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Thursday, September 15, 2011

Choosing Extension Cords for Plug-In Strobes


Before considering an expensive battery pack or generator for your big lights, don't overlook the obvious. Extension cords are a cheap and reliable way to get power to your studio strobes.

A couple hundred feet of power cable and a little advance scouting will solve the vast majority of your location needs. But be careful not to skimp. Here's how to choose a good extension cord for your strobes.
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The Short Version

For those not into the technicals, here's the cheat sheet: Buy the thickest cord you can, and don't run longer spans than necessary. Also, don't combine splitters with long cord runs.

Cords are pretty cheap. I bought the 12 AWG, 100-foot stinger (meaning, the female end lights up when the cord is live) above from Costco for ~$25. A small price to pay for remote power, and safe for even my juice-hungry Profoto Acutes.

Way better to be safe than sorry.


The Detailed Version

Note: All units below are expressed in US terms. Your voltages and power requirements may differ in other countries.

There are actually a few variables to consider when using AC extension cords to power big lights remotely. Four things you'll need to think about when choosing a cord for strobes are:

1. Strobe power requirements
2. The gauge of your extension cord
3. Distance you need to cover
4. Safe operating voltages for your flash


Strobe Power Requirements

This will be expressed in amps, and will loosely track with the maximum power rating of your flashes. For instance, AlienBees only need 6 amps continuous, while a Profoto 8A can draw 8 amps just to run two modeling lights -- the flash itself will add to that considerably.

And even the 6 amp draw of the AlienBees is a little deceiving. When the flashes are recharging, they will momentarily draw much more power. Long story short, make sure you understand what your flash need to drink so you can get a straw (extension cord) thick enough to feed it.


Gauge of the Cord

This is the thickness of your straw. A smaller number represents a thicker wire, i.e., 12 gauge is thicker (and better) than 16 gauge. In the U.S., an extension cord will say something like "12 AWG", which stands for 12 American Wire Gauge.

The wire should note a safe amperage level, too. If it does not, you can look up safe amperage levels for different gauges of wire easily.

For instance, my 12 AWG extension cord will safely carry the 20 amps that my Profoto Acutes need. But If I were shooting AlienBees, I could get away with a 16 AWG or even an 18 AWG cord.


Distance

The distance you are going to transport the electricity comes into play. Wire has resistance, and for a given thickness a longer wire will have more resistance. Thinner wires (say, 18 AWG vs. 12 AWG) also have more resistance.

Some of the voltage of your AC outlet will be lost across this resistance. A long run of thin wire can leave you with significant voltage drops, which could be harmful to your flash. How much current you are running will also factor into the drop. The more current needed, the more voltage drop over a given line.

There is a great online calculator for this, here. (Many thanks to reader Gary Nach for the link.)

So technically, you can run 13 amps through a 16 AWG cord. Meaning it is safe for the cord. But doing that over a span of 200 feet would rob you of about 22 percent of your voltage. Which could be very risky for your particular model of flash.

That calculator is a very neat tool, and it especially shows the dangers for your big flashes when running power excessive distances.

Speaking of that, pay special attention when combining long cord runs with splitters. You have to add up all of the current requirements of everything plugged into the splitter. So use short cords with splitter, or go with multiple cords.


Operating Voltages

The technical data in your strobe's manual will list the safe operating voltages. Most are designed to account for less-than-ideal voltages. For instance, your flash might be happy down to 90 volts.

But you have to remember that your mystery meat AC outlet on location might not have ideal voltage levels. And any voltage drop issues from long cord runs would exacerbate that problem.

Long story short, invest a few more dollars to get a nice, thick extension cord for your AC strobes. Cords are cheap and strobes are expensive. And when you are not using them for location power, your cords will also be versatile enough to handle other heavier current uses.


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32 Comments:

Blogger Jason said...

When shooting in nicer parks or around lakes where the city/county has mad alot of improvements, added boat ramps etc, I ALWAYS check around gazebos, bathrooms, picnic areas and look to see if any trees have landscape lighting. More often than not, there is a plug close by. I carry a 50 foot cord in my van and most of the time I have not even needed that much cord. A 100ft cord is MUCH less weight than a Vagabond 2 when you dont have an assistant.

September 15, 2011 12:07 AM  
Blogger Michael said...

You might also want to consider having a GFCI outlet added to a short cord & plug. This comes in handy if the outlet you are using doesn't have one and you are outside or if there is a possibility of water nearby. This is for safety reasons.

If you cannot find one, they are easy to make. They are cheaper than a lawsuit but will cost more than the extension cord.

September 15, 2011 12:22 AM  
Blogger Dean Bradshaw said...

Great post! Would love to see a post about choosing a generator to power studio packs. Thanks for being awesome David!

September 15, 2011 12:40 AM  
Blogger JoelB said...

And remember to always, always, unspool the entire cord when using it, even if you don't need the entire length.

The power loss is turned in to heat and if the cord is spooled it can't dissipate this heat and bad things might happen.

September 15, 2011 12:40 AM  
Blogger rmbwebs said...

There's a lot of cord manufacturers out there putting smaller-gauge cables inside really thick jackets to make it seem more heavy-duty.
Always check the gauge of the cable first. Smaller number is better! If you can't find the rating on the package, don't buy that one!

September 15, 2011 12:44 AM  
Blogger Jesse said...

and PLEASE remember that the electricity is traveling the entire length of the chord. If you are 10 feet from the outlet, and using a 100' chord, you are getting the same losses as if you were 99 feet from the outlet. Somehow this seems to escape people.

September 15, 2011 12:53 AM  
Blogger Frank F. said...

If you want to know the best way I've seen to keep that 100 foot cord from getting all tangled up see http://www.youtube.com/watch?v=eaEv9wm6gy0

September 15, 2011 1:26 AM  
Blogger Per Rutquist said...

Most countries outside the US have 220 Volts. This means half the current and only one quarter of the losses compared to 110 Volts. Long extension cords are less of a problem here.

September 15, 2011 6:59 AM  
Blogger www.robhammerphotography.com said...

A 100ft. extension cord is much lighter then the vagabond. However, the vagabond doubles as a power source/sand bag.Put the vagabond in a small backpack and it's much easier to carry. Then you don't have to worry about anyone tripping over a 100 ft. of chord, and knocking over your light.

September 15, 2011 7:39 AM  
Blogger Simon said...

I did not read through all the comments, but one thing you left out is: if it doesn't have a ground pin, don't buy it.

It's an obvious thing to say but it can't hurt to repeat it if it can save someone or someone's flashes!

September 15, 2011 8:01 AM  
Blogger Jonathan Histed said...

In the UK: if you are designing fixed mains voltage wiring it is currently 230 V+10%-6% (though this is slated to change to +-10%) at some point. You measure it with a voltmeter and you usually get 240V: this is all for harmonisation with The EU! Anyway, 2 serious points: (1) If you need to get a lot of juice a long way: you usually find it is cheaper to use a really beefy cable/flex for most of the way, then split it down (with a suitable distribution board, with suitable fusing to protect the thinner, on going flexs/ cables), than having a lot of "medium" wire going the full distance. It is a huge difference in cost and amount of copper. For lighting this will become apparent if you are setting up "hot" stage lights for example. You move the dimmers near to the lights... Also: when you start running large distances, it is voltage drop that dictates your flex/ cable thickness. So do check what the minimum working voltage is for the device on the end: lastly: you need to have sufficient current carrying capacity, that if there is a fulat condition, your safety device will trip, in the time allowed by your local design regulations. Again, this really does matter, and is enshrined in the rules of thumb as to how long cables can be, which often are explicit if you try buying made up longer than normal cables: they simply won't be sold. This can occur over surprisngly short lengths like 10--15 meters. It is not just a theoretical point.

September 15, 2011 8:26 AM  
Blogger newsshooter said...

You have impeccable timing. I was just thinking about this for lighting up a large building for an upcoming job. I know most of it from using power tools while working construction, but a refresher is always helpful

And to Frank F., thanks for the link, I always wondered how people did rolled cords like that.

September 15, 2011 9:26 AM  
Blogger NJTrout said...

Also look for a cord that is UL listed (US) or certified by your countries safety certification.

September 15, 2011 9:39 AM  
Blogger Tom Legrady said...

I've seen people who use the method suggested by @Frank F. While it is an easy-to-learn method for keeping ropes or cables neat, you won't find any professionals doing that. Look at sailors, electricians, theatre and rock-band lighting guys.

For one thing, once you get into thicker guages it becomes difficult to do anything but roll it onto a spool.

The almost-best way is to hold one end of the rope or cable, pull the other hand out a 'standard' distance, and move that point to the first hand, forming a loop, making sure to get rid of any twist in the cable.

You must have seen a cable coiled without getting rid of the twists ... usually by coining between hand and elbow. As soon as it comes off the arm, the cable coils up like spaghetti into unmanageable clumps. getting rid of the twist as you coil avoids that, and produces a neat coil that holds its shape when you store it.

In the theatres I've worked at, they have piece of wood protruding from the wall in a storage area, on which to hang cables. We used to attach long shoelaces or thin rope to the end of the cable, to tie around the bundle. A modern solution sometimes uses velcro. If you're going to store the coil in some other manner, it might be worth having two shoelaces, so opposite sides of the coil can have the bundle tied snug.

A slight improvement on the coil is to reverse the twist on each loop. With the simpler method, you have a little bit of a twist with each coil. If you were to grab one end and toss the rest of the coil ... say from a sinking boat to a rescue vessel ... the accumulated twist accross a large number of loops can cause a problem.

If you have one loop go 'on top' of the previous loop, and the next loop 'go under', the twists cancel out in pairs. Instead of 'a little twist', 'a little more twist', 'a little more twist', it's 'a little twist', 'no twist', 'a little twist', 'no twist'. So you canb toss your cable across the room or across the parking lot and have no twist or clumping.

September 15, 2011 9:43 AM  
Blogger colin j. said...

On the topic of safety, remember to tape down your cords as well.

September 15, 2011 11:33 AM  
Blogger Prelo said...

In addition to heavy gauge cords, I'd advise adding a circuit tester -about $4 at any hardware store - to the location kit. Its a cheap way to monitor wiring as it indicates dead circuits, bad wiring, and in particular open grounds.

September 15, 2011 1:00 PM  
Blogger Steven Noreyko said...

"Stinger" does not normally mean a cord that has a light-up end. It just means "extension cord" in film production jargon.

Stinger:
A single extension cord. Most often referred to a single 'hot' extension that is left lying around for occassional use. (Grip/Lighting)
http://www.filmland.com/glossary/Dictionary.html

Also - This post reads kinda like warnings from Homeland Security: "Be vigilant! Bad Things will happen if you use cheap cords!" What bad things?

Seriously - what are examples of what happens if you underpower your AC strobes?

In practice, I've had more problems on location with popped electrical breakers because of plugging in too many packs to a low powered circuit. BTW - If you're using more than a couple packs, look for 30amp circuits.

Last point - Profoto lists input power requirements for the Acute2 1200 generator as "Input Power Supply 100-127V/200-240V, 50/60Hz (nominal)" It does not list the AMP draw.

Although there is this:
Fuse Requirement for 2 units at max. speed setting Slow blow 10A/230V, 16A/120V
Automatic mains fuse type C, D, E 16A/230V 20A/120V

Is this your reference for the 20 amps you mention in relation to Profoto Acutes?

I was under the impression that a typical Profoto generator pulls about 9 amps when on recycling on "fast" and 4-5 amps when recycling on "slow" (not including amps for modeling lights)

September 15, 2011 1:22 PM  
Blogger DaveL said...

Great article! Now I am wondering how many of my alien bees (I have the poor-man's b400's) can I use in 1 15 amp circuit? I would assume they would all have their maximum draw at the same time, right?

Also, I prefer to use 4 of 25' cords to 1 100' cord - I find them much easier to coil up (but finding heavy 25' cords isn't as easy!).

September 15, 2011 2:12 PM  
Blogger John said...

All good advice for extn cords in general, not just for AC strobes. Having just lived for a while on generator power during hurricane Irene, I'm freshly reminded that cautions re voltage drops are just as relevant at home; for example, some appliances (notably refrigerators, according to my appliance repairman) suffer badly under low voltage, so not a good idea to run the reefer on 500 feet of extension cord (even 12-gauge).

September 15, 2011 3:01 PM  
Blogger Wing Tang Wong said...

Awesome tip, David, especially about the gauge of the cable and the losses with distance.

Another factor to take into account is the splitting of amperes when splitting at the ends or if you are extending from a split.

@Frank F, love the video. Looks like a great way to manage long unruly cables!

September 15, 2011 3:55 PM  
Blogger Daniel Sullivan said...

WHAT!!! No tech info on the photo setup? I know it's no free floating light bulb shot, but it's still a quality image and worthy of an explanation. Can I guess? I say 3 light setup, one from behind, on directly overhead and one on axis.

September 15, 2011 5:56 PM  
Blogger David Hobby said...

@Daniel-

Lighting? It is not even a photo. It's CGI.

After all of the bellyaching over the "appropriateness" of the Messi shot, I gave up photography altogether and render every surface after building a wireframe model.

I sold all of my SB-800s on Craigslist last week for $60 each.

September 15, 2011 6:00 PM  
Blogger Dale Pearce said...

I recommend the a.c. extensions that you will find on most film lighting trucks: A variety of 15, 25 and 50 foot, 12 Gauge SJ Stingers. These heavy duty cables have a flexible neoprene jacket that makes them virtually impervious to tangling. and cold wThey also remain flexible when it's cold. It's much easier to handle a couple of 50 footers or four 25's than a big, heavy 100 footer. Professional Stingers are always black by the way.If they accidentely appear in the shot, they might not be noticed. Orange and yellow scream "here I am."

September 15, 2011 6:16 PM  
Blogger Richard Davis said...

I thought the detail on the 'Messi' composite was great! So maybe we're not all shooting ad campaigns the care that went into lighting the elements of that shot and using doubles for the setup to save time with the talent was also great advice.

Wonder how many extension cords Gary Land used and how he (his crew) coiled them?

September 16, 2011 9:46 AM  
Blogger Aud1073cH said...

I really hate the daisychain (actually a simple crochet pattern) method for wrapping cables. - (sorry FrankF and newsshooter) Ever try to lay a cord straight against a length of wall, that was previously wrapped this way? Good luck!

I much prefer the under-over coil (mentioned at the end of Tom Legrady's post). Some people on YouTube call it a "roadie wrap"

Each wrap of the coil twists the cable a 1/4 turn. with the under-over method, alternating coils twists cancel each other out. This gives you a straight cord when you unwrap it.

I've used this method on cords as small as 26 or 30 AWG, and as large as 4/0 (copper as thick as your thumb).


Also, If you plan on using cords outdoors, look for the water and oil resistant jackets. Some SJ are not rated. Look for SJO, SJOW, SO, and SOOW jackets.

September 16, 2011 11:58 AM  
Blogger Andrew said...

Good idea on the GFCI, Michael. I believe you can buy a cord with GFCI built in. It would definitely be worth the extra dollars to avoid injury to self or others.

September 16, 2011 11:34 PM  
Blogger MTBtrials said...

Great post David!

I build CNC machines when I am not shooting weddings, so as someone with a good understanding of wire gauge and voltage drop, hats off to you for a well thought out and written article.

My reason for posting has to do with a very unique circumstance (read: hacks). Long story short, I was shooting a wedding where there were 120vac duplex outlets (read: household outlets) with a dimmer inline. The setup was supposed to be for the christmas lights around the stage, but I saw an outlet and thought "Score!" until my alienbee was acting erratically. I asked the band about it and they informed me about the dimmers in the circuit. I happened to have a voltage meter in my car and read 50vac at the plug I was trying to run my bees off of.

Just an anecdotal story to remind all my fellow strobies that not all wall sockets are created equally.

September 17, 2011 1:20 AM  
Blogger Jonathan Histed said...

@tom legrady: I agree: Franks method is horrible.

When working in theatre we would use Tom's method: I would add a couple of small points, to his very good descrition: rather than slavishly going "turn one way then the other" as I've seen people do, many cables have been "right norzed up" previously by beind wound over someone's elbow to hand to elbow: putting semi permanent twists in it, as the copper bends. When you come to coil this properly, it acts like a snake, with a mind of its own. What you do, having removed as much twist as possible by paying the cable out before you start, letting it untwist, is to note as you coil it the natural loop diameter it is tending to hang in, and go with that, rather than forcing a diameter which matches your own arm length. Secondly, as you twsit each coil, as you put it onto the coil that is so far formed in the other hand, rather than going "1 twist one way, 1 the other", go for the "least resistance", and as you coil each one, let it twist to whichever side its most keen to do. The cable will inevitably have an asymetric bias if someone else has coiled it badly before, so you may find it needs "3 one way, 1 the other"... and later on in the coil, it be 1 to 1... this is easy to do if you pay attention to the cable's tendancy, rather than forcing your own views on it. This also works when coiling rope. If you are coiling a fat multicore cable (e.g. 3 inches + thickness), you lay it on the floor, standing above it, giving a coil diamter much larger than you could hold in your hand. (e.g. 1 or 2 meters), and once done, two people can pick it up and move it.
Lastly taping cables down : I always hate this. Trip hazards are bad, but careful routing often avoides the problem, and often a taped cable is still a trip hazard, and you get sticky residue on the cable etc. (and it's wasteful fo material too): instead often a mat over it works far better, and there are often mats around that can be borrowed, e.g. in entrance lobbies. This is much harder to trip over than a taped cable, which is still a trip hazzard.

September 17, 2011 6:37 AM  
OpenID Radu Dumitrescu said...

Great post. I've seen lots of ridiculously long recycle times and hot wires from bad or badly chosen (too thin) extension cords.

Most people that don't have too many technical skills just think all cords are the same.

September 17, 2011 11:40 AM  
Blogger Raymond St Arnaud said...

Consider doing a follow up for those that might wish to use hot lights with extension cords. The danger here is keeping long cables coiled up when working with short distances from the outlet. A coiled power cable increases the resistance in the wire and the power cable will heat up to a very high level. I've forgotten most of my theory on opposing electromagnetic fields. A refresher would be nice.

September 17, 2011 1:50 PM  
Blogger Amos Terry said...

Totally off topic, but I was sent this link and thought it was pretty hilarious... Guy wants to trade a car for nikon gear. http://newyork.craigslist.org/fct/pho/2602025276.html

September 19, 2011 11:56 PM  
Blogger Mario said...

If you´re not sure what wire is in your cord, you can always buy a length of TSJ 3x12 cable at your hardware store and put on the end connectors. If you don't know how to, most hardware store guys will explain or might even do the job for you.

The big advantage is that you can be sure of what you're getting inside the insulation. And you can also customize your connectors as far as the hardware store inventory will allow.

I always carry around a hardware store variety multimeter and polarity checker, comes in real handy for mystery outlets. A few seconds and you can check what it's putting out, how stable is it, and whether it's wired right or not.

September 20, 2011 8:37 AM  

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