Adding New Electrical Circuits - Installing A Sub-Panel

Updating Old House Wiring:

Adding A Sub-Panel:
An Overview

In This Article: A brief summary of the major steps involved in adding a subsidiary breaker panel to an existing main breaker box.	Related Articles: Index of Electrical Articles Part 1 - Cutting A Hole In A Plaster Wall Part 2 - Changes At The Main Panel Part 3 - Installing The New Panel
Skill Level: 3+ (Intermediate and higher)	Time Taken: About 6 Hours

By Bruce W. Maki, Editor

New Sub-Panel - A Quick Summary:

Route the sub-feed cable from the main panel to the new sub-panel. BUT... size matters, and length matters too. See below for more info.
Mount the sub-panel, either surface mounted to plywood, or flush mounted between the studs.
Connect the sub-feed to the sub-panel lugs.
Connect the sub-feed to the appropriate 2-pole breaker in the main panel.
Connect the branch circuits to the sub-panel.
Test everything, call for "rough" electrical inspection.
Install wallboard, then complete the finish electrical work as needed.
Get the electrical finish work inspected.

Start:

I have some friends who own a century-old house, and for the past five years I've been helping them remodel the place from top to bottom. When the wall and ceiling plaster was removed, we took the time to replace the old knob-and-tube wiring.

Many years ago (the 1950's or early 60's) the house had been divided into two apartments, and the wiring was partially updated at that time. A subsidiary electrical panel (commonly called a sub-panel) was installed on the second-floor to service the upstairs apartment. This remote breaker box was fed with a 6-3 cable (that's 3 conductors, each a Number 6 size) with no ground wire.

The sub-panel had been installed in an old attic, but the attic had been converted to a bedroom. The room had no closet, and the previous homeowner installed closet rods above the sub-panel, so the panel was blocked by hanging clothes. The National Electrical Code requires a 3' x 3' clear area in front of a panel, so this arrangement was a code violation, albeit a minor one.

We decided to solve several problems by simply replacing the old sub-panel with a new sub-panel of the same capacity, and then re-wiring all of the existing circuits. The new sub-panel would be located in a hallway instead of a bedroom. Perhaps the trickiest part of installing a new sub-panel in an old house is running the sub-feed cable from the main panel to the sub-panel. Since we had stripped the ceiling and walls below, we were able to easily run a new 6-3G cable from the basement to the second-floor hallway.

Size Matters:

Sub-Panel:

There are two main factors to sub-panel size:

The number of available slots for circuit breakers.
The amperage capacity (ampacity) of the conductive parts within the panel.

We used a 125 Amp capacity panel with spaces for 20 breakers, but we did not use the full amperage capacity.

Sub-Feed:

The size of the conductors (individual wires) inside a sub-feed cable dictates how much current (measured in Amps) can flow through that cable. We used 6-3G cable, which is normally capable of handling 50 Amps. But... there is a maximum length to that cable, 55 feet, before there is a possible problem of a voltage drop happening.

Sub-Feed Breaker:

As with all electrical wiring, the circuit breaker amperage capacity must be equal to or less than the ampacity of the cable that it serves. We used a 50 Amp 2-pole breaker.

Running The Sub-Feed Cable:

The logical question is: Why not route the new sub-feed cable in the exact location of the original cable?

The original cable ran on the outside of an interior wall, beside a drain pipe and supply pipes for the upstairs kitchen. The entire bundle of utilities was protected with a U-shaped cover made from 1x6's. These pipes and wires interfered with our main-floor kitchen remodeling project, so when the walls were opened up we routed the new sub-feed cable through the wall cavity, where it belongs.

I didn't get a chance to shoot any pictures of running this cable. Some important points are:

The cable needs to be secured to the framing every 4 feet, or closer.
When the cable needs to run through a joist or stud, the cable needs to be placed far enough back from the face so that a nail or screw won't accidentally puncture the insulation. I believe the rule is to place the cable at least 1Ľ" behind the face, or apply a metal protective plate to the face of the stud.
When holes are drilled through framing members, the holes need to be located in regions that will not weaken the structure. This typically means that holes need to be drilled in the middle third of a joist, as measured vertically.

Mounting The Sub-Panel:

After cutting a rectangular hole in the wall, we secured the sub-panel to the studs with 1Ľ" sheet metal screws.

After installing a 3/4" cable clamp, we inserted the sub-feed cable into the panel.

Connecting The Sub-Feed To The Sub-Panel Lugs:

After the sub-feed cable jacket was stripped, we connected the four wires:

The hot wires, red and black, were each connected to a lug on the bus bars.
The neutral wire (white wire) was connected to the neutral bus bar.
The ground (bare) wire was connected to the ground bus.

At The Main Panel -
Connecting The Sub-Feed To The Supply Breaker:

We ran the 6-3G sub-feed into the main breaker panel.

Inside the main panel:

The neutral and ground wires were connected to the neutral/ground bus bar.

The hot wires were connected to a 50 Amp 2-pole circuit breaker. The breaker was snapped into place so it connected to the hot bus bars.

Then we removed two knock-outs from the cover panel, to accommodate the new breaker.

Connecting The Branch Circuits:

At the sub-panel:

We ran a new 14-2G cable into the panel.

After the cable jacket was cut away, we secured the ground wire in the ground bus.

Note that one screw in the ground bus can hold two or three wires.

We stripped the end of the neutral wire and connected it to the neutral bus bar.

Note that each screw on the neutral bus bar can only hold one wire.

The hot wire was stripped and connected to a 15 Amp circuit breaker.

The breaker was connected to the panel.

This type of breaker simply hooks onto a metal tab and clamps onto the hot bus bar.

But Wait, There's More...

After we installed all the individual circuit breakers, we were ready for the "rough-in" electrical inspection.

Then we installed the drywall. When the wallboard was finished and painted, we finished the electrical work by installing the switches, receptacles, and light fixtures. When all the electrical work is done the electrical inspector can be called for the final electrical inspection.

Read The Detailed Articles:

Part 1 - Cutting A Hole In A Plaster Wall
Part 2 - Installing The New Panel
Part 3 - Changes At The Main Panel

Warnings And Cautions:

I strongly recommend anybody who plans on doing their own electrical work to do more research besides just reading these articles. I am not an electrician and I certainly do not know everything about wiring. I could be wrong. The electrical codes in your area may be different than in my area.

While minor electrical changes and repairs may not require a permit, larger projects usually do. Consult your local Building Department (look in the "local government" section of your phone book) before making your own major electrical changes. Local electrical inspectors usually are available during their early morning hours to answer your code questions, normally free of charge. If you've done your homework, you can speak intelligently and learn about lots of valuable rules and regulations.

Read HammerZone's disclaimer.

Adding A Sub-Panel: An Overview