Panelboards are found in nearly every building and facility type. They represent a significant element in power distribution systems and it is common for most professionals and non-professionals to come in contact with them regularly. Panelboards are covered by NEC Art. 408. The typical panelboard consists of the ‘can’, the ‘interior’ or bussing, and ‘circuit breakers.’ Panelboards can be found in ratings as high as 1200 amps and provide an excellent and compact means for distributing large quantities of feeders and branch circuits. There are some fundamental characteristics to be aware of when in design and during construction. Here is a brief look.
- Location: Ideally, locate panelboards as close as possible to the loads they feed and in a dedicated space such as an electrical room. For low voltage panels at 208/120V, locate panels to limit excessive branch circuit runs and associated voltage drop. Consider the analogy of a tree trunk (as the feeder) and the branches (as the branch circuits). In general you get better economies of power from the feeder than from individual branch circuits. Get those panels where you need them to limit branch lengths. However, remember to try to stay away from restrooms, janitor closets, and similar areas.
- Interrupting rating: Otherwise known as AIC or “amperes interrupting current” is a circuit breaker rating that indicates the maximum amount of current that the breaker can safely interrupt during a fault. The panel ‘AIC’ rating is that of the lowest rated device in the panel. Multiple breakers can combine to achieve a ‘series rated’ combination but that is a topic too involved for this discussion. Pro tip: AIC is a significant cost driver that shouldn’t be overlooked when estimating equipment costs.
- Accessories: Many panelboards can accommodate lighting controls, metering, surge protection, sub feed and feed through lugs. You might need or want these.
- Ratings: Of course this sounds obvious. Voltage, current, etc. However, make sure to match a 3 wire system to 3 wire panel and same for 4 wire system. A 3 wire system may function using a 4 wire panel but the code (and likely your inspector) wont permit it. Panelboards with three phase high leg systems require the high leg be marked per the NEC.
- Mounting: Flush or surface? Remember to provide at least a 6″ deep wall for flush mounting a standard sized panel which is typically 5-3/4″ deep. The ‘semi-flush option’ resulting from the panel sticking out 2 inches beyond the wall isn’t pretty and usually requires some trim out to help reduce the silly appearance.
- Enclosures: Various NEMA ratings are available. Indoors, NEMA 1. Outdoors, NEMA 3R. There are explosionproof panelboards and also corrosion resistant panelboards for locations such as marinas and chemical process areas. Many times it is not possible to keep the panels remote from the environment (see item 1 above) due to excessive distance.
- Height: Although not a common issue, the maximum breaker mounting height is 6′-7″ as per NEC Article 404.8. So, consider this when installing the taller varieties. Impress your friends at cocktail parties with that one.
- Branch circuits quantities: Since the 42 circuit rule was eliminated in recent code cycles, panelboards of the lighting and appliance type can have as many branch circuits as available from the manufacturer (in most cases) provided item #7 (above) is met. More often than not the panelboard runs out physical breaker space before ampacity. Being able to add additional breakers can provide some much needed flexibility. However, with all good things comes moderation. Pro tip: Always include spare breakers and/or prepared space when specifying new panels. Half of these will likely be used before the panel is ever put into service.
- Working clearance: 30″ width minimum is always required and can be measured from either side of the can. The voltage level and condition dictate the required depth in front of the panel (measured from the face) as outlined in NEC Article 110.26. For 208V systems 36″ is the maximum depth required. 480V systems require 36″, 42″, or 48″ depth depending on whether the area in front of panel is clear, has grounded system, or other live parts. There is also dedicated equipment space required above the panel within which no foreign systems are allowed. Refer to NEC Article 110.26 for some detailed explanations and nice diagrams (annotated version of the NEC).
- Arc flash, incident energy, and PPE (personal protective equipment): Arc flash warning labels are required by NEC. Detailed arc flash hazard analysis which includes incident energy, hazard level, and corresponding PPE provides the most appropriate information. After an arc flash hazard analysis is completed the corresponding hazard levels, identified on the arc flash labels, will alert a worker to the corresponding personal protective equipment that should be worn.
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