The power grid

Why it matters

The grid is the largest machine ever built, and electrical engineers design, plan, protect and operate it. Even if you work on electronics, the grid is where your power supply's input comes from and where solar inverters and EV chargers connect. Knowing the path from plant to outlet, and why each step exists, is basic professional literacy.

The big picture

1. Generation: power plants (gas, coal, nuclear, hydro) spin synchronous generators, typically at 13.8 to 25 kV. Wind turbines and solar farms connect through power electronics (inverters).
2. Step-up substation: a transformer next to the plant raises the voltage to transmission level.
3. Transmission: high-voltage lines (in North America typically 115 to 765 kV) carry bulk power long distances.
4. Transmission substations step down to sub-transmission (often 34.5 to 138 kV) for regional delivery.
5. Distribution substations step down again to primary distribution voltages, commonly 4 to 35 kV, on feeders running along streets.
6. Service transformers (on poles or pads) make the final step to customer voltage: in North America, homes get 120/240 V split-phase; commercial buildings often get 208Y/120 V or 480Y/277 V three-phase.
7. Inside the building: the service entrance, meter, main panel, branch circuits and finally the outlet, all governed by the National Electrical Code (NFPA 70).

The grid runs at 60 Hz in North America (50 Hz in most of the world), and the system is almost entirely three-phase AC.

Why high voltage?

Line losses are I^2 R. For the same power, raising the voltage lowers the current, and losses fall with the square. For 100 MW at unity power factor on a three-phase line:

I = P / (sqrt(3) * V_LL)
at 138 kV:  I = 100e6 / (1.732 * 138e3)  = 418 A
at 13.8 kV: I = 100e6 / (1.732 * 13.8e3) = 4,184 A

Ten times the voltage means a tenth of the current and one hundredth of the I^2 R loss on the same conductor. Transformers make AC easy to step up and down, which is why AC won for most of the grid. High-voltage DC (HVDC) links are used for very long distances, undersea cables and connecting grids that are not synchronized.

Balancing supply and demand

Electricity is mostly used the instant it is generated (batteries and pumped hydro store only a small share). When load exceeds generation, the generators slow and the frequency drops; when generation exceeds load, it rises. Operators keep frequency very close to 60 Hz with governors, automatic generation control and reserves. In North America, the grid is divided into large synchronous interconnections (Eastern, Western, and ERCOT in Texas, plus Québec), run day to day by balancing authorities and grid operators (ISOs and RTOs), under reliability standards from NERC.

Who does what

• Generation companies run plants.
• Transmission owners and operators run the high-voltage network and markets.
• Distribution utilities run feeders and connect customers.
• Electricians work on the customer side under the NEC; utility line workers work on the utility side under OSHA 1910.269 and utility safety rules.

Safety and licensing

Transmission and distribution equipment carries lethal voltages and can kill without contact through arc flash and arcing across air. Never approach downed or overhead lines or open utility equipment. Work on energized electrical equipment is governed by NFPA 70E, Standard for Electrical Safety in the Workplace (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and requires employer training, qualification and supervision. Power and utility design is signed off by licensed professional engineers (PE; see NCEES, https://ncees.org). Checked on 2026-10-23.

Recap

• Plant (13.8 to 25 kV), step-up, transmission (115 to 765 kV), substations, distribution (4 to 35 kV), service transformer, 120/240 V or three-phase service.
• High voltage means low current and much lower I^2 R loss.
• Frequency reflects the balance of generation and load and is held near 60 Hz.