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Understanding Service Panels

Learn how power gets from the service entrance into the house.

By Mike Litchfield, Michael McAlister

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.

Three-Wire Service Drop
A typical 3-wire service drop consists of two insulated hot conductors wrapped around a stranded bare aluminum wire with an internal steel messenger cable—the bare aluminum wire also serves as a neutral.

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

Service Entrance to the Eaves Side
When the service drop approaches over eaves, the service riser is sent up through the eaves and the point at which the riser emerges from the roof jack is caulked.

Service Entrances

Service Entrances
Older homes may have overhead service, but underground service is increasingly common. The utility’s responsibility ends where the service-drop cables are spliced to the service conductors running to the meter. In some areas, Code requires that underground conductors be housed in conduit.

 

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:

  • Hard hat
  • Arc-rated long-sleeve shirt and pants or arc-rated coverall
  • Arc-rated face shield or arc flash suit hood
  • Safety glasses or safety goggles
  • Hearing protection (ear canal inserts)
  • Heavy-duty leather gloves (over) rubber insulating gloves
  • Leather work shoes

Personal Protection Equipment
Reproduced with permission from NFPA70E-2012, Electrical Safety in the Workplace, Copyright© 2011, National Fire Protection Association. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

 

Safety Warning

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.

service conductors
Avoid touching the service conductors that attach to the main breaker—they stay hot even when the main breaker is turned off.

 

use a panel lockout
Lock ’Em Out: Once you’ve shut off power in a service panel, tape the panel shut and post a warning sign of work in progress. Pros prefer to use a panel lockout, however, which limits panel access to the person holding the key; electrical suppliers and home centers sell them.

 

Safety Alert

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.

turn off the subpanel disconnect
Always turn off the power before working on an electrical system. If you are working in a subpanel, turn off the subpanel disconnect in the service panel.

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.

test the noncontact tester on a receptacle or device that you know is hot
First test the noncontact tester on a receptacle or device that you know is hot. Here, the tester indicates that subpanel breakers are energized.

Test breakers after power to the subpanel has been shut off.
Test breakers after power to the subpanel has been shut off.

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.

test the hot feed
Carefully remove the subpanel cover and touch the tester to the insulated portion of a hot feed or to a main lug.

verify earlier readings
Use a multimeter, a more sensitive tester, to verify earlier readings. First touch tester probes to Phase A lug (hot) and to the neutral bus.

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:

  • Phase A lug (hot) to neutral lug
  • Phase B lug (hot) to neutral lug
  • Phase A (hot) to Phase B (hot)

If all of these tests do not detect voltage, you can work safely in the subpanel.

Test Phase B lug (hot) and the neutral bus.
Test Phase B lug (hot) and the neutral bus.

Test Phase A lug and Phase B lug. If
Test Phase A lug and Phase B lug. If the tester shows no voltage on any of these tests, the subpanel is de-energized and safe to work in.

 

 

Wiring Complete, 3rd Edition

Excerpted from Wiring Complete, 3rd Edition (The Taunton Press, 2017) by Michael Litchfield and Michael McAlister

Available in the Taunton Store and at Amazon.com.

Previous: Electrical Breaker Panels Next: Leviton’s Load Center

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