80 ft of 6 gauge wire conducting 100 AMPS?

[gfretwell]

The short answer is the wire will be a 780 watt toaster.

[Dave J]

I have a subpanel that is about 80 feet away from the main panel. It uses 6 gauge copper THHN wires conducting 220V current, in a 3/4" raceway. What will happen if I upgrade the breakers to 100 A and try to use close to 100A of 220V. Thanks, Dave J.

[shoppa]

750 watts over 80 feet of wire isn't so bad.

NEC lists the ampacity of 6 gauge wire as 55 amps or 70 amps depending on environment and raceway. 100 amps is certainly not up to code but probably won't cause an instant fire!

Upgrading to a larger wire size may well require a larger raceway, too.

Tim.

[SJF]

Yep! Wrong. Two things. My wire tables show 0.39 Ohms per thousand. Also, the formula for power consumed in a resistor is I *squared* times the resistance. Hence, 100 x 100 x 0.39 x 160/1000 = 624 Watts *total*, or

624/80 = 7.8 Watts *per foot* of conduit.

[John Grabowski]

What do you think will happen? I think it will burn up the insulation on the wire.

[George]

P=I^2*R

[clifto]

VOLTS dropped across the wire. 7.5 volts times 100 amps gives 750 watts dissipated in the wire.

[Terry]

That at first reading seems surprising? However; Using I^2 * R = 780 watts. Then 100 * 100 * R = 780 And R = 780/10,000 = 0.078 ohms If 0.078 ohms represents 80 times 2, feet (two conductors totalling 160 feet) then 0.078/160 = 0.49 ohms (per 1000 feet). This is close to 0.395 ohms per 1000 feet given in one available wire table. So the cable dissipating (780/160 = 4.9) nearly 5 watts per conductor foot and about 10 watts per 'cable' running foot will get hot!

NB: Referring to wire tables in an older version of Northern Electric's 'Electrical Conductors Handbook', also presuming we are referring to Number 6 AWG (American Wire Gauge) and copper conductors.

The AMPACITY of #6 AWG (Copper) is listed in various configurations ranging from bare and weather resistant conductors in free air, to "Not more than 3 conductors in a raceway or cable" as ranging from 55 to 100 amps.

So IMHO doubt that anything will actually 'burn off' but will not be in accordance with your current electrical codes for new construction?

The other consideration however will be voltage drop; 80 feet is quite a distance! Also using the 780 watt figure, of a total unity power factor VA of 100 * 220 = 22,000 watts; 780/22,000 = 3.5% will be lost as heat.

[Dave J]

Not likely. Resistance of 6 gauge wire is 0.47 ohms per 1,000 feet. I have 160 feet (going both ways), which makes resistance equal to 160*0.47/1000 = 0.075 ohm. At 100 amps, a 0.047 ohm resistor would produce 7.5 watts of power total, a negligible amount.

Is my calculation wrong?

Dave J

[Rick]

insulation on

equal

Power is current SQUARED times resistance...

[Doug Miller]

**typo** (you clearly meant "0.075 ohm")

Yes, and very badly so.

Voltage = current times resistance. Power = current SQUARED times resistance. So you're low by a factor of 100.

[HorneTD]

He can pull three number four AWG THHN /THWN conductors in the 3/4 inch raceway. Since the ampacity of #4 AWG is 85 Amperes he is permitted to install a ninety ampere breaker.