7.5 Panelboards, Switchboards & Distribution Equipment

7.5 Panelboards, Switchboards & Distribution Equipment

Quick Reference: The ICC E3 content outline lists Panelboards and Switchboards as its own job task inside Wiring Methods and Distribution Systems, and it carries roughly the same weight as feeder calculations. The task description is specific: determine code compliance of panelboards and switchboards and verify the location, clearances, and rating. Those three words — location, clearances, rating — are the whole checklist, and they map to NEC Article 408, NEC 110.26, and NEC 240.24.

1. Panelboard Rating and Overcurrent Protection (NEC 408.30, 408.36)

Two separate rules govern the numbers on a panel schedule, and candidates routinely collapse them into one.

NEC 408.30 — Panelboard rating. A panelboard must have a rating not less than the minimum feeder capacity required for the load calculated in accordance with Article 220. This is a load test: it compares the panelboard's bus rating against the calculated demand load the feeder must carry.

NEC 408.36 — Overcurrent protection. Each panelboard must be protected by an overcurrent protective device having a rating not greater than that of the panelboard. The protective device is permitted to be located anywhere on the line side of the panelboard — it does not have to be a main breaker inside the panel. A 225 A main-lug-only panelboard fed from a 200 A feeder breaker upstream is compliant; a 100 A main-lug-only panelboard fed from a 200 A feeder breaker is a violation.

+-----------------------------------------------------------------------------+
|            PANELBOARD PROTECTION LOGIC (NEC 408.30 AND 408.36)              |
+-----------------------------------------------------------------------------+
|  TEST 1 (408.30):  Panelboard bus rating  >=  calculated feeder capacity     |
|  TEST 2 (408.36):  Upstream OCPD rating   <=  panelboard bus rating          |
|                                                                             |
|  The OCPD may be:  a main breaker in the panel                              |
|                    a main fused switch ahead of the panel                    |
|                    the upstream feeder breaker in the distribution board     |
+-----------------------------------------------------------------------------+

Recognized modifications to 408.36

• Transformer-supplied panelboards (408.36(B)). Where a panelboard is supplied through a transformer, the overcurrent protection required by 408.36 must be located on the secondary side of the transformer. The narrow exception is a panelboard supplied by a transformer whose primary device protects the secondary under 240.21(C)(1) — that is, a 2-wire-to-2-wire or delta-delta 3-wire transformer.
• Delta breakers (408.36(C)). A 3-phase disconnect or overcurrent device shall not be connected to the bus of any panelboard that has fewer than three-phase busbars. This kills the field practice of bolting a 3-pole device onto a single-phase panel.
• Back-fed devices (408.36(D)). Plug-in-type overcurrent devices or plug-in main lug assemblies that are back-fed and used to terminate field-installed ungrounded supply conductors must be secured in place by an additional fastener that requires other than a pull to release. On plan review, look for the retaining-clip note on any back-fed PV, generator, or main-lug detail.

[!WARNING] The dead rule candidates still cite: the 42-circuit limit. The "lighting and appliance branch-circuit panelboard" classification and the 42-overcurrent-device maximum were deleted from the NEC in the 2008 cycle and are not in the 2020 or 2023 editions. A 60-space panelboard is not a violation on its face. The governing limit today is the panelboard's own listing and marking under NEC 110.3(B) — reject only if the schedule exceeds what the manufacturer's label allows.

2. Identification: Circuit Directories and Source of Supply (NEC 408.4)

NEC 408.4 is the single most commonly violated rule in Article 408, and it is pure plan-review territory because it is satisfied or failed on the drawings.

408.4(A) Circuit Directory or Circuit Identification. Every circuit and circuit modification must be legibly identified as to its clear, evident, and specific purpose or use. The identification must include sufficient detail to distinguish it from all other circuits, and must be on a circuit directory located on the face of or inside the panel door for a panelboard, or at each switch or breaker for a switchboard or switchgear. Two explicit prohibitions:

• Identification cannot depend on transient conditions of occupancy — "Jim's office" fails; "Room 214 receptacles" passes.
• Spare positions that contain unused overcurrent devices must be described as "spare" and the directory must be updated when a spare is put into service.

408.4(B) Source of Supply. All switchboards, switchgear, and panelboards supplied by a feeder in other than one- or two-family dwellings must be permanently marked to indicate the device or equipment where the power supply originates. A panel schedule that says "Panel 2B — 225 A, 208Y/120 V" but never says "fed from MDP, breaker 7" is incomplete under 408.4(B).

3. Bus Arrangement and the High Leg (NEC 408.3, NEC 110.15)

NEC 408.3(E) fixes conductor and busbar phase arrangement so that a technician in any building reads the same panel the same way: A, B, C from front to back, top to bottom, and left to right, as viewed from the front of the equipment.

On a 4-wire delta-connected system (240/120 V high-leg delta), one phase has a higher voltage to ground than the other two — nominally 208 V to ground on a 240/120 V system. Two rules govern it:

[!CAUTION] Plan review catch: a 240/120 V high-leg delta panel schedule that assigns 120 V single-pole loads to the B-phase column is a rejectable error — the high leg cannot supply line-to-neutral loads. The high leg reads about 208 V to ground, which will destroy 120 V equipment. Look for the schedule note prohibiting single-pole 120 V loads on the high leg.

4. Neutral Terminations, Unused Openings, and Wire-Bending Space

• NEC 408.41 — Grounded conductor terminations. Each grounded conductor must terminate within the panelboard in an individual terminal that is not also used for another conductor. The single exception covers the grounded conductors of circuits made up of parallel conductor sets, which are permitted to land on a common terminal or busbar. Double-lugged neutrals are the classic failure: opening one device then leaves the second circuit's neutral floating at line potential through the load.
• Unused openings. Unused openings for circuit breakers and switches must be closed using identified closures or other approved means that afford protection substantially equivalent to the wall of the enclosure (NEC 408.7, mirroring the general rule in 110.12(A)).
• NEC 408.55 — Wire-bending space. Bending space at the top and bottom of a panelboard enclosure is sized from Table 312.6(B), and side wire-bending space is sized from Table 312.6(A). On large feeder terminations this is what forces an oversized gutter or a 20-inch-wide enclosure; a 400 A panel with 500 kcmil feeders will not physically comply in a minimum-width can.

5. Location and Clearances (NEC 240.24, NEC 110.26)

The ICC task description says verify the location, clearances, and rating. Location and clearance live outside Article 408.

Location limits for overcurrent devices (NEC 240.24)

Working space and dedicated space (NEC 110.26)

• Depth comes from Table 110.26(A)(1) by voltage to ground and condition. For 151–600 V: Condition 1 = 3 ft, Condition 2 = 3 ft 6 in., Condition 3 = 4 ft. Concrete, brick, and tile walls count as grounded surfaces, which pushes an otherwise Condition 1 layout into Condition 2.
• Width is the width of the equipment or 30 in., whichever is greater, and the space must permit at least a 90-degree opening of equipment doors (110.26(A)(2)).
• Headroom must be 6 ft 6 in. or the height of the equipment, whichever is greater, for service equipment, switchboards, switchgear, panelboards, and motor control centers (110.26(A)(3)).
• Dedicated equipment space (110.26(E)(1)(a)) is the footprint of the equipment, extending from the floor to a height of 6 ft above the equipment or to the structural ceiling, whichever is lower. Foreign systems — sprinkler piping, chilled water, sanitary drains, ductwork — are prohibited from that envelope. Note the deliberate split: sprinkler protection piping is permitted in the room, but not in the dedicated space above the equipment.

6. Worked Plan Review: 208Y/120 V Panel Schedule Audit

Submittal: Panel LP-3, 225 A main-lug-only, 208Y/120 V, 3-phase 4-wire, 42 spaces, fed from a 250 A breaker in MDP. Calculated demand load 168 A. Schedule shows circuits labeled "Lights," "Recepts," and "Spare." No source-of-supply note. Two 20 A circuits share one neutral terminal screw. Panel is shown in a janitor's closet with mop storage on the wall opposite, 30 in. clear in front, and a chilled-water line routed 3 ft above the enclosure.

Findings:

1. 408.36 violation. The 250 A upstream breaker exceeds the 225 A panelboard rating. Either reduce the feeder OCPD to 225 A or specify a 250 A-rated panelboard. (408.30 is satisfied — 225 A bus covers the 168 A calculated load.)
2. 408.4(A) violation. "Lights" and "Recepts" do not identify a clear, evident, and specific purpose. Require room numbers or equipment tags.
3. 408.4(B) violation. No source-of-supply marking. Require "Fed from MDP-1, CKT 7."
4. 408.41 violation. Two grounded conductors on one terminal. Require individual neutral terminals.
5. 240.24(D) violation. Mop and supply storage makes this the vicinity of easily ignitible material.
6. 110.26(A)(1) violation. 30 in. depth is short; a 208Y/120 V panel is 120 V to ground, which falls in the 0–150 V row of Table 110.26(A)(1) requiring 3 ft in all conditions.
7. 110.26(E)(1)(a) violation. The chilled-water line at 3 ft above the panel is inside the 6 ft dedicated space.

7. Plans Examiner Verification Checklist: Panelboards & Switchboards

• Bus vs. Load: Panelboard rating is not less than the Article 220 calculated feeder capacity (408.30).
• Bus vs. OCPD: Upstream overcurrent device rating does not exceed the panelboard rating (408.36); transformer-fed panels are protected on the secondary (408.36(B)).
• Circuit Directory: Specific purpose per circuit, no occupancy-dependent labels, spares marked (408.4(A)).
• Source of Supply: Feeder-supplied panels and switchboards marked with the originating equipment (408.4(B)).
• High Leg: On 4-wire delta, high leg on phase B (408.3(E)) with orange or other effective marking (110.15) and no 120 V loads assigned to it.
• Neutrals: One grounded conductor per terminal (408.41).
• Back-Fed Devices: Retaining fastener specified for back-fed breakers and main-lug assemblies (408.36(D)).
• Location: Handle height <= 6 ft 7 in.; not in easily ignitible storage, dwelling/guest bathrooms, or over stair steps (240.24).
• Clearances: Table 110.26(A)(1) depth by voltage to ground and condition; 30 in. minimum width; 6 ft 6 in. headroom; dedicated space clear to 6 ft above.