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Section E3705 Conductor Sizing and Overcurrent Protection
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Ampacities for conductors shall be determined based in accordance with Table E3705.1 and Sections E3705.2 and E3705.3. [310.15(A)]
ALLOWABLE AMPACITIES
| CONDUCTOR SIZE | CONDUCTOR TEMPERATURE RATING | CONDUCTOR SIZE | |||||
| 60°C | 75°C | 90°C | 60°C | 75°C | 90°C | ||
| AWG kcmil | Types TW, UF | Types RHW, THHW, THW, THWN, USE, XHHW | Types RHW-2, THHN, THHW, THW-2, THWN-2, XHHW, XHHW-2, USE-2 | Types TW, UF | Types RHW, THHW, THW, THWN, USE, XHHW | Types RHW-2, THHN, THHW, THW-2, THWN-2, XHHW, XHHW-2, USE-2 | AWG kcmil |
| Copper | Aluminum or copper-clad aluminum | ||||||
| 14a | 15 | 20 | 25 | — | — | — | — |
| 12a | 20 | 25 | 30 | 15 | 20 | 25 | 12a |
| 10a | 30 | 35 | 40 | 25 | 30 | 35 | 10a |
| 8 | 40 | 50 | 55 | 35 | 40 | 45 | 8 |
| 6 | 55 | 65 | 75 | 40 | 50 | 55 | 6 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 | 4 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 | 3 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 | 2 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 | 1 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 | 1/0 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 | 2/0 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 | 3/0 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 | 4/0 |
| 250 | 215 | 255 | 290 | 170 | 205 | 230 | 250 |
| 300 | 240 | 285 | 320 | 195 | 230 | 260 | 300 |
| 350 | 260 | 310 | 350 | 210 | 250 | 280 | 350 |
| 400 | 280 | 335 | 380 | 225 | 270 | 305 | 400 |
| 500 | 320 | 380 | 430 | 260 | 310 | 350 | 500 |
| 600 | 350 | 420 | 475 | 285 | 340 | 385 | 600 |
| 700 | 385 | 460 | 520 | 315 | 375 | 425 | 700 |
| 750 | 400 | 475 | 535 | 320 | 385 | 435 | 750 |
| 800 | 410 | 490 | 555 | 330 | 395 | 445 | 800 |
| 900 | 435 | 520 | 585 | 355 | 425 | 480 | 900 |
For SI: °C = [(°F) - 32]/1.8.
- See Table E3705.5.3 for conductor overcurrent protection limitations.
For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities specified in Table E3705.1 by the appropriate correction factor shown in Table E3705.2. [310.15(B)(2)]
TABLE E3705.2 [Table 310.15(B)(2)(a)]
AMBIENT TEMPERATURE CORRECTION FACTORS
| For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities specified in the ampacity tables by the appropriate correction factor shown below. | ||||
| Ambient Temperature (°C) | Temperature Rating of Conductor | Ambient Temperature (°F) | ||
| 60°C | 75°C | 90°C | ||
| 10 or less | 1.29 | 1.20 | 1.15 | 50 or less |
| 11-15 | 1.22 | 1.15 | 1.12 | 51-59 |
| 16-20 | 1.15 | 1.11 | 1.08 | 60-68 |
| 21-25 | 1.08 | 1.05 | 1.04 | 69-77 |
| 26-30 | 1.00 | 1.00 | 1.00 | 78-86 |
| 31-35 | 0.91 | 0.94 | 0.96 | 87-95 |
| 36-40 | 0.82 | 0.88 | 0.91 | 96-104 |
| 41-45 | 0.71 | 0.82 | 0.87 | 105-113 |
| 46-50 | 0.58 | 0.75 | 0.82 | 114-122 |
| 51-55 | 0.41 | 0.67 | 0.76 | 123-131 |
| 56-60 | — | 0.58 | 0.71 | 132-140 |
| 61-65 | — | 0.47 | 0.65 | 141-149 |
| 66-70 | — | 0.33 | 0.58 | 150-158 |
| 71-75 | — | — | 0.50 | 159-167 |
| 76-80 | — | — | 0.41 | 168-176 |
| 81-85 | — | — | 0.29 | 177-185 |
For SI: 1 °C = [(°F) - 32]/1.8.
Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled for distances greater than 24 inches (610 mm) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table E3705.3. [310.15(B)(3)]
Exceptions:
- Adjustment factors shall not apply to conductors in raceways having a length not exceeding 24 inches (610 mm). [310.15(B)(3)(2)]
- Adjustment factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit, or rigid nonmetallic conduit having a length not exceeding 10 feet (3048 mm) and the number of conductors does not exceed four.[310.15(B)(3)(3)]
- Adjustment factors shall not apply to type AC cable or to type MC cable without an overall outer jacket meeting all of the following conditions:
- Each cable has not more than three current-carrying conductors.
- The conductors are 12 AWG copper.
- Not more than 20 current-carrying conductors are bundled, stacked or supported on bridle rings. [310.15(B)(3)(4)]
- An adjustment factor of 60 percent shall be applied to Type AC cable and Type MC cable where all of the following conditions apply:
- The cables do not have an overall outer jacket.
- The number of current-carrying conductors exceeds 20.
- The cables are stacked or bundled longer than 24 inches (607 mm) without spacing being maintained. [310.15(B)(3)(4) Exception]
| NUMBER OF CURRENT-CARRYING CONDUCTORS IN CABLE OR RACEWAY |
PERCENT OF VALUES IN TABLE E3705.1 |
|---|---|
| 4-6 | 80 |
| 7-9 | 70 |
| 10-20 | 50 |
| 21-30 | 45 |
| 31-40 | 40 |
| 41 and above | 35 |
The temperature rating associated with the ampacity of a conductor shall be so selected and coordinated to not exceed the lowest temperature rating of any connected termination, conductor or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both. Except where the equipment is marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table E3705.1. [110.14(C)]
Except where the equipment is marked otherwise, termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:
- Conductors rated 60°C (140°F);
- Conductors with higher temperature ratings, provided that the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used;
- Conductors with higher temperature ratings where the equipment is listed and identified for use with such conductors; or
- For motors marked with design letters B, C, or D conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used provided that the ampacity of such conductors does not exceed the 75°C (167°F) ampacity. [110.14(C)(1)(a)]
Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for:
- Conductors rated 75°C (167°F).
- Conductors with higher temperature ratings provided that the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or provided that the equipment is listed and identified for use with such conductors. [110.14(C)(1)(b)]
Separately installed pressure connectors shall be used with conductors at the ampacities not exceeding the ampacity at the listed and identified temperature rating of the connector. [110.14(C)(2)]
Conductors in NM cable assemblies shall be rated at 90°C (194°F). Types NM, NMC, and NMS cable identified by the markings NM-B, NMC-B, and NMS-B meet this requirement. The allowable ampacity of Types NM, NMC, and NMS cable shall not exceed that of 60°C (140°F) rated conductors and shall comply with Section E3705.1 and Table E3705.5.3. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and calculations provided that the final corrected or adjusted ampacity does not exceed that for a 60°C (140°F) rated conductor. Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table E3705.3. Where more than two NM cables containing two or more current-carrying conductors are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table E3705.3. (334.80 and 334.112)
Where used as a branch circuit or feeder wiring method within the interior of a building and installed in thermal insulation, the ampacity of the conductors in Type SE cable assemblies with ungrounded conductor sizes 10 AWG and smaller shall be in accordance with the 60°C (140°F) conductor temperature rating. The maximum conductor temperature rating shall be permitted to be used for ampacity adjustment and correction purposes, provided that the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor. [338.10(B)(4)(a)]
All ungrounded branch-circuit and feeder conductors shall be protected against overcurrent by an overcurrent device installed at the point where the conductors receive their supply. Overcurrent devices shall not be connected in series with a grounded conductor. Overcurrent protection and allowable loads for branch circuits and for feeders that do not supply the entire load associated with a one-family dwelling or the entire load associated with an individual dwelling unit in a two-family dwelling shall be in accordance with this chapter.
Branch-circuit conductors and equipment shall be protected by overcurrent protective devices having a rating or setting not exceeding the allowable ampacity specified in Table E3705.1 and Sections E3705.2, E3705.3 and E3705.4 except where otherwise permitted or required in Sections E3705.5.1 through E3705.5.3. [240.4, 240.21, and 310.15(B)(7)(2)]
Cords shall be protected in accordance with Section E3909.2. [240.5(B)]
The next higher standard overcurrent device rating, above the ampacity of the conductors being protected, shall be permitted to be used, provided that all of the following conditions are met:
- The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord- and plug-connected portable loads.
- The ampacity of conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
- The next higher standard device rating does not exceed 400 amperes. [240.4(B)]
Except as specifically permitted by Section E3705.5.4, the rating of overcurrent protection devices shall not exceed the ratings shown in Table E3705.5.3 for the conductors specified therein. [240.4(D)]
TABLE E3705.5.3 [240.4(D)]
OVERCURRENT-PROTECTION RATING
| COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||
| Size (AWG) | Maximum overcurrent- protection-device ratinga (amps) | Size (AWG) | Maximum overcurrent protection-device ratinga (amps) |
| 14 | 15 | 12 | 15 |
| 12 | 20 | 10 | 25 |
| 10 | 30 | 8 | 30 |
Air-conditioning and heat pump equipment circuit conductors shall be permitted to be protected against overcurrent in accordance with Section E3702.11. [240.4(G)]
The standard ampere ratings for fuses and inverse time circuit breakers shall be considered to be 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350 and 400 amperes. (240.6)
Circuit breakers and switches containing fuses shall:
- Be readily accessible. [240.24(A)]
- Not be located where they will be exposed to physical damage. [240.24(C)]
- Not be located where they will be in the vicinity of easily ignitible material such as in clothes closets. [240.24(D)]
- Not be located in bathrooms. [240.24(E)]
- Not be located over steps of a stairway.
- Be installed so that the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, is not more than 6 feet 7 inches (2007 mm) above the floor or working platform. [240.24(A)]
Each occupant shall have ready access to all overcurrent devices protecting the conductors supplying that occupancy. [240.24(B)]
Overcurrent devices shall be enclosed in cabinets, cutout boxes, or equipment assemblies. The operating handle of a circuit breaker shall be permitted to be accessible without opening a door or cover. [240.30(A) and (B)]
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