2021 IECC Sections C405 and C406
Lighting was the big electrical load in Lesson 10, but Section C405 has more to say: meters, transformers, motors, elevators, voltage drop, receptacle controls, and energy monitoring. After those, Section C406 asks every new building to earn extra efficiency credits on top of the basic requirements. The credit tables are dense, so this lesson decodes them one row at a time and ends with a worked example. Local adoption governs which edition applies (see Lesson 1).
A building on the prescriptive path complies with Sections C402 through C406 and Section C408 (C401.2.1). The total building performance path (Lesson 12) works from Section C407 instead. Most of Section C405 stays mandatory there, except the lighting power allowance in C405.3, but the C406 credit requirement is not on its list. Inside Section C405, each dwelling unit in a Group R-2 building must have its own electrical meter (C405.6). The total voltage drop across the customer-owned service conductors, feeder conductors, and branch circuit conductors combined may not exceed 5 percent (C405.10). Both are simple to check on the single-line diagram and the panel schedules before anything is built.
Low-voltage dry-type distribution transformers must meet the efficiencies in Table C405.7, tested by the DOE 10 CFR 431 procedure and verified by an approved certification program or manufacturer data (C405.7). The table lists a minimum by size and phase: a 75 kVA single-phase unit needs 98.50 percent, a 75 kVA three-phase unit needs 98.60 percent, and a 15 kVA single-phase unit needs 97.70 percent. Fourteen kinds of transformer are exempt, including sealed and nonventilating units, drive, rectifier, auto-, regulating, and welding transformers. Motors must meet Tables C405.8(1) through C405.8(4) with the same kind of verification (C405.8). Air-over, component-set, liquid-cooled, submersible, and inverter-only motors are exempt. In the field, the check is the nameplate or the submittal, not the price.
The luminaires in an elevator cab, not counting signals and displays, must produce at least 35 lumens per watt. Cab ventilation fans that have no air conditioning of their own may draw no more than 0.33 W per cfm at maximum speed. Controls must shut off the fans and lights when the elevator has been stopped, unoccupied, with its doors closed for more than 15 minutes (C405.9.1). Escalators and moving walks need automatic controls that slow them when unused, or a variable voltage drive as the alternative, and escalators must recover energy when resisting overspeed going down (C405.9.2).
At least 50 percent of the 125-volt, 15- and 20-amp receptacles in enclosed offices, conference rooms, copy and print rooms, breakrooms, classrooms, and individual workstations must be automatically controlled. So must at least 25 percent of the branch circuit feeders for modular furniture that is not shown on the construction documents (C405.11). The controlled receptacle is either the top half of a split receptacle or a separate one within 12 inches of each uncontrolled receptacle, and all controlled outlets are permanently marked. Three control methods are allowed: a time-of-day schedule with an independent schedule for each portion of the building of not more than 5,000 square feet and one floor and an override of no more than 2 hours; an occupant sensor that turns receptacles off within 20 minutes of the space emptying; or a signal from another control or alarm system that does so within 20 minutes. Plug-in devices do not comply. Equipment that must run 24 hours a day and spaces where shutoff would endanger safety or security are exempt (C405.11.1).
New buildings with 25,000 square feet or more of gross conditioned floor area must measure, record, and report energy use (C405.12). An R-2 building or a tenant space with its own utility services and meters and under 5,000 square feet of conditioned floor area is excepted. Table C405.12.2 splits the metering into six categories: total HVAC, interior lighting, exterior lighting, plug loads, process loads (any single load over 5 percent of the building's peak connected load), and building operations and other loads such as elevators and automatic doors.
New buildings must achieve a total of 10 credits from Tables C406.1(1) through C406.1(5), where the table is chosen by use group: (1) Group B, (2) Groups R and I, (3) Group E, (4) Group M, and (5) all other groups (C406.1). Credits come from any combination of eleven kinds of measure: more efficient HVAC (C406.2), reduced lighting power (C406.3), enhanced lighting controls (C406.4), on-site renewable energy (C406.5), a dedicated outdoor air system (C406.6), efficient water heating (C406.7), enhanced envelope (C406.8), reduced air infiltration (C406.9), energy monitoring (C406.10), fault detection and diagnostics (C406.11), and efficient kitchen equipment (C406.12). Where a building has more than one use group, the credits from each group are weighted by floor area to get a weighted average. Tenant spaces need at least 5 credits chosen from C406.2, C406.3, C406.4, C406.6, C406.7, or C406.10, and are deemed to comply where the whole building earned its credits through C406.5, C406.8, or C406.9. A previously occupied tenant space that complies under Chapter 5 (Section C501) is excepted (C406.1.1).
Each table has one row per option and 17 climate zone columns: 0A and 1A share the first column, 0B and 1B the second, then 2A, 2B, 3A, and so on to 8. Houston is Zone 2A (Lesson 2), the third column, so count from the left with care. NA means the option is not available. In Zone 2A the heating options are marked NA in every table. The figure reads down the 2A column of the Group B table. The same option is worth different amounts in different tables: reduced lighting power is 9 credits for Group B, 15 for Group M, 8 for Group E, and 2 for Groups R and I.
A 20,000 square foot building in Zone 2A has 15,000 square feet of Group B offices and 5,000 square feet of Group M retail. Reduced lighting power is worth 9 credits in the Group B table and 15 in the Group M table. The weighted average is (15,000 x 9 + 5,000 x 15) / 20,000 = (135,000 + 75,000) / 20,000 = 10.5 credits, so lighting alone reaches 10, provided the lighting qualifies. Compare an all-Group B building of the same size: lighting alone gives 9 and falls 1 short, so the designer might add the 2-credit fault detection system for a total of 11.
HVAC credits (C406.2) require equipment that exceeds the Section C403.3.2 minimums by 5 or 10 percent, for heating or cooling or both. Lighting power (C406.3) requires connected interior power below 90 percent of the allowance from Lesson 10 for the base credit, and below 85 percent for extra credits by an equation in C406.3.2. The office in Lesson 10 was at 84 percent. Dwelling and sleeping units must also use lamps of at least 65 lumens per watt or luminaires of at least 45 lumens per watt for 95 percent of the lighting (C406.3.3). Enhanced digital lighting controls (C406.4) mean continuous dimming, individually addressed luminaires (or groups of not more than four), no more than eight luminaires in a daylight zone control group, a digital system with load shedding and reconfiguration, a written sequence of operations, and functional testing under C408.
The basic renewable credit (C406.5.1) needs on-site systems rated at least 0.25 W per square foot of conditioned floor area, so a 40,000 square foot building needs 40,000 x 0.25 = 10,000 W, or else supplying 2 percent of annual energy for HVAC, water heating, and C405 lighting. A dedicated outdoor air system (C406.6) is an independent 100 percent outdoor air system with total energy recovery and supply-air reset. Envelope credit (C406.8) needs a total UA at least 15 percent below the reference. Infiltration credit (C406.9) needs a whole-building test by an independent third party showing no more than 0.25 cfm per square foot at 75 Pa. The energy monitoring credit (C406.10) is for buildings not already required to monitor by C405.12, and the fault detection credit (C406.11) is for buildings not already required to have that system by C403.2.3 (100,000 square feet or more). It needs sensors that sample HVAC performance at least every 15 minutes and automatically report faults with repair priorities.
In the field
Credits are promises that have to be found in the building. When GCHS estimates or reviews a new Group B clinic, plan review asks which credits the design claims and where each one is documented. The inspector then verifies each claim: equipment nameplate efficiencies for an HVAC credit, the connected lighting schedule against the allowance, the third-party test report for infiltration, and the fault detection sequence and sampling rate. A claim with no paper trail is a missing credit, and the building can fall below 10.
1. A new Group R-2 building with 12 dwelling units is designed with a single master electrical meter for the whole building. What does Section C405.6 require?
2. Under Section C405.10, what is the most the total voltage drop across service, feeder, and branch circuit conductors combined may be?
3. What is the minimum efficacy for luminaires in an elevator cab, not counting signals and displays, under Section C405.9.1?
4. An elevator cab has a ventilation fan and no air conditioning of its own. Which control does Section C405.9.1 require?
5. In a new office building, at least what share of the 125-volt, 15- and 20-amp receptacles in enclosed offices must have automatic receptacle control under Section C405.11?
6. A designer uses a time-of-day control to meet the automatic receptacle control rule. Which arrangement follows Section C405.11.1?
7. Two new Group B office buildings are permitted, one with 30,000 square feet of gross conditioned floor area and one with 18,000. Under Section C405.12, which must have the energy monitoring system?
8. A new building uses a three-phase, 75 kVA low-voltage dry-type distribution transformer. What minimum nominal efficiency does Table C405.7 require?
9. Under Section C406.1, a new commercial building on the prescriptive path must achieve a total of how many credits from Tables C406.1(1) through C406.1(5)?
10. A new Group B office in Climate Zone 2A chooses reduced lighting power (C406.3) and a fault detection and diagnostics system (C406.11). Using Table C406.1(1), how many credits do the two options earn together?
11. A 20,000 square foot building in Climate Zone 2A has 15,000 square feet of Group B and 5,000 square feet of Group M. Reduced lighting power is worth 9 credits in the Group B table and 15 in the Group M table. What is the weighted average credit under Section C406.1?
12. A tenant space in a new shopping center must earn 5 credits under Section C406.1.1. Which option may the tenant pick to reach them?
13. A building has an interior lighting power allowance of 5,376 W. Under Section C406.3.1, which connected interior lighting power earns the base reduced lighting power credit?
14. A new building has 40,000 square feet of conditioned floor area. Using the floor-area option in Section C406.5.1, what is the minimum on-site renewable system rating for the basic credit?