2023 NEC Articles 300 and 310
Two questions decide most wiring-method problems in commercial work: is the wiring protected where it goes, and is the conductor big enough once the heat and the crowding are counted. Article 300 answers the first, and Article 310 answers the second. This lesson covers protection in framing and fire-rated assemblies, the burial depths in Table 300.5(A), and how to use Table 310.16 with the ambient temperature and adjustment factors from the 2023 NEC. Local adoption governs which edition is enforced (Lesson 1).
Where a cable or raceway passes through bored holes in wood joists, rafters or studs, the edge of the hole must be at least 1-1/4 in. from the edge of the member. If that cannot be done, a steel plate or bushing at least 1/16 in. thick must cover the wiring, and no plate is needed for RMC, IMC, PVC, RTRC or EMT (300.4(A)(1)). Wiring run parallel to framing or furring follows the same 1-1/4 in. distance where nails or screws are likely to hit it, with the same 1/16 in. plate or sleeve as the fix (300.4(D)). In commercial work this comes up most in wood-framed Type V buildings, and around wood blocking and nailers in any building.
Wiring above a suspended ceiling cannot be supported by the ceiling grid, and support wires that are not secure cannot be the only support (300.11(B)). In a fire-rated floor-ceiling or roof-ceiling assembly, wiring may not be secured to or supported by the ceiling assembly or its support wires, and an independent means of support is required (300.11(B)(1)). Openings around electrical penetrations of fire-resistance-rated walls, floors and ceilings are firestopped with approved methods so the rating is kept (300.21). The listed assembly tells you the method, and the building code sets the rating.
Direct-buried cable, conduit and other raceways meet the cover in Table 300.5(A), measured from the top of the wiring to the top of finished grade, concrete or similar cover (Note 1). The columns are: 1, direct burial cable; 2, RMC or IMC; 3, EMT, listed direct burial nonmetallic raceway with no concrete, and other approved raceways; 4, residential branch circuits of 120 V or less with GFCI and 20 A protection at most; 5, irrigation and landscape lighting of 30 V or less. Columns 1 to 3 cover most commercial work.
| Location | Col 1 cable | Col 2 RMC, IMC | Col 3 EMT, PVC |
|---|---|---|---|
| All locations not listed below | 24 in. | 6 in. | 18 in. |
| In trench below 2 in. of concrete | 18 in. | 6 in. | 12 in. |
| Under 4 in. exterior slab, no vehicles, slab 6 in. beyond | 18 in. | 4 in. | 4 in. |
| Under streets, roads, alleys, driveways, parking lots | 24 in. | 24 in. | 24 in. |
| Airport runways and adjacent areas | 18 in. | 18 in. | 18 in. |
Other rules ride with the table. Where a Column 1 to 3 method is used for a Column 4 or 5 circuit, the shallowest depth applies (Note 4). Underground cable under a building goes in a raceway (300.5(C)). Cables that emerge from grade are protected by an enclosure or raceway from the required cover depth, never more than 18 in. down, up to 8 ft above grade (300.5(D)(1)). Underground service conductors not encased in concrete and buried 18 in. or more get a warning ribbon 12 in. above them (300.5(D)(3)). The inside of an underground raceway is a wet location, so the conductors must be a wet-location type (300.5(B)). Where rock stops you, the wiring goes in metal or listed nonmetallic raceway under 2 in. of concrete down to rock (Note 5).
Table 310.16 gives ampacities for conductors rated 0 to 2,000 V, in raceway, cable or earth, when all four things are true: the conductor is rated 60, 75 or 90 degrees C, the ambient is 30 degrees C (86 degrees F), there are not more than three current-carrying conductors, and the rest of the rules are met. Change any condition and the table value must be corrected. Here are the copper rows most used.
| Copper size | 60 C | 75 C | 90 C |
|---|---|---|---|
| 12 AWG | 20 A | 25 A | 30 A |
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
| 1/0 | 125 A | 150 A | 170 A |
| 4/0 | 195 A | 230 A | 260 A |
Where the ambient is not 30 degrees C, multiply the ampacity by the factor in Table 310.15(B)(1)(1) for the conductor's temperature rating (310.15(B)(1)). A Houston attic or mechanical room in summer commonly lands in the 41 to 45 degree C band (105 to 113 degrees F).
| Ambient | 60 C | 75 C | 90 C |
|---|---|---|---|
| 31 to 35 C (87 to 95 F) | 0.91 | 0.94 | 0.96 |
| 36 to 40 C (96 to 104 F) | 0.82 | 0.88 | 0.91 |
| 41 to 45 C (105 to 113 F) | 0.71 | 0.82 | 0.87 |
| 46 to 50 C (114 to 122 F) | 0.58 | 0.75 | 0.82 |
Raceways or cables in direct sun on a roof, where the bottom of the wiring is less than 3/4 in. above the roof, add 33 degrees C (60 degrees F) to the outdoor temperature before choosing the factor (310.15(B)(2)). XHHW-2 conductors are not subject to that adder.
When more than three current-carrying conductors share a raceway or cable, each conductor's ampacity is reduced by the Table 310.15(C)(1) percentage, and paralleled conductors each count (310.15(C)(1)).
| Current-carrying conductors | Percent of ampacity |
|---|---|
| 4 to 6 | 80 |
| 7 to 9 | 70 |
| 10 to 20 | 50 |
| 21 to 30 | 45 |
| 31 to 40 | 40 |
| 41 and above | 35 |
What counts matters. The count includes spare conductors, but not conductors tied to loads that cannot run at the same time (Table note). A grounding or bonding conductor is not counted (310.15(F)). A neutral that carries only the unbalanced current is not counted, but the neutral of a 3-wire circuit taken from a 4-wire wye system is counted, and so is the neutral on a 4-wire wye circuit that serves mostly nonlinear loads such as LED drivers and computers (310.15(E)). Raceways 24 in. or shorter are exempt from adjustment (310.15(C)(1)(b)).
Correction and adjustment can be applied to the ampacity of the conductor's own temperature rating, such as the 90 degree C column for THHN, as long as the result does not exceed the ampacity at the termination's temperature rating (310.15(A), 110.14(C)). Terminals for circuits of 100 A or less, or marked for 14 to 1 AWG, are treated as 60 degrees C unless the equipment is listed and marked otherwise. Terminals for larger circuits are treated as 75 degrees C (110.14(C)(1)). So the answer is the smaller of two checks: the derated value, and the table value at the termination temperature.
Two 208 V three-phase, 3-wire circuits feed shop equipment at 45 A each, noncontinuous, on 50 A breakers. They run in one EMT through an attic with a design temperature of 110 degrees F, using THHN copper.
The answer is 6 AWG THHN on 50 A breakers. If the loads were nonlinear and each circuit carried a counted neutral, there would be eight conductors and 70 percent, so 6 AWG would drop to 75 x 0.87 x 0.70 = 45.7 A, which still covers 45 A but barely.
In the field
A GCHS rehab of a small clinic runs a new feeder for rooftop units. The plan review checks two things. First, that the underground stub-up from the parking lot uses the right cover, 24 in. under the paving and a steel elbow where it rises. Second, that the conduit fill sheet counts every current-carrying conductor and lists the roof ambient and the height of the conduit above the roof. In the field, look for EMT sitting right on the roof and for neutrals that were left out of the count on LED and computer circuits.
1. A Group M shopping center runs 480 V direct burial UF cable to a parking lot light pole under the paved lot. What minimum cover does Table 300.5(A) require for the cable under the paving?
2. Rigid metal conduit carrying 208Y/120 V feeders is buried in an open landscaped area beside a Group A-3 community center. What minimum cover does Table 300.5(A) require, Column 2, all locations?
3. A Group B office needs a feeder in Schedule 40 PVC conduit listed for direct burial, with no concrete around it, run under an open lawn. What minimum cover does Table 300.5(A) give for this nonmetallic raceway?
4. PVC conduit for a Group B building lies under a 4 in. thick exterior concrete slab that carries no vehicles and extends at least 6 in. beyond the conduit on each side. What minimum cover applies to the PVC in Column 3?
5. A trench crosses beneath the footprint of a new Group R-2 apartment building at a slab that will be poured later. What does 300.5(C) require of underground conductors under the building?
6. Direct burial cable buried at 24 in. rises to a wall-mounted disconnect on a Group B building. How far must the protection extend, under 300.5(D)(1)?
7. An underground 400 A service lateral in PVC serves a Group B strip center. It is buried 24 in. and is not encased in concrete. What does 300.5(D)(3) require?
8. An electrician runs MC cable through a hole in a wood stud where the hole edge is 3/4 in. from the stud edge in a Group R-2 building. What does 300.4(A)(1) require?
9. In a Group B tenant space, an electrician clamps an EMT run to the ceiling support wires of a rated floor-ceiling assembly. What does 300.11(B)(1) say?
10. A 12 AWG THHN copper conductor in EMT (three current-carrying conductors) runs through a Group B attic at 43 degrees C. All terminations are rated 60 degrees C. What is the highest ampacity that may be used?
11. Nine current-carrying 12 AWG THHN copper conductors share one EMT in a Group B office at 30 degrees C. Using the 90 degree C column, what is the adjusted ampacity of each conductor under Table 310.15(C)(1)?
12. Three 4-wire, 208Y/120 V circuits, each with three phase conductors and a neutral, run in one EMT to a Group B office where most of the load is LED drivers and computers. How many conductors are counted, and what percentage applies?
13. A 3-phase, 3-wire feeder in a Group B building has three phase conductors, one grounding conductor and, as the drawings show, two spare conductors in the same raceway. How many conductors are counted for Table 310.15(C)(1)?
14. EMT runs across the roof of a Group B building, held 2 in. above the roof surface and in direct sun. Under 310.15(B)(2), what temperature adder is applied to the outdoor temperature?