How to Install a Subpanel

PHOTOS BY MIKE ANDERSON

When you imagine a dream garage, I’ll bet it includes an air compressor, a welder, a table saw or some other large tool. But many garages have only a single 15- or 20-amp circuit — far too little power to run these tools without overloading the circuit. To make your dream a reality, you need more power. In many cases, the best solution is to install a garage subpanel.

This can dramatically increase your garage’s power capacity, enabling you to power multiple tools at once without tripping a breaker or blowing a fuse, and it enhances safety because you can shut off power at the panel in an emergency.

Subpanel basics
A subpanel is a panel located downstream from the one that’s closest to the utility meter. It is wired differently from a standard main panel—the neutrals are isolated from the grounds, whereas a typical main panel’s neutral and ground busses are connected (more on this later). Subpanels are typically installed for three reasons. First, a subpanel conserves wiring, as you’ll only need to run one cable (the serviceentrance cable) to power the panel
(as opposed to the numerous cables necessary to power multiple independent circuits). Another reason is convenience — it’s a lot easier to reset a tripped breaker that’s nearby. Finally, subpanels are often installed as a supplement to the main service panel if all of the circuit slots in the main panel are full. (Make sure the main panel receives enough amperage to support the new subpanel and its related circuits.)

When determining where to install a subpanel, be sure to follow the National Electrical Code (NEC) requirements. In front of the subpanel, you’ll need free working space that’s 36 in. deep and at least as wide as the subpanel (or 30 in. wide — whichever is greater). The space above the panel must be completely open to the structural ceiling (no shelving, etc.), and the space below must be open to the floor (no lawnmowers, trash cans, workbenches, etc.).

Subpanel wiring
A subpanel requires two insulated hot wires fed from a breaker of the main service panel as well as an insulated neutral and a grounding wire. All of these wires (contained within the sheathed service-entrance cable) must connect into the subpanel and tie all the way back to the main service panel.

A subpanel doesn’t require a main breaker, though there’s nothing wrong with incorporating one for convenience. In fact, you may find that a panel with a main breaker is actually less expensive than one without a main breaker. Because more panels are produced with a main breaker, the price per unit tends to be lower.

If you opt to install a subpanel without a main breaker, you’ll find two big lugs at the top of the panel in lieu of the main breaker. In this type of setup, the main breaker for the subpanel is considered the breaker in the main service panel that feeds the subpanel’s service-entrance cable.

In most main panels you’ll find that the neutral bus bars (the common bars that are isolated from the panel case) are connected via a metal tie bar. In a subpanel, however, the neutral has to be totally isolated from the ground, so you must remove this tie bar.

Why isolate the neutral in a subpanel? By code, the neutral for a residential installation is only bonded to the ground at the main service panel.At all other points throughout the house, there is no connection between the  bare (or green) grounding conductor and thewhite neutral conductor. Under normal conditions, the grounding conductor carries no current. If you bond the neutral and the ground at the subpanel, stray currents from the neutral return could go through the equipment ground on the electrical devices fed from this subpanel. But if you isolate the neutral and the ground at the subpanel, stray currents would go back to the main panel and to the service ground.

The exception to this grounding rule is when you’re installing a subpanel in a detached building that does not share a common ground with the main service. In these situations, you’ll need to drive a ground rod at the subpanel.

Installing circuits
When installing individual circuits in a garage subpanel, it’s important to know what each circuit will power and how much electricity each device will draw (see “Determining Power Needs,” below). As a rule of thumb for garages, never install a circuit that’s less than 20 amps — even a circular saw under load can trip a 15-amp breaker. (Remember that a 20-amp circuit will require 12-gauge wire; 14-gauge is commonly used in a 15-amp circuit.) Code dictates that each circuit be protected by a ground fault circuit interrupter (GFCI) either incorporated into the breaker itself or as part of the receptacle.

Wiring circuits into a subpanel is similar to installing them in a main panel, but you attach the neutral and ground wires differently. Because the neutral and ground busses in the subpanel are isolated, you’ll need to connect the grounds to one bus and the neutrals to the other (see photo). Once you’ve wired the circuits, label them so you know which receptacle each breaker controls. Remember to have your state inspector check the installation for code compliance; then get ready to power up the garage of your dreams.

Determining Power Needs
To determine how much power your garage will need, you must know how much amperage the devices in the garage will draw. The amperage is typically listed in the owner’s manual, but if you can’t find the figure, here’s a simple way to calculate it: Divide the device’s wattage by its voltage. For example, an exhaust fan that’s rated at 100 watts and 120 volts would draw .83 amps.