Basic wire sizing guide for US 120 and 240 volts

Contributed by: Moshmont
Submitted: 18-05-2004

Useful info:

# = American Wire Gauge (AWG), the lower the number the larger the wire guage.
"Service cable" is large insulated stranded copper cable (usually refers to single #4 wire and up).

Most house hold circuits (Typically 15 - 30 amp) can safely handle 95% to 100% of it’s max rating - but only for an hour or so at a time. Loads that require long periods of 'on' time (like an air conditioner, ballasts, etc) should not exceed 80% to 85% of the max rated load of the wire/cable. The known safe capacity that the National Electric Code (NEC) recommends at 100%, is actually 80% of load.

99.9% of the time when you get into "insulated" #8 and bigger wire, it’s most likely going to be stranded (just like it’s big sister "Service cable"). You can usually get your basic sheathed xx/2 and xx/3 cable up to 6 guage. If you need larger than #8 or #6 though, you will have to buy service or "service type" cable. This can cause two problems. One: very high cost on long runs. Two: the cable might not fit the appropriately-sized breaker on a long run. This is why it’s wise to opt for putting your ballast on 240V when 2400+ watts of light power is needed on a single circuit.

#4 and above = Cable
1/0 and 2/0 are Service cable

120V (US) (@ 80% max load)
(50ft run or less)
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For every extra 50 feet of cable/wire up to #8 normally you upgrade to the next size, consult you local codes if your unsure about double and triple length runs.

Ex: #6 is sometimes mandatory for a 200 foot 12 amp run but can be used up to 300 feet on a 12 amp circuit.

Each time an additional plug is used in line of the run using 80% safe load, subtract an additional 2% from the over all power usage (80% to 78%).

Ex: One plug into the wall counts as your one 'free' plug.

WARNING: extension cords ARE included into the total length from breaker box (+25 feet and one gauge up), if intended for continuous use at said MAX safe power usage.

In addition, you need to make sure you getting what is actually equal to said gauge (if your making you own cord from something like SJO cable).

Recently, I have found that some places go by size and not current. A 12 gauge standard wire is actually the size of 10 gauge solid. This is to make up for it not being a solid connector. Bring something with you to compare wire size with what's printed/stamped on the sheeting. It should be one gauge bigger in size than what's on the sheeting.
Ex: If you have a 1000W light and are using a 12 amp circuit, you should use a 15amp #12 extension cord no longer than 25 feet.

This info isn’t complete and probably doesn’t apply to many, cuz if your thinking this big you should already have a general understanding of codes and loads.

#4 (approx 65-75A each) used for 100-115 amp service
#2 (approx 90A each) used for 125-150 amp service
#1/0 (approx 150A each) used for 200 amp service
#2/0 (approx 175A each) typically for industrial or vary long run with a large load. 300-350 amp service
#0/3 (approx 200A each) typically for industrial or vary long run with a large load. 400 amp service

Service cable is specifically designed for extra service lines and or extra long (In structure or over-head) runs. 1/0 Gauge I believe is the only service cable (or cable) sold connected as x/3 (retail), provides a path fore both hots, the neutral and ground.
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