Voltage Levels and Why They Step Down

7 min · difficulty 2/10

Power equals voltage times current. That single relationship explains the shape of the whole grid.

Moving power over long distances wastes some of it as heat in the wires. That loss grows with the square of the current. If you raise the voltage, the current needed to carry the same power drops, and the loss drops fast. So transmission runs at very high voltage to move power across a region with little waste.

High voltage is dangerous and useless to a toaster, so the system steps it down in stages. A substation transformer brings transmission voltage down to a distribution level. A service transformer near the customer brings it down again to the level inside a building. Each step is a transformer doing the same job at a different scale.

Voltage step-down ladder A descending three-tier ladder. The top tier is Transmission at 115 kV, the middle tier is Distribution primary at 12.47 kV, the bottom tier is Utilization at 120/240 V. A transformer sits on the conductor between each tier, labeled Substation transformer for the first step and Service transformer for the second. Gold arrows show power flowing down the ladder. Side notes read High voltage, low current at the top and Low voltage at the customer at the bottom. The header shows the equation P = V x I with the note Loss is proportional to I squared. P = V x I Loss rises with I² Substation transformer Service transformer Transmission 115 kV Distribution primary 12.47 kV Utilization 120/240 V High voltage, low current Low voltage at the customer
Power equals voltage times current, and line loss rises with the square of current, so the grid moves power at high voltage and steps it down through a transformer at each stage until it reaches the 120/240 V a building uses.

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Question 1 of 2

Why does long distance transmission use high voltage?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.