Why are the power lines on the poles so small in gauge but for example my 200 amp service has a much thicker wire going into the panel. ?
In: Engineering
Wire gauge is about amperage – the higher the amps, the larger gauge you need.
Wattage (the amount of power you are using) = Volts * Amps.
The power lines above your house have **much** higher voltage than what your appliances use – 110 kV in the overhead lines vs 110V in your household wires (assuming you are American). The box on the poll is a transformer, which steps the voltage down from transmission levels to what your house can use.
So, back to our equation, the super high voltage means fairly low amperage.
Low amperage means thin wires.
The wires between the poles are at much higher voltage (7.2KV or 14.4KV in the US). This allows smaller wires to save weight and money, but that’s why you have transformers on the poles.
People have discussed distribution voltages and Ohm’s law at length already, which is all true, but there are 2 other things that are missing from the conversation.
First, the wires on the pole are larger than you think. They tend to be uninsulated aluminum with a steel core for strength. Between not being insulated and being quite high, they look smaller than they really are.
Second thing is that because they’re uninsulated and because they’re out in free air, the utilities are allow much higher current than you are for a given cable gauge. That’s because electrical insulation also acts as thermal insulation, greatly reducing heat dissipation, and the codes for house wiring make additional conservative assumptions that this cabling may be stuffed into a conduit or grouped with a bunch of other wires in a confined space that further reduces heat dissipation. Thus the allowances for your house are very conservative, while the utility’s bare wires in the open air have excellent heat dissipation, even if it’s hot and sunny outside.
You spec the gauge of wires based on the current they will carry and you spec the insulation based on the voltage (and environment). The higher the current, the thicker the wire and the higher the voltage, the thicker the insulation.
Ohm’s Law tells us that there is an inverse relationship between voltage and current for the same power. If you double voltage (again, for the same power), you halve the current.
The lines going from pole transformer to pole transformer are going to be in the 7-14-ish kilovolt range. This means that for a 24 kW load at 7.2 kV, you’d only be drawing about 3.3A. However, for the service drop from the pole transformer to the house, that same 24 kW load would require 200A.
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