Where to use each wire type and size, and in which application.
Wire types and sizes
Gauge and current capability
The current capability of the larger wire gauges varies a bit with temperature, but here are the approximate specs...
8 45A (if not encased, otherwise 40A))
6 60A (if not encased, otherwise 55A))
4 80A (if not encased, otherwise 70A)
2 100A (if not encased, otherwise 95A)
A key point here is that 2/0 copper wire is allowed by NEC 2008 and IRC 2009 to carry 200 amps. This is significant because electrical panels are often 200A and 2/0 tends to be the best value for money of all the high current wires because it is sold in the highest quantity.
Two 2/0 wires in parallel are allowed by code to carry 300 amps, ie the current is de-rated to account for any uneven load sharing. In practice however, having two parallel wires will reduce voltage drop much more than the official de-rating suggests.
In my case I am using single run 2/0 wires to a house sub-panel, but I am using two sub-panels mounted next to each other in the house. As well as other benefits described below, this avoids any de-rating.
NM cable (NM stands for Non Metallic) (sometimes called Romex) is for Internal house wiring. It cannot be used outside or in wet or damp locations.
UF cable (UF stands for Underground Feeder) is for Outside use. It can be used it wet locations and can even be directly buried in the ground, but it CANNOT be used in conduit because of heat build-up issues.
W/G means that the cable also has a ground wire (usually not separately insulated).
The number after the dash gives the number of insulated conductors (ie not counting the ground wire if present).
Cable called "NM 12-2 W/G" has two insulated 12 gauge wires (a black Live and a white Neutral) plus a bare copper Ground wire and is only for use inside your dry house.
Cable called "UF 14-3 W/G" has three insulated 14 gauge wires (a black Live-1, a red Live-2, and a white Neutral) plus a bare copper Ground wire and is suitable for use outside in your yard or in other wet areas.
The term THWN (Thermoplastic High Water-resistant Nylon-coated) refers to the insulation round individual conductor wires, as does the term THHN (Thermoplastic High Heat-resistant Nylon-coated). For use outside in the ground inside conduit you need wire that is rated as THWN. Confusingly, most (but not all) wire called THHN is dual rated to meet the THWN spec as well as the THHN spec. You need to check the THHN wire you buy to make sure it also meets the THWN spec.
Recommended wire gauges
Only ever use copper wire (not Aluminum).
Additional considerations over and above the wire gauge current capability spec come into effect when deciding what gauge wire to choose. Firstly the wire has too be able to take the current for the breaker that is used on that circuit. If you have a 30 amp breaker then you must make sure that all the wire on that circuit can handle 30 amps (as determined by NEC 2008 and IRC 2009). Another consideration is that even though a particular gauge of wire can safely handle the breaker current, you will still get a voltage drop along the wire and for a typical long run that may be more voltage drop than you are willing to accept. The last factor is to choose a wire type that is sold in the highest quantity and is therefore the best value. Here are my recommendations on what wire to use for different breaker currents in their typical applications, together with HomeDepot pricing.
15A House lighting (dry areas) 120V
Interior Copper NM 14-2 with Ground $44 for 250'
20A House power sockets (dry areas) 120V
Interior Copper NM 12-2 with Ground $65 for 250'
20A Yard power/lights 120V
Sch 40 Conduit, with Solid Copper THWN 12 AWG, 3 insul wires 3 x $65 for 500'
(Note for comparison that 14 gauge is $42 for 500' and 10 gauge is $110 for 500')
20A Shed 240V
Sch 40 Conduit, with Solid Copper THWN 12 AWG, 4 insul wires 4 x $65 for 500'
30A Big Electric boiler 120V
Interior Copper 10-2 and earth $142 for 250'
55A Big Electric range 240V
Interior Copper 6-3 and earth $??
40A Well house 240V
Currently I have Exterior
with 3 insul wires & earth
4 x $2.17 per foot
But a WellHouse only really needs L1+L2 at 40A, ie 8 AWG.
200A House sub-panel 240V
with 3 insul wires & earth $2.17 per foot x 3 +
Separate runs to each sub-panel (I have two sub-panels).
If money is tight, fit one run of 2/0 (200A) and provide conduit for a second sub-panel later.
Lowes is a good supplier of 2/0 wire. If you buy 500 feet then it is $1.94 per foot.
Panel earth stake
Exterior copper #4 (one daisy chained wire
with no joins)
It is ok by code to use eg #4 bare copper for the earth for a 200A service. It wants to be uninsulated so it is in contact with the ground all the way.
I have two 200A panels in the house, so need to use two #4 bare copper wires.
A 200' roll of #4 copper wire is $197 from HomeDepot.
Details are here .
Copper is a rather volatile commodity on the world market. Lots of the world's copper supply is being used up in China's building boom. When placing a large order for all the wiring in your house it is worth trying to time your purchase when copper prices dip. Below are live charts that gives the price over the last 3 years and 3 months. There will probably be a small delay before any change in the world commodity price changes the price of a bundle of wire in Home Depot, but it absolutely will cause a change. A high price for copper is $4 per pound, but the hope is that it's below $3 per pound. In the 2008 crash it was $1.50 per pound, and if it ever hits that again then I will do a copper roof!