Understanding Service Panels
Learn how power gets from the service entrance into the house.
Power from the utility company is typically delivered through three large conductors, which may enter the house overhead or underground. Overhead service wires are called a service drop. The drop runs to a weatherhead atop a length of rigid conduit. When fed underground, service conductors are installed in buried conduit or run as underground service-entrance (USE) cable.
Whether it arrives overhead or underground, 3-wire service consists of a service neutral and two energized (hot) service conductors—which are also called Service Phase A and Service Phase B—two phases of alternating current (AC). Three-wire service delivers 120v relative to ground, and 240v between the energized conductors.
Service Entrance to the Eaves Side
Personal Protection Equipment
A qualified person testing or working in a potentially energized electrical panel must wear personal protection equipment (PPE) as specified in NFPA 70E® PPE Hazard Level 1, Table 130.7 (C) (16). “The PPE requirements of 130.7 are intended to protect a person from arc flash and shock hazards.” This protective equipment includes the following:
In service panels, a small area around the main breaker stays hot even after the breaker is set to the off position. The only way to cut all power in a service panel is to pull the meter—which we don’t recommend and which is illegal in many jurisdictions. Typically, only the utility company can cut a meter seal or pull a meter.
If you are at all uncertain whether a panel is energized, do not remove its cover. Call a licensed electrician. Many jurisdictions forbid panel work by nonlicensed electricians.
Testing: The Key to Working Safely
Cut power inside a service panel by switching off the main breaker, or by removing the main fuse in a fuse box. This de-energizes the hot buses. Note: Incoming hot service conductors—and the lugs where they terminate—will still be energized. Unscrew the panel cover and set it aside.
To cut power to a subpanel, flip off the subpanel breaker in the service panel—and lock the service panel so no one can turn the subpanel breaker back on. That should de-energize the subpanel so it can be safely opened and worked in.
But always test, never guess. As shown in the photos, we recommend using two testers.
Testing with a noncontact tester
Start with a noncontact tester, which can detect voltage through wire insulation. First test the tester to make sure it is working correctly.
Before removing the subpanel cover, touch the tester to several breakers in the ON position. If the tester tip doesn’t glow, the panel is probably dead. But keep testing to be sure: Remove the cover and test again inside the panel. Touch the tester to insulated portions of the two main feeds, and to the main lugs.
Testing with a multimeter
Test lugs with a multimeter. Be careful not to touch the bare-metal probes of this tool. Set your multimeter on AC VOLTAGE and touch its probes to hot and neutral bus lugs in this sequence:
If all of these tests do not detect voltage, you can work safely in the subpanel.
Excerpted from Wiring Complete, 3rd Edition (The Taunton Press, 2017) by Michael Litchfield and Michael McAlister