Do-It-Yourself Repairs & Basic Wiring Projects: How to Install a Subpanel

By Terry Peterman, the Internet Electrician

In this situation, the original panel was full to the point of some circuits that were added since construction having to be ‘double-lugged’ under existing breakers.

Figure 1 - Original Panel

My customer had just purchased the house, and he needed to renovate his basement in order to accommodate his collection of reptiles, spiders, amphibians and other creatures. It made for one of the most interesting conditions I've ever had to work under!

Figure 2 - Cuddly Snake!

Figure 3 - Original Panel Cover Removed

Planning

The first step should always be planning, as a project such as this is fairly complex.  Take the time to think through the necessary steps to complete the job, ensure that you have the appropriate tools and materials, and that all appropriate safety requirements are being followed.

Remember - Safety First!  If you are at all unsure about tackling a project like this, leave it to a reputable electrical contractor.

Good lighting is critical when working on this type of project, so I would recommend that you arrange for an external light source, such as a good battery operated light, or a portable generator outside. 

Run in an extension cord to a portable work light, or a trouble light.  This will allow you to work in the original panel to make the required changes with the main breaker off, which will de-energize the bus bars in the panel.

If the planned renovations are in the opposite end of the house from the original panel, you may want the subpanel closer to that area.  This would result in using a longer length of the heavier, more expensive wire to feed the sub, but you would use less wire in all the branch circuits added in the renovation. 

In this case, the service board has plenty of room to the left of the original panel, and the room that the panel is in doesn’t have a finished ceiling, so running new wires to the sub will be easy to do from anywhere in the basement that the renovation will take place. 

Now prepare the panel for mounting by removing all covers. 

Figure 4 -  Remove the Subpanel Cover

Preparing the Subpanel

Mount the panel at a comfortable height for working on, and for re-setting breakers etc. A good rule of thumb is about 5 feet from the floor to the center of the panel.

Figure 5 - Mounting the Subpanel

Use the top key-hole screw slot in the back of the panel to hang it, and using a level, hold the panel in place, and use the remaining mounting holes to secure the panel to the mounting board. (At least 4-#10 wood screws for a panel this size).

Removing Circuits

I’ve chosen to remove a 3-wire (two 15A breakers) that feed some kitchen counter split receptacles, and two 2-wire branch circuits (as these were double-lugged already) to make room for the 2 pole 40A breaker to feed the new subpanel.

Figure 6 & 7 - Removing Circuits

Cutting in the Cables

Next, cut in the cables that were removed from the existing panel into the new tub, and the new subfeed cable interconnect between the two panels. 

I am using #8AWG 3-conductor NMD-90 wire for the interconnect (Range cable).  Make sure you use approved cable connectors where you bring cables in through the knock-outs in the panels. 

Notice that I installed the subpanel with the main breaker compartment at the bottom.  This was to accommodate the existing and the future branch circuits that will enter the subpanel in the top and the sides of the panel.

Figure 8 - Branch Circuits into New Subpanel

Figure 9 - Installing Subfeed into New Subpanel

I chose these circuits, as the wires were long enough to cut into the new subpanel without spicing. 

This particular panel is a Westinghouse ‘Nova-Line’ panel, and Westinghouse has since been bought out by Cutler-Hammer. 

The breakers used in this panel are called ‘quads’, and ‘tandems’ or DNPL style.  They are a ‘push-in’ breaker, as opposed to a ‘bolt-in’ style. 

These panels will also accommodate a ‘BR’ style breaker, but they take up one whole breaker space alone, as they are twice as wide. 

One thing to check is that some of the older Westinghouse breakers will not fit in the new Cutler-Hammer panels, as the mounting tabs are slightly different. 

A new Cutler-Hammer breaker will fit in the Westinghouse panel, but in the case here, the breakers I removed from the original will not fit in the new tub. 

I will use a 15-40-40-15 DNPL breaker to feed the subpanel.  The two 40A breakers in the middle (split across both hot busses of the panel) will feed the new sub, and the two 15A circuits on the outside will be used to feed the existing circuits that will now have their own breakers, instead of being double-lugged (which is against any and all rules that I’m aware of!).

The Subpanel

Figure 10 - Subfeed Connected to Subpanel

Figure 11 - Making Room for Subpanel Breakers

The panel I chose to use here is a Cutler-Hammer model # CPM-120.  This is a panel with a main breaker, and space for 20 BR style breakers, or 40 circuits in the DNPL style.  This will give us more than enough breaker space for the planned renovation, as this subpanel is bigger than the original. 

We also would not require a main breaker in this subpanel because a disconnect exists right beside it in the original panel.  I used this panel with a main breaker because it is more inexpensive (about 30% less) than one with main lugs. (They tell me that because they manufacture more panels with a main breaker, the cost goes down, due to volume).  That aside, it makes no difference whether you feed the buss bars in the subpanel via main lugs, or through a main breaker; the results are the same. 

The 100Amps main is providing over-current protection for the buss bars, and not any of the branch circuits, and the subpanel current is limited to 40Amps by the subpanel feed breaker in the main panel.

Making Your Connections

At this point, you could make your connections to the subfeed breaker in the main panel, and re-install the panel cover. 

Leave the subfeed breaker off, but now you could turn the main breaker in the original panel back on, and restore power to the house and get the lights, heat, etc. back working again.

Figure 12 - Installing Subfeed into Subpanel

When installing cables into a panel, connect the bare ground wires first, then the neutral (white) wires to the neutral buss, and finally, install the breakers and connect the hot wires (red or black) to the breaker lugs.

Always take the time to make the wires look neat and orderly, as it not only looks good, but saves time tracing wires, if necessary, in the future.

The same goes for the connections to the main lugs (or in this case the main breaker) in the subpanel.  Make sure that you bring the subfeed cable into the main breaker compartment of the subpanel.

Figure 13 - Subfeed Connected to Breaker

Neutral Bonding Jumper

This would be a good time to talk about the neutral bonding jumper. 

In most panels you will find that the neutral buss bar (the common bar that is mostly isolated from the panel case), has either a long brass machine screw, or a metal strap of some kind that connects the neutral bar to the panel case, or ground. 

In a subpanel, the neutral must be totally isolated from ground, so you must remove this screw or strap.  The neutral and the ground can only be bonded in one location in a residential service, and that is in the main panel.

Figure 14 - Bonding Jumper Removed

Figure 15 - Bonding Jumper

The sub-feed cable then connects like this:

• bare ground wire to a case ground lug, the white neutral wire to an appropriately sized lug on the now isolated neutral bar, and the red and black hot wires to the lugs of the main breaker in the subpanel. 

As is the case in the main panel, you are reverse feeding the main breaker, which then feeds power through to the panel’s hot buss bars that the branch circuit breakers connect to. (Clear as mud, or did I lose you now?)

Final Connections

Now connect the branch circuit wires in the same order:

• Bare grounds to the panel case, white neutrals to the neutral bus bar, then install your branch circuit breakers and connect the hot wires to the breaker lugs.

Once all the connections are made, you can install the main compartment barrier, and then remove the appropriate knockouts in the panel cover, and install it.

If no sparks fly, and the circuits that you moved to the subpanel now work, you’ve done a great job, and you now have the room you need to accommodate the additional circuits that will be added during the upcoming renovation project.

Figure 16 - The Final Product

Labeling the Panel

The final step is to label the panel in order to identify which breaker feeds what; a step that is all too often over-looked, but any good inspector will catch you on this one, and make you label the circuits before giving his/her final acceptance.

Figure 17 - Labeling the Panel

Now my job here is done, and I can get out of here before my customer's Burmese Python starts “Wrappin’ himself around me! 

Figure 18 - Customer Dean Harper, Owner of "Wrappin' About Reptiles" and His Python Friend