiring and therefore good for 20 amperes but that's not a given.

The best plan is to buy a simple outlet tester and find separate circuits by turning off breakers on at a time and trying to locate all the outlets on that breaker. That can be a pain and requires turning EVERYTHING off except ceiling light fixtures (every code I know of requires separate circuits for ceiling mounted fixtures). But the more separate circuits you find, the more wattage you've got available. There are limits to this strategy, eventually you could overload the feeders coming into the circuit breaker panel but I wouldn't worry about using up to 4 separate circuits for lighting in either residential or commercial.

I would only follow my own advice in a reasonably modern building or house. In an old structure you could run into just about anything - including plug fuses and really cruddy wiring. Even the existence of outlets with a ground pin wouldn't tell you anything reliable about how modern the wiring was, the outlets could have been changed to avoid using grounding adapters

Michael's a bit more conservative than me - I'd worry more about panel box capacity rather than otherwise unloaded circuit capacity. Building a distro box that plugs into a dryer or range outlet is a good gambit - but if you're not electrically knowledgeable let a real gaffer or electician build it for you.
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#4 JD Hartman

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Posted 08 January 2007 - 05:33 PM

I've found that in the USA, a 15 ampere breaker will carry a 2000 watt fixture without tripping, most of the time. One of the first things to do on location, is locate the breaker panel or panels and make certain that you have access (covers unlocked). Access the available power (size in amps of the panel, single or 3 phase). The voltage available will dictate what lighting fixtures can be used. If the panel is 120/208v or 120/277v, a 6k HMI isn't going to strike if the ballast reguires a 220 volt input (or be damaged on 277 volts). A mistake like that, can make for a very bad first day of shooting.
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#5 Craig Knowles

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Posted 08 January 2007 - 05:35 PM

Thanks guys. Exactly the lesson I needed.

Edited by Craig Knowles, 08 January 2007 - 05:36 PM.

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#6 JD Hartman

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Posted 09 January 2007 - 09:14 AM

Hal Smith said, "Building a distro box that plugs into a dryer or range outlet is a good gambit - but if you're not electrically knowledgeable let a real gaffer or electician build it for you."

I've got to disagree with that idea and I know that these "converters" are already being sold. A 220 range or Dryer outlet will have two hots and a ground, but no Neutral. While the neutral and the ground on most branch circiuts are connected to the same buss bar in the panel, the NEC espressly forbids using the ground as a current carrying conductor. It is only to be used as an equipment ground. Doing what you suggest is a bad practice. However in many homes, you will find that the waser and dryer have their own dedicated circuit. One more source of power for your lights. This is usually true of the refrigerator and the dishwasher as well.
As far as gounded recepticals, buy a plug in outlet tester, about \$10 (made by Ideal, Greenlee, other), to see that receticals are properly wired and grounded. Always have some ground lift adapters available, pig noses, and if the home has fuses, be sure to bring spares of the correct amperage and type.
My ideas and those of the others, barely scratch the surface of the things to think about and be prepared for. Many things are learned through experience.
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#7 timHealy

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Posted 09 January 2007 - 09:40 AM

I've found that in the USA, a 15 ampere breaker will carry a 2000 watt fixture without tripping, most of the time. One of the first things to do on location, is locate the breaker panel or panels and make certain that you have access (covers unlocked). Access the available power (size in amps of the panel, single or 3 phase). The voltage available will dictate what lighting fixtures can be used. If the panel is 120/208v or 120/277v, a 6k HMI isn't going to strike if the ballast reguires a 220 volt input (or be damaged on 277 volts). A mistake like that, can make for a very bad first day of shooting.

American residential homes generally have 15 amp breakers. One may find 20 amp breakers once and a while but 20 amp breakers are more commercial in nature.

One has to read the breakers in the breaker box at the location to be sure. It is the only way. Typically that is done during the tech scout.

A 15 amp breaker may be able to hold a 2000 watt fixture but don't count on it. If you absolutely need it, one had better find a 20 amp power source to be sure. A 20 amp light draws 16.4 amps.

A specific question for JD:

JD, Where have you run into a 120/277 volt electrical system? Having been a film eelctrician for almost 20 years, I have never seen that. I'm not criticising. I'm just curious.

Best

Tim

Building a distro box that plugs into a dryer or range outlet is a good gambit - but if you're not electrically knowledgeable let a real gaffer or electician build it for you.

Personally, I do think this is a great idea for low budget filmmakers. In fact I did it myself when I was in film school. An older film student knowledgable about electriciity and who already had some professional experience built it for me.

It worked like a charm.

I agree that some 220 volt 50 amp plugs may not have a neutral, there are some that do. If it does, one is good to go.

The only improvement I would make is to install 20 amp breakers in each edison breakout for safety.

Best

Tim
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#8 Frank Barrera

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Posted 09 January 2007 - 10:29 AM

Specifically, I've got an interior location shoot coming up where we will be plugging into on-location outlets in a conference room area of an office building. We're bringing 3 1Ks. How do I determine how many I can hook up to a particular outlet without risk of blowing a fus

There should always be some type of location contact who should know at least where the service panel is. But in a professional building you should always speak with the house electrician or engineer. Your usage of their power is THEIR responsibility not yours. They cannot allow a curcuit to trip that might have a working computer on it. Often they will run you a couple of extension cords from other offices as per your requirements. And if it's a modern building they most definetly would have all 20 amp lines.
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#9 JD Hartman

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Posted 09 January 2007 - 03:19 PM

American residential homes generally have 15 amp breakers. One may find 20 amp breakers once and a while but 20 amp breakers are more commercial in nature.

One has to read the breakers in the breaker box at the location to be sure. It is the only way. Typically that is done during the tech scout.

A 15 amp breaker may be able to hold a 2000 watt fixture but don't count on it. If you absolutely need it, one had better find a 20 amp power source to be sure. A 20 amp light draws 16.4 amps.

A specific question for JD:

JD, Where have you run into a 120/277 volt electrical system? Having been a film eelctrician for almost 20 years, I have never seen that. I'm not criticising. I'm just curious.

Best

Tim

I've been in homes that had many 20a circuits. I guess it depends on how cheap the builder was. In any case a 15a breaker isn't going to trip at exactly 15amps.

Where do you see 277v? In factories and commercial buildings, typically for lighting. Where did I run into 208v? In a school, where the DP went on the tech. scout and I wasn't invited. It was a good thing that the budget didn't cover the 6k HMIs she wanted, because I would have had no way to power them from the tie-in.
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#10 timHealy

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Posted 10 January 2007 - 09:24 PM

I've been in homes that had many 20a circuits. I guess it depends on how cheap the builder was. In any case a 15a breaker isn't going to trip at exactly 15amps.

Where do you see 277v? In factories and commercial buildings, typically for lighting. Where did I run into 208v? In a school, where the DP went on the tech. scout and I wasn't invited. It was a good thing that the budget didn't cover the 6k HMIs she wanted, because I would have had no way to power them from the tie-in.

Hey JD,

I understand that one may see higher voltages in commercial applications, but you spoke of a "120/277" volt system in a thread about US residential applications. So that is where my confusion lies. Typically a single phase residential system would be 120/240 volts. As we all know a typical three phase system which most homes do not have run at 120/208. Many commerical application would have something like 270/480 for special applications (lighting. motors) but I really don't deal with those voltages unless using the Lighting Strikes 100k soft sun or the stunt men winches used on Spideman II.

A 15 amp breaker may not trip at exactly at 15 amps but one is cutting it close with a 2k. Sooner or later, and appropriately when needed the most, it won't work.

You can get 6k HMI's that run off 208 or 240 volts. Some older ballasts have switches and newer ones are automatic. You have to check the ballast one is using.

best

Tim

Ahh in re reading the first post he mentions "household/office" outlets. my mistake.

Edited by timHealy, 10 January 2007 - 09:27 PM.

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#11 Paul Bruening

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Posted 10 January 2007 - 11:30 PM

We all use practical rules of thumb to survive the scary world of household and commercial juice. My father's house is rated at the normal 110/220. Yet, often the 110 legs would meter as high as 132 volts. I can't believe his various systems could handle all that. Nothing ever burned up or out. So, what's my point? It can get really sloppy out there in the real juice world.
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#12 Donnie Lewis

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Posted 12 January 2007 - 02:44 AM

I've a related question, if you may. Let's say you have a grounded plug, can you use a 2 prong adaptor to plug it in an ungrounded socket?
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#13 JD Hartman

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Posted 12 January 2007 - 11:28 AM

Yes, you can. Many older homes still have un-grounded receptacles. You should carry some ground lift adapters with you.

With regards to voltage readings over 125vac, I had read that it was common for some utilities to raise the delivery voltage in the winter months. Something related to added revenues.
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