Determining Wire Size

You've got a zillion Christmas lights and they need power.  If only it was as easy as stringing a bunch of extension cords together.  Sometimes this actually works, but as your light display grows you have to start factoring in the size and length of all that wire.  

You can think of electrical wire a bit like water pipes.  If you wanted to fill a swimming pool, which would you rather use:  a 5/8" water hose or a 2" fire hose?  The bigger the hose, the more water it can carry.  Same is true for an electrical wire.  If you have to power a large load, you need a large wire.  But there's another factor most people don't consider: friction.  Just because you have a 2" water hose to fill that pool, if the hose is several thousand feet long, the friction of the water against the inside of the hose will give you just a fraction of what you want at the output.  The same is for electricity except you have voltage drop over long distances. 

What you get at the end of that extension cord is a factor of both wire size and wire distance.  If the wire is too small and the load is too large, you can end up with the wire heating up (because of too much electron friction inside).  Likewise, if you have a small load but it is several hundred feet away, you'll have less voltage to use at the end of that long wire. 

To keep you even more confused, wire size is backwards from the way we normally think (no doubt some sort of conspiracy by electricians.)  In most cases, the smaller the wire gauge, the bigger the wire.  As an example:  16 gauge wire is bigger than 18 gauge wire.  This holds true until you get to really big wire... larger than 0 (zero) gauge and logic changes, but you shouldn't have to worry about anything that big. 

Shortcuts:  Wire Sizing,  Wire Types, Power Formulas, Electrical Safety

Wire Sizing:

Always try to oversize your wire or extension cords.  Trying to get by with wire that's too small for any reason will lead to trouble.  Use the following chart as a good starting point. 

Ext. cord length	Amperage Required
	0-2 amps	2-5 amps	5-7 amps	7-10 amps	10-12 amps	12-15 amps
25 ft.	16 ga.	16 ga.	16 ga.	16 ga.	14 ga.	14 ga.
50 ft.	16 ga.	16 ga.	16 ga.	14 ga.	14 ga.	12 ga.
100 ft.	16 ga.	16 ga.	14 ga.	12 ga.	12 ga.	10 ga.
150 ft.	16 ga.	14 ga.	12 ga.	12 ga.	10 ga.	-
200 ft.	14 ga.	14 ga.	12 ga.	10 ga.	-	-

Backstage Handbook by Paul Carter has these recommendations:

	Gauge for Rated Load in Amperes
Length	2	3	4	5	6	8	10	12	14	16	18	20
25'	18	18	18	18	18	18	18	16	16	16	14	12
50'	18	18	18	18	16	16	14	14	12	12	12	12
100'	18	16	16	14	14	12	12	10	10	10	8	8
150'	16	14	14	12	12	10	10	8	8	8	6	6
200'	16	14	12	12	10	10	8	8	6	6	6	6
250'	14	12	12	10	10	8	8	6	6	6	4	4
300'	14	12	10	10	8	6	6	6	6	4	4	4
400'	12	10	8	8	6	6	4	4	2	2	2	2

All extension-cord jackets are marked with a code that indicates (among other information) the American wire gauge (AWG) as well as the jacket material and its properties, according to standards established by the National Electrical Code.

Wire Types

Then there's the challenging of deciphering that odd code on the side of most of your extension cords. 

 

In the picture above, The AWG 12-3 is telling you the American Wire Gauge (AWG) is 12 and there are 3 wires inside.  The SEOW means... well, see below:

• O: Oil-resistant, usually synthetic-rubber jacket, more flexible in cold temperatures
• OO: Oil-resistant synthetic-rubber jacket and inner-conductor insulation
• S: Standard service (synthetic-rubber insulated, rated for 600v)
• SE: Extra-hard usage, elastomer
• SEOW: Oil-resistant and weather-resistant elastomer jacket, rated for 600v (photo above)
• SJ: Service junior (synthetic-rubber insulated, rated for 300v)
• SJO:  Same as SJ but Neoprene, oil resist compound outer jacket, rated for 300v
• SJOW: Oil-resistant and weather-resistant synthetic rubber, rated for 300v
• SJOOW: Oil-resistant and weather-resistant synthetic rubber (jacket and conductor insulation), rated for 300v
• SJT:  Hard service thermoplastic pr rubber insulate conductors with overall plastic jacket, rated for 300v
• SJTOW: Oil-resistant and weather-resistant thermoplastic, rated for 300v
• SJTW: Thermoplastic-jacketed, weather-resistant, rated for 300v
• SO:  Extra hard service cord with oil resistant rubber jacket, 600v
• SOOW: Same as SOW but with oil resistant rubber conductor insulation and suitable for outdoor use.
• SOW: Rubber jacketed portable cord with oil and water resistant outer jacket
• SPT-1: All rubber, parallel-jacketed, two-conductor light duty cord for pendant or portable use, rated for 300v
• SPT-2:  Same as SPT-1, but heavier construction, with or without third conductor for grounding purposes, rated for 300v
• SPT-3:  Same as SPT-2, but heavier construction for refrigerators or room air conditioners, rated for 300v
• ST: Extra-hard usage, thermoplastic (PVC), 600v
• STO:  Same as ST but with oil resistant and thermoplastic outer jacket, 600v
• STOW:  Same as STO but with oil and water resistant thermoplastic outer jacket, 600v
• SV:  Vacuum cleaner cord, two or three conductor, rubber insulated, rubber jacket, 300v
• SVO:  Same as SV except neoprene jacket, 300v
• SVT:  Same as SV except all thermoplastic construction, 300v
• SVTO:  Same as SVT except with oil resistant jacket, 300v
• THHN:  600v nylon jacketed building wire
• THW:  Thermoplastic vinyl insulated building wire, moisture and heat resistant
• THWN:  Same as THW but with nylon jacket
• W: Extra-hard usage, weather-resistant

Power Formulas

For the math wizards out there:

Power:
Watts = Volts * Amps
Amps = Watts / Volts

Sine Wave (VAC):
RMS Volts = 0.707 x Peak Volts
RMS Volts = 1.11 x Average Volts
Peak Volts = 1.57 x Average Volts
Peak Volts = 1.414 x RMS Volts
Average Volts = 0.637 x Peak Volts
Average Volts = 0.9 x RMS Volts

Electrical Safety

Now we need to talk about electrical safety.  Again, if you have any questions or concerns at all about electricity, please consult a licensed electrician.  Remember, this stuff can kill you.

When working with electricity, you always run the risk of electric shock, burns or fire.  A good preventive measure is to buy electrical cords that have the UL and OSHA labels on them.  Those labels tell you the cords have met rigorous standards and been subjected to spot-testing to ensure their reliability.  Remember, extension cords suffer routine wear and tear that can compromise their safe operation.  For instance, although many extension cords are rated water resistant, they should not be left underwater for extended periods of time because minor nicks and abrasions on the insulation can allow water to seep into the cord's interior, cause shorts and lead to all kinds of unanticipated fireworks.

Another extension cord challenge concerns plug ends.  It's tempting to grab a cord and yank it out of an outlet instead of pulling on the plug itself.  Eventually the cord jacket will separate from the molded plug and probably break the wire connection inside the plug.  You will eventually end up with a hidden short that gives someone a nasty surprise.