D2P and EWP panelboards are factory sealed. Replacements of breakers are allowed but the addition of new breakers is not possible. These panels have a two cover design which is composed of the junction compartment (upper chamber) and the breaker compartment (lower chamber). Each compartment can be accessed independently. D2Ps and EWPs do not use a bus bar system; hence, breakers are “hard-wired” in the breaker compartment. The chambers are separated by a thick wall with openings for wire passage. Sealing cement is installed in each opening after wire are passed through, making the division between the upper and lower chambers factory sealed.
Unless the customer is certain that no additional loads will be added in the panel, it is always recommended to fill all spaces/circuits with circuit breakers. Having excess breakers which can be used for future loads is far more economical than ordering a whole new panel assembly.
Seen below are actual photos of a D2P panelboard in various stages of completion, clearly showing the two chambers divided by a thick wall with sealing cement installed in the wire passages. Once the wire passages are sealed, additional wires cannot be added, therefore additional breakers cannot be added. Note the 24 circuit panel with 8 spaces left empty, compared with the fully loaded 24 circuit panel complete with breakers and wires.
The technical question and answer above were featured in the April 25, 2015 edition of the ECM Wednesday Weekly online newsletter.