8717 Views 3 Replies Latest reply: Feb 2, 2012 5:56 PM by Richard_Comerford_Editor_Electronic_Products
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# LIGHT STRINGS and bulbs

I am trying to build a Marquee flashing light system. I can buy the controller to give me the affect required. I need to run four circuits of lights a total of 68 feet each. I would like to use the spools of cord with sockets. My question is. how long a run for each can I do if I plan to use the American light - Long Life Bulbs B75C this is an INDOOR application. what is the CHEAPEST light strings we can use knowing our runs will be about 70 feeet each and with the 7.5 watt bulbs that would make it about 525 watts per line. HELP me I am just a novice trying to build a display for a retail store'THANK YOUJim Ferris

• 1 posts since
Jul 19, 2011
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Jan 12, 2012 4:12 PM (in response to nathalie)
LIGHT STRINGS and bulbs

I know I'm a bit late and I'm having a little trouble understanding your request.

Some points to remember, you should not load a circut for more than 80% of capacity if the circuit is going to be on for more than 3 continuous hours a day.  So, you could have 120*15A = 1800 W or 120 * 20 A - 2400 W circuits in terms of power.  There could also be other devices on that circuit as well.  80%  valuues are 1440 W and 1800 W

525*4 = 1100 W and 1920 W.

##### 25 * 4 = 1100 W so it could be tight if there is other stuff on the circuit.

Wiring depends on voltage drop,  You should use wire, such that a 3% voltage drop could be tolerated at the light the furthest away.  Since this is lighting, I'd shoot for >0% and <5%.  No more than 5%.

The load is a little strange for a string of lights separated by some distance.  An approximation would be the entire load where the string plugs in.  So, you could  basically size the extension.  If the strings were puurchased, then the string wiring was already addressed.

There are some calculators floating around on the net.  The FUNDAMENTAL formula is R = pL/A where p is the resistivity of copper or aluminum, A is the cross-sectional area.  Wire tables use circular mills.  Then there is L.  L is not the distance to the load, but the path length which twice L.  Once going to the load and once coming back.

The formulas are made to look easier,  If you do it fundamentally, make sure the units cancel out.

A wire table will give you cross sectional area in circular mills by using the wire guage as input.  They roll a lot of the conversions into what looks to be p.   feet (length), mills (cross sectional area), ohm-cm (Resistivity), 2x the distance.  Everything needs to work out in the wash and units have to cnacel.

• 2 posts since
Jan 16, 2012
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Jan 16, 2012 2:28 PM (in response to nathalie)
LIGHT STRINGS and bulbs

Nathalie,

If you use this formula to find out current draw that total amps for all 4 light strings is 17.5 amps

Amps = Watts / volts   2100/120 = 17.5 amps

Most breakers in commercial buildings are rated at 20 amps for single phase.  The color code for your wires is Black is the hot (small prong on the plug with a gold colored screw )  the white wire is the neutral and the fat prong with the silver colored screw.  The green is the ground and goes to the green colored screw.  It would appear that our friend Kiss is an engineer (and right on the money with his answer to)  I am a maintenance electrician by trade  so I have worked with engineers to make their plans reality.   I try to keep things in civilian terms rather then what I speak in my trade other wise I get the dreaded thousand yard stare where I know that I have lost them in my explanation....   you may want to check with a local sparky (electrician ) to make sure your display is up to code.  They can spank you if it is not ( the municipality in which your display is )   ask for a consultation on the code requirements.  Low voltage lighting my be something to look into as (12 volt) the code is more relaxed.  Have you looked into LED lighting.

• 31 posts since
Sep 7, 2011
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Feb 2, 2012 5:56 PM (in response to nathalie)
Re: LIGHT STRINGS and bulbs

Hi Jim,

Assuming your using American Lighting - Globe Light String LSC12 with about 68 to 70 American Lighting - Long Life Bulbs B75C per run on this project, the power should be OK. But I strongly suggest that you work with a licensed electrician on this project or, better yet, turn the whole project over to one. The fact that this lighting will be in a retail establishment means that if anything does go wrong with the lighting, the lawyers will hang you on a scaffold of gold. This is definitely a job for a licensed electrician.

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