Power Transmission

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Last Updated: Friday, 11 August 2023 21:37
Published: Tuesday, 19 July 2022 19:57
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Power Transmission:

In Electrical Engineering, power generation and transmission is one of the most fascinating subjects. It's not in fashion anymore, but it's the one that's most relevant in your day to day electrical work at home. Whether it's electric plugs, electric panel in garage or grounding wires, they all require some basic knowledge of power transmission.

 

Electrical wires:

The wires that you see at home in your attic are the ones that transmit power to all the outlets in your home. Their thickness, shape, etc vary depending on what is going to be running off that wire.

Material: Most wires are made of copper or aluminum.

Aluminum has lots of disadvantages compared to Copper:

  • Both have low resistivity, but copper has lower resistivity than aluminum (Cu=1.7*10^-8 ohm-m, Al=2.6*10^-8 ohm-m), so copper is almost exclusively used in wires now a days.
  • The only downside of copper is it's higher price compared to Aluminum (used to be 2X the price of copper, so older homes used Aluminum). Aluminum was being used in long distance transmission due to it's lower cost, and being lighter.
  • Aluminum is softer and so easier to get cut, which may potentially cause fires if there are combustible materials around it). Copper has more strength with enough flexibility.
  • Aluminum is more susceptible to damage and heats up quicker. Aluminum heats up more and expands more which may again be a fire hazard.
  • Aluminum also rusts, and the resulting oxide is non conducting resulting in even higher resistance.
  • Copper can easily be soldered and requires smaller conductors, which is a big plus for chips.

So, copper preferred overall for any kind of wiring. Even Silicon chips now use copper wires instead of aluminum. The covering around the wire is made of plastic or silicon

The most common type of wiring used in homes is non-metallic (NM) cable, commonly called "Romex," after the popular brand name. New NM cable (known as NM-B which are more heat resistant than older NM ones) contains two or more insulated conducting wires and usually a bare ground wire. All of the wires are encased in a flexible plastic jacket or sheathing. These cables are rated for 600V. Most common one are 14-2G meaning it's 14 gauge and 2 wire (ground wire is not counted, even though it's present there).

Gauge (AWG => American Wire Gauge): Gauge refers to resistance of wire, and hence how much current it can carry safely. It's unit is AWG or just G. Higher the gauge of wire, higher the resistance and thinner the wire. Temperature of wires go up as they carry more current (as they get heated up more). Thinner wires provide more resistance to current, so I^2*R power dissipation is higher, resulting in higher heat and less area to dissipate. Lower the Gauge thicker the wire. Note that wires don't really have max amp rating, as they can carry really large currents before they melt. We don't want to get to that point. We want to limit currents to safe temperatures as 90C and below. How much temperature the wire can be allowed to go to depends on what is around that wire, and if that can safely dissipate that heat. 90C is considered max temp we usually allow wires to go to, as beyond that, things around it may get too hot to catch fire. Think about houses in USA which are made of wood, and have insulating materials around it, which may potentially catch fire at high temps. Below we see temperature vs current for various Gauge:

A simple chart for Gauge vs thickness: https://www.powerstream.com/Wire_Size.htm

  • 16G => carries 14A @60C and 18A @ 90C (common in extension cords). This is the most common gauge found in wires that we buy off the shelf in stores. They are used commonly in building PCs, electronics, etc. They can withstand a lot of abuse, without breaking. This is also the cheapest wire, as it's widely available.
  • 14G => carries 20A @60C and 25A @ 90C (common inside houses). Usually home wires won't run at 25A currents, as most appliances consume < 15A. So, these wires won't reach temps over 60C.
  • 12G => carries 25A @60C and 30A @ 90C => not common, but people buy it to be on the safer side.

16G to 12G wires are from 1mm to 2mm in thickness (diameter), and have resistance from 13 ohms/km to 5 ohms/km.

Resistance of 1km long Cu wire with 1mm radius =  ρ * L / Area = 1.7*10^-8*1000m / Π * (10^-3)^2 = 5 ohms/km (matches closely with what's taken from link above. In silicon chips for transistors < 100nm , these Cu wires are radius=0.1um=10^-7 m. That gives Res=5*10^-8 / km = 50 ohms/um.

Max amp is rated from 20A to 40A for chassis wiring, as we don't want temps to exceed 100C. As can be seen, it's difficult to transmit DC voltage over the wire as the voltage droop would be V=I*R=20A*10ohms/km=200V/km. So, very high voltages would be needed to transmit them over long distances, as the droop would be unacceptable with lower voltages.

Skin Effect: The biggest downside of a thick wire is that AC current flowing thru it is not able to utilize the whole cross section of the wire due to "skin effect". Skin effect causes most of the AC currents to flow on the outside cross section of the wire, so for high freq AC current as 500Hz or so, it may be wiser to use a thinner wire since thick wire gives same performance as a thin wire.

Wire Coloring: There are 3 wires in 1 cable -

  1. live (incoming current), Color = black, red or blue (usually black for 1 phase supply coming into the house)
  2. neutral (return current): Color = white or gray
  3. ground: Color = green

Neutral and Ground are connected together at transformers, bot not inside the house. So, potential at neutral wire is close to that of ground wire, so you may not get big shock touching the neutral wire. But live wire is always at 120V AC (0 to 120V in a sinusoidal wave), so you should never touch live wire as it's very high potential. Same amount of current is flowing in live wire and neutral wire (as live wire brings current in, while neutral wire provides the return path, thus completing the circuit). If we touch live wire, we form a resistance from 120V to 0V (our leg touching the gnd is 0V), providing huge current thru our body. But if we touch neutral wire, it may be at most at 10V AC, which forms a resistance from 10V AC to 0V, thus providing much lower current to flow thru our body. FIXME => Attach circuit diagram below.