What table would you find the allowable ampacities for not more than three copper conductors in raceway or cable?

This app uses the calculation requirements found in the 2017 National Electrical Code® to determine the ampacity of insulated conductors and certain cable-type wiring methods. This calculator also complies with the 2020 NEC requirements. Certain installations (such as cable trays and medium-voltage conductors) are not included in this app. Insulated single conductors are assumed to be installed in conduit or direct buried. All calculations assume a voltage of no more than 2000 volts and that the appropriate NEC® requirements are followed by the installer, engineer or other responsible party. For more information on ampacity calculations, please consult our technical resources.

The primary two metals used for electrical conductors are copper and aluminum. Each has a different ampacity for a specified gauge size, and these ampacities are found in NEC® Table 310.15(B)(16) for the calculations in this app. For example, 3/0 AWG copper has an ampacity of 200 amps at 75℃, while 3/0 AWG aluminum has an ampacity of 155 amps at 75℃.

WIRE / CABLE TYPE:
Insulated wire types like THHN/THWN-2, XHHW-2 and USE-2/RHH/RHW-2 have an insulation temperature rating of 90℃, and any temperature correction factors or ampacity adjustment factors can be applied to the insulation temperature rating. The resulting value is the ampacity of the wire installed throughout the building or other installation.

However, electrical equipment (such as panelboards or circuit breakers) typically has an additional limiting temperature rating that must be considered when determining the ampacity of the wire that connects to the equipment. As an example, 600-volt panelboards have a maximum temperature limit of 75℃. Based on NEC 110.14(C)(1), conductors sized 14 AWG to 1 AWG are assumed to be connected to equipment limited to 60℃, and conductors 1/0 AWG and larger are assumed to be connected to equipment limited to 75℃. The installed equipment may have different temperature limitations, so be sure to check the actual ratings.

Certain cable types may also have a specified maximum temperature limit. For example, Type NM-B is limited to the ampacity values in the 60℃ column, even though the insulation on the conductors is required to be rated at 90℃.

This app assumes installation in conduit or direct burial when individual insulated wire types are chosen. The app limits the final ampacity of the wire to the most conservative of: the corrected and adjusted ampacity; the equipment temperature limitation default values as described in NEC® 110.14(C)(1); or the cable type temperature limitation.

Note: The calculator will permit choosing NM-B and UF-B up to 2 AWG. NM-B is permitted to be manufactured with conductors as large as 2 AWG, but multiconductor UF-B is limited to conductors no larger than 6 AWG by the UL standard.

AMBIENT TEMPERATURE:
The environment surrounding a wiring method can affect the ampacity of the conductors. NEC® Table 310.15(B)(16) ampacities are based on an ambient temperature of 86℉. If the temperature is greater than 86℉, the ampacity from the table must be corrected based on the values found in NEC Table 310.15(B)(2)(a). If the temperature is lower than 78℉, the ampacity is permitted to be corrected in accordance with the values in that table.

NUMBER OF CURRENT-CARRYING CONDUCTORS:
The number of current-carrying conductors in a wiring method can affect the ampacity of the conductors. NEC® Table 310.15(B)(16) ampacities are based on no more than three current-carrying conductors. If there are more than three current- carrying conductors, the ampacity from the table must be adjusted based on the values found in NEC Table 310.15(B)(3)(a). Adjustment factors range from 80% for 4 to 6 conductors, all the way to 35% for 41 or more conductors.

Do not include equipment grounding conductors, and do not include neutral conductors that meet the exception conditions in NEC 310.15(B)(5). Do include “spare” conductors that are intended to be energized at some point in the future. If there are multiple sets of conductors in a wiring method that supply electrical components that cannot be simultaneously energized, only the number of conductors that will be energized at any one time must be counted. Use the most conservative conditions anticipated to avoid overheating the conductors and creating a hazardous condition. 

NUMBER OF PARALLEL RUNS:
There are times when it is necessary or desired to run two or more conductors in parallel to carry the required current for a circuit. Paralleled conductors are permitted if sized at 1/0 AWG or larger. The number of parallel runs is the number of separate conductors installed within each phase, polarity, etc. There are several restrictions for the installation of paralleled conductors that can be found in NEC® 310.10(H).

OUTPUT PAGE:
The calculated ampacity value is the minimum of:  the corrected and adjusted ampacity; the equipment temperature limitation default values as described in NEC® 110.14(C)(1); or the cable type temperature limitation. The initial input values are also shown for verification, and they will be emailed along with the calculated ampacity if the email option is chosen. The insulation temperature rating is the rated temperature of the insulation surrounding the conductor metal, and the termination temperature rating is based on the default values shown in NEC 110.14(C)(1). If a cable wiring method was chosen, the temperature limit is 60℃ for NM-B and UF-B, and it is 75℃ for SER, SEU and MC cable.

Table 310.15(B)(2)(b) Ambient Temperature Correction Factors Based on 40°C (104°F)

For ambient temperatures other than 40°C (104°F), multiply the allowable ampacities specified in the ampacity tables by the appropriate correction factor shown below.

Table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current-Carrying  Conductors in a Raceway or Cable

* Number of conductors  is the total number of conductors  in the race­way or cable  adjusted  in accordance  with 31 0.15(B)(5) and (6).

Informational Note No. 2:   See 366.23(A) for adjustment factors for conductors in sheet metal auxiliary gutters and 376.22(B) for adjustment factors for conductors in metal wireways.

(1) Where conductors  are installed in cable trays, the provisions of 392.80 shall apply.(2)  Adjustment factors  shall  not apply  to conductors in raceways having a length not exceeding 600  mm (24 in.).(3) 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, rigid polyvinyl chloride conduit (PVC), or reinforced thermosetting resin conduit (RTRC) having a length not exceeding 3.05 m (10 ft), and if the number of conductors does not exceed four.

(4)  Adjustment factors shall not apply to Type AC cable or to Type MC cable under the following  conditions:

1. The cables do not have an overall outer jacket.
2. Each cable has not more than three current-carrying conductors.
3. The conductors are 12 AWG  copper.
4. Not more than 20 current-carrying conductors are installed without maintaining spacing, are stacked, or are supported on”bridle rings.”

(5)  An adjustment factor of 60 percent shall be applied to Type AC cable or Type MC cable under the following conditions:

1. The cables do not have an overall outer jacket.
2. The number of current carrying conductors exceeds 20.
3. The cables are stacked or bundled longer that 600 mm (24 in) without spacing being maintained.

(b) More Than One Conduit, Tube, or Raceway. Spacing between conduits, tubing, or raceways shall be maintained.
(c)  Circular Raceways Exposed to Sunlight on Roof­tops.  Where conductors or cables are installed in circular raceways exposed to direct sunlight on or above rooftops, the adjustments shown in Table 310.15(B)(3)(c) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 31 0.15(B)(2)(a) or Table 31 0.15(B)(2)(b).

Informational Note:  One source for the average ambient temperatures in various locations is  the ASHRAE Hand­book – Fundamentals.

Table 310.15(B)(3)(c)  Ambient Temperature Adjustment for Circular Raceways Exposed to Sunlight on or Above Rooftops

Informational Note to Table 31 0.15(B)(3)(c): The temperature adders in  Table 310.15(B)(3)(c)  are based on the results of averaging the ambient temperatures.

(4)  Bare  or Covered  Conductors. Where bare or covered conductors are installed with insulated conductors, the temperature rating of the bare or covered conductor shall be equal to the lowest temperature rating of the insulated con­ductors for the purpose of determining  ampacity.

(5)   Neutral Conductor.

(a)  A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required  to be counted when applying the provisions of 310.15(B)(3)(a).(b)  In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conduc­tors and shall be counted when applying the provisions of 310.15(B)(3)(a).

(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic cur­rents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.

(6)  Grounding or Bonding  Conductor. A grounding or bonding conductor shall not be counted  when applying the provisions of 31 0.15(B)(3)(a).

Table 310.15(B)(16) (formerly  Table 310.16) Allowable Ampacities of Insulated  Conductors  Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through  194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)*

*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 30°C (86°F).
**Refer to 240.4(D) for conductor overcurrent protection limitations.

Table 310.15(B)(17) (formerly Table 310.17) Allowable Ampacities of Single-Insulated Conductors Rated Up to and Including 2000 Volts in Free Air, Based on Ambient Temperature of 30°C (86°F)*

*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 30°C (86°F).
**Refer to 240.4(D) for conductor overcurrent protection limitations.

Table 310.15(B)(20) (formerly  Table 310.20) Ampacities of Not More Than Three Single Insulated Conductors, Rated Up to and Including 2000 Volts, Supported on a Messenger, Based on Ambient Air Temperature of 40°C (104°F)

*Refer to 310.15(B)(2)  for the ampacity correction factors where the ambient temperature is other than 40°C (104°F).

Table 310.15(B)(21) (formerly  Table 310.21) Ampacities of Bare or Covered Conductors  in Free Air, Based on 40°C (104°F) Ambient, 80°C (176°F) Total Conductor Temperature, 610 mm/sec (2 ft/sec) Wind Velocity

Table 310.60(C)(4) Ambient Temperature Correction Factors

For ambient temperatures other than 40°C (104°F), multiply the allowable ampacities  specified in the ampacity  tables by the appropriate factor shown below.

Table 310.60(C)(67) Ampacities of Insulated Single Copper Conductor Cables Triplexed in Air Based on Conductor Temperatures of 90°C (194°F) and 105°C (221°F) and Ambient Air Temperature of 40°C (104°F)*

*Refer to 310.60(C)(4) for the ampacity correction factors where the ambient air temperature  is other than 40°C (104°F).

Table 310.60(C)(68) Ampacities of Insulated Single Aluminum Conductor Cables Triplexed in Air Based on Conductor Temperatures of 90°C  (194°F) and 105°C (221°F) and Ambient Air Temperature of 40°C  (104°F)*

*Refer to 310.60(C)(4) for the ampacity correction  factors where the ambient air temperature  is other than 40°C (104°F).

Table 310.60(C)(69) Ampacities of Insulated Single Copper Conductor Isolated in Air Based on Conductor Temperatures of 90°C (194°F) and 105°C (221°F) and Ambient Air Temperature of 40°C (104°F)*

*Refer to 310.60(C)(4)  for the ampacity correction  factors where the ambient air temperature  is other than 40°C (104°F).