Is this subpanel arrangement right?

expert EXPERT April 26, 2007

Yes, the arrangement of subpanels that you are talking about is possible, as long as each subpanel and feed wire is adequately protected by fuses or circuit breakers. As your inventory says that your home was built around 2003, your distribution panel almost certainly uses breakers. The following example demonstrates one way that adequate protection can be provided. Imagine thata new subpanel were added right beside the existing panel. This new subpanel might be rated for 60 amps, which is a common size. First, two existing 15 amp circuits should be removed from the main panel and relocated to the subpanel. This will make room for a 240V breaker, which will be used to protect the feed wire to the subpanel. In our example, we'll be installing a #8 AWG wire between the main panel and the subpanel. We will install a 40 amp double-pole(240V) breaker into the newly-created space in the distribution panel, and connect the feed wire to this. The other end of the wire connects to the bus bars of the subpanel. Moving to the subpanel, we can now install two new 15 amp circuits to serve our extra electrical needs in the house, and a 20 amp double-pole (240V) breaker to protect the feed wire to the garage. This subpanel now contains four 15 amp 120V circuits and one 20 amp 240V circuit. We can now connect a #12 AWG, three conductorfeed wire to the 20 amp breaker and run the wire out to the garage, connecting it to a panel with a rating of 30 amps, containing two 15 amp, 120V circuits. Looking at the various levels of protection, you can see that the 40 amp breaker in the main panel makes sure that the current flowing through the feed wire remains at or below the rating of the feed wire and the rating of the subpanel. The 20 amp breaker in the subpanel provides the same protection for the garage feed wire and subpanel. All of this assumes that the additional load does not exceed the total circuit breaker rating of the main panel. In this panel, we have increased the sum of the circuit breaker ratings by 50 amps, since we've added a double 40 amp breaker(2 X 40=80 amp), and removed two 15 amp breakers (2 X 15=30 amp). The total of all the the circuit breakers should remain below the total rating stamped or printed on the panel. In a 200 amp panel, this is typically 600 amps.