choosing wire sizes

[mr peanut]

I know know this is a stupid beginner question, but I'm a stupid beginner.

I have always understood conductor sizing to be a function of power requirements. I have done considerable searching and have found no specs for wire sizes in terms of watts. They are all amps. It seems to me that a wire conducting 1 amp at 1 volt is less likey to overheat than one conducting 1 amp at 600 volts. Am I wrong about this?

Where can I find guidelines for choosing wires (eg AWG 18, 24,28, etc.) in terms of expected power loads?

 

[gerty]

Here ya go:https://en.wikipedia.org/wiki/American_wire_gauge
Wire is rated in amps not watts, it also has a voltage rating for the insulation, but that's another post.

 

[MrAl]

Hi,

To go a little farther, if you do want to spec wire in terms of wattage then
you have to specify the operating voltage such as 12v, 120v, 240v, etc.
You can then divide the power (watts) by the voltage and get the required
amps, then look up a wire that can do the amps.

For example, say you have 1200 watt heater, and your home voltage is
120vac. Divide 1200 by 120 and we get 10 amps, so we look up a wire that
can handle 10 amps, at least. 15 amps even better.
On the other hand if your heater is 1200 watts and your home voltage is
240vac then divide 1200 by 240 and we get 5 amps, so you would then
need a wire that can handle at least 5 amps (10 amps even better).

You can always go higher in amp rating too, as long as you dont mind
the heavier wire.

 

[ecerfoglio]

Hi,

To go a little farther, if you do want to spec wire in terms of wattage then
you have to specify the operating voltage such as 12v, 120v, 240v, etc.
You can then divide the power (watts) by the voltage and get the required
amps, then look up a wire that can do the amps.

For example, say you have 1200 watt heater, and your home voltage is
120vac. Divide 1200 by 120 and we get 10 amps, so we look up a wire that
can handle 10 amps, at least. 15 amps even better.
On the other hand if your heater is 1200 watts and your home voltage is
240vac then divide 1200 by 240 and we get 5 amps, so you would then
need a wire that can handle at least 5 amps (10 amps even better).

You can always go higher in amp rating too, as long as you dont mind
the heavier wire.

Beware of the power factor (aka Cos fi):

In AC, Power is voltage times current times cos fi.

The power factor can be equal to one (a resistor, a heater, etc) or much lower - requiring more current for the same power and voltage

Typical power factors:

Heaters, incandescent lamps, resistors: ==> 1

Induction motors, at full load: ==> 0.8 (at light loads the motor has a lower PF)

Fluorescent lamps ("tube" type, with inductive ballast) ==> 0.5

 

[Hero999]

Assuming this is related to his other thread it's about enameled wire used in a transformer.

It's only the potential difference between adjacent turns that matters with transformers.

For normal electrical cables it's the potential difference between the conductor and 0V or the chassis that matters. Most cables have a breakdown voltage of >10 their rated voltage.

Edit:
Also the current rating needs to be reduced if the wire is to be coiled, like in a transformer - I'd derate by at least a factor of two.

 

[mr peanut]

I did post

a transformer question. It seems like ages ago and I forgot my old user name.
What was it? How did you figure out it was my post?

 

[crutschow]

It seems to me that a wire conducting 1 amp at 1 volt is less likey to overheat than one conducting 1 amp at 600 volts. Am I wrong about this?

Yes. An amp is an amp is an amp. It doesn't make any difference to the wire what the operating voltage is as related to its current capacity. It's the current through the wire resistance that cause a voltage drop in the wire and heats the wire. That's why a larger wire with lower resistance can carry more current.

 

[Willbe]

conductor sizing to be a function of power requirements.
have done considerable searching and have found no specs for wire sizes in terms of watts.
guidelines for choosing wires (eg AWG 18, 24,28, etc.) in terms of expected power loads?\
Right here.

Based on ambient temp. of 30°C, Table 310-16 of the '99 NEC shows that #10 AWG copper can carry 30A with 60°C insulation, 35A with 75°C and 40A with 90°C insulation.

Since heating is proportional to I^2, if you plot I^2 vs. ins. temp. using these I values you get almost a straight line.

If the wire is uninsulated you can sort of extrapolate this line up to the melting temp of #10 AWG copper and that's how much current an uninsulated wire can carry, but compare that with fusing current tables from the Internet.

At the other end, you can figure out how much current #10 can carry without any appreciable temp. rise by plugging in 30°C into your calculation.

Teflon insulation can stand very high temps. A soldering iron tip at 900°F doesn't melt it. Residences don't seem to use teflon wire insulation.

#10 comes in at 1000'/Ω so at 30A this wire is dissipating (30^2) x 1mΩ = 900 mW/ft, which gives a 30°C rise above ambient, but this does not take into account the increase in resistance due to the temp. increase.

These are all steady-state capacities. There is a MIL-SPEC (Military Specification) somewhere that shows transient current capabilities vs. wire size.

 

[mr peanut]

Thanks everyone.

I'm clear now. My original thought - that it was power that heats the wire - appears to be wrong.

I got mixed-up because resistor capacities are specified in watts and I was thinking of wires as resistors.

 

[Hero999]

If the wire is uninsulated you can sort of extrapolate this line up to the melting temp of #10 AWG copper and that's how much current an uninsulated wire can carry, but compare that with fusing current tables from the Internet.

Don't forget that the resistance of copper increases with temperature so the relationship probably isn't linear - especially at higher temperatures.

 

[Willbe]

https://www.powerstream.com/Wire_Size.htm

 

[Dean Huster]

"I'm clear now. My original thought - that it was power that heats the wire - appears to be wrong."

Well, it IS power that heats the wire: P = I²R

The wire has resistance, and the more current that passes through it, the more power it dissipates and the hotter it gets. It's just that the current will be dependent upon the voltage which is why you can get away with using #12 wire to operate a water heater at 240v but you can't if you're working with 120v because the current will be double.

#12 wire can take a lot more than the maximum of arount 25 or 30 amps the NEC specifies, but it'll get too hot to be safe. Ampacity is based upon insulation melting temperature more than anything and is affected by whether the wire is free-air, stuffed into conduit with other wires, etc.

Dean

 

[Sceadwian]

It dramatically depends on the insulation type as well and the environment the wire is in. A straight piece of 12 gauge wire in free air is a WHOLE lot different than a winding or something enclosed in PVC pipe.

 

[Hero999]

https://www.powerstream.com/Wire_Size.htm

I've seem similar tables to those before but they assume the conductor is at a fairly low temperature, i.e 90°C. Like all metals the resistance of copper increases with temperature at small increases in temperature, i.e increasing the temperature from 60°C to 120°C the resistance probably change much but increasing it to 500°C will increase the resistance significantly.

So for low temperatures described in the common electrical guides the temperture doesn't matter much but given that copper melts at 1084.62°C I don't think you can linearly extrapolate to that using the figures provided in the tables.

 

[Willbe]

For copper, the resistance at 100°C is 1.3x the resistance at 0°C.

 

[Dean Huster]

So for low temperatures described in the common electrical guides the temperture doesn't matter much but given that copper melts at 1084.62°C I don't think you can linearly extrapolate to that using the figures provided in the tables.

It has absolutely nothing to do with the melting point of copper! It's the melting point of the insulation! Do you realize how effective #18 (AWG) lamp cord (zip cord) is when you're trying to run 50 amps through it? Works for a few seconds ...

Dean