Fused Disconnect Installed with a Load Center
When a fused disconnect is installed upstream of a load center, the fused disconnect is the “first means of disconnect”). The panel with the fused disconnect is the service panel and must have its own grounding electrode conductor (GEC) connected to a grounding electrode. It’s in this cabinet that the neutral and grounding bus bars, and the metal panel cabinet will all be connected to the GEC.
In this configuration, the load center, even if it has a main disconnect, is just a sub-panel, and the neutral bus bar should float (be electrically isolated from the metal cabinet), and should not bond to the GEC/grounding electrode. A single neutral conductor should connect the (floating) neutral bus bar in the load center to the fused disconnect.
- If the fusible disconnect and the load center are remote from each other, the load center should have its own GEC and grounding electrode.
- If the fusible disconnect and the load center are installed at the same building, only the service panel (fusible disconnect) would be connected to a grounding electrode.
- If for some reason the fusible disconnect service and the load center are installed at the same building and each has a grounding electrode installed, the electrodes would need to be directly connected by a conductor of adequate size to avoid creation of stray electrical current traveling through soil between grounding electrodes at different electrical potential.
- Never operate a disconnect when the cover is open. Modern disconnects have door interlocks that prevent the handle from being turned on when the door is open, but they can fail or they can be defeated.
- Always wear gloves and safety glasses around electrical equipment;
- Turn your face and body away from the disconnect before operating the handle.
- Never turn a disconnect off while the equipment it feeds is running, unless it is an emergency.
Types of Panelboards
The National Electrical Code® (NEC®) defines a panelboard as a single panel or group of panel units designed for assembly in the form of a single panel, including buses and automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front.
Main Breaker Panelboard
…has a main disconnect installed between the lugs where the service entrance cables connect and the supply bus bars.
Main Lug Panelboard
…has no main disconnect installed. Power to the panel is controlled from a main disconnect at another location, typically the electrical service.
Lighting and Branch Circuit Panelboards (NEC)
A lighting and appliance branch- circuit panelboard must have more than 10% of its overcurrent protective devices (OCPD) protecting lighting and appliance branch circuits.
A lighting and appliance branch circuit is a branch circuit that has a connection to the neutral of the panelboard and has overcurrent protection of 30 amperes or less in one or more conductors.
A lighting and appliance branch- circuit panelboard is limited to no more than 42 OCPD (poles) in any one cabinet or cutout box.
Power (Distribution) Panelboards
A power panelboard is one that has 10% or fewer of its OCPD protecting lighting and appliance branch- circuits.
A panelboard has 42 OCPDs but only four OCPD are rated at 30 amps or less. Since four is less than 10% of 42, this panelboard doesn’t qualify as a lighting and appliance branch- circuit panelboard.
Dangers of Fused Neutrals
Check to make sure that the neutral conductor is not connected through a fuse. If the neutral conductor is connected to a fuse that blows, the system will appear to be turned off because electricity will not flow, but the load side of the system will still be energized. Someone could accidently come into contact with energized electrical components because they mistakenly think the system is not energized.
Panelboard Working Clearances
WIDTH: minimum 30". The 30" clearance width of the working space in front of the panel doesn’t have to center on the panel. The 30" can be measured from either side of the panel as long as the door can open a minimum of 90 degrees.
DEPTH: 3 feet.
HEIGHT: 6 feet- 6 inches
TESTING A BACKFED DISCONNECT
Test for voltage at the line side, then test for voltage at the load side. The line side is the only side that should be energized with the disconnect off.
Backfed disconnects must be attached to the load center with a screw, clamp, clip, or other device specifically made for the purpose of preventing the disconnect breaker from being accidentally pulled loose from the bus bars of the load center.
Wherever feeders enter or leave a load center through concentric knockouts, a bonding bushing must be installed to ensure electrical continuity of the bonding/equipment grounding system. Bonding bushings should be connected to the neutral bus bar.