Sub-panels differ from your main service panel in several important ways. First, a sub-panel is a “Main Lug” panel and has no main breaker or service disconnect in them. The disconnect for the sub-panel is a 2-pole breaker installed in the main circuit breaker panel. Second, and this is very important, the sub-panel's neutral bar is not “Bonded” over to the sub-panel's box. In fact, bonding the neutral bar to the box is a direct violation of the NEC.
The Anatomy of a Sub-Panel
As you can see from the above picture the anatomy of a sub-panel is much like that of a main-breaker panel minus the main disconnect.
The two large, black wire (1 and 2) are equivalent to the phase wires on the main-breaker panes and the White wire (3) is equivalent to the service neutral. These three wire, along with the green system grounding wire, make up sub-feeder from the main-breaker panel to the sub-panel.
The Green, system grounding wire connects to the top of the sub-panels ground bar (4), and the individual bare, branch circuit ground wire connect to this bar.
This particular sub-panel has three neutral bars (5, 6, and 7) to which the branch circuit white, neutral wires. When installing a “Main Lug” panel as a sub-panel, make sure to remove the “Bonding” screws, if any, from the “Neutral Bars”.
The already installed branch circuit breakers (8) are covering up the panels bus bars except for the very of the “B-Phase” bar which can be seen at the empty breaker slot (9).
Selecting the sub-feeder cable size.
Sub-panel installed in house
When installing the sub-panel in your home to extend the circuit breaker capacity of your main-breaker panel or to handle the branch circuits in an area of heavy electrical loads, the ampacity and size of the sub-feeder cable is based on NEC Table 310-15(B)(6).
As you can see from this table, you need to run a sub-feeder size AWG 4 CU or AWG 2 AL for a 100 Ampere sub-panel. Some jurisdiction interpret this table very strictly and may not accept these ampacity. If you have any doubts what is acceptable to the AHJ in your area, call the inspector or increased the feeder cable by one size—in this example that would mean going to an AWG 3 CU or AWG 1 AL wire size.
A Word About using Aluminum. The NEC and most AHJ permit the use of aluminum wire for service-drops, feeders, and sub-feeders, if you choose to go with aluminum make sure that the circuit that you install in the main-breaker panel and the lugs in the sub-panel are rated for use with aluminum wiring.
Underground feeder to a sub-panel in an outbuilding.
If the underground feeder is ran in conduit, the size of the individual conductors are based on the ampacity given in NEC Table 310.16 under the 75°C column i.e. a 100 Ampere sub-panel would require AWG 3 Copper. When running these conductor in PVC conduit, the minimum conduit size should be 1 and ¼ inches. If direct burial cable, Type—UF, cable is used instead of individual conductors in a conduit, the size of the cable conductors will be based on the 60°C column of NEC Table 310.16. For a 100 Ampere sub-panel you would need to go with AWG 2 which has an ampacity of 95 Amperes but the code permits it to be protected up to 100 Amperes.
A main breaker required
If the sub-panel is installed in an outbuilding and contains more than six branch circuit, circuit breakers, the panel must have a main-breaker installed in it. In this case the feeder wire would connect to that breaker instead of to the sub-panels main lugs.
Bonding and grounding
If the panel is installed in an out building, two, 8-foot grounding electrodes must driven at least 6-feet apart. The two ground rods must be connected to the panes grounding bar with an AWG 6 Copper wire. The rods must be driven flush with the grade and the ground rod clamps must be rated for direct burial, a “DB” stamped on them indicates direct burial.
If a water pipe is used instead of the two driven ground rods, the system ground wire must connect to the metal water within five feet of where it enters the structure.