Return to electrician calculators

NEC Table 310.15(B)(16) Conductor Size, OCPD, Voltage Drop,
 and Equipment Grounding Conductor Size Calculator

by electrician2.com

This calculator applies 2005 NEC Table 310.16 and accompanying code sections and notes to find the wire size and minimum and maximum overcurrent protective devices (OCPD) for branch circuits and feeders per Section 240.4(B), and (D) and solves many common design problems.  

See Note 11 and examples at bottom of page.

Written by Gerald C Newton November 2006.  Updated to 2011 NEC on 04/03/2011: Ampere values updated and ambient temp expanded.  Fixed sizing grounding conductor problem for when conductor sizes are increased to lower voltage drop 04/12/2014.

Select Material

Ambient
Temperature C
Number Current Carrying Conductors in Raceway or Cable or bundled   see Note 9

Continuous Load
(in Amperes)

NonContinuous Load
(in Amperes)

Special Conditions

Select Conductor Insulation Temperature Select Termination Temperature C. OCPD and assembly is rated for Continuous Load  see Notes 1-5 Circuit is Branch Circuit that supplies multiple receptacles see Note 7

Yes

Yes

See Note 10 See Note 10

No

No

Note: NM cable shall have a 90 degree C insulation (used for derating) but shall be used at the 60 degree C ampacity (334.80).  See Note 12 and example 6. Circuit is bundled or in raceway 24 inches or less in length Circuit is Type AC or MC cable that complies with Note 6.
If circuit is 15, 20, 30 40, or 50 amperes branch-circuit supplying two or more outlets or receptacles See Table 210.24.

Yes

Yes

No

No

CLICK BUTTON FOR OUTPUTS BELOW

Conductor Size

OCPD Minimum Size

OCPD Maximum Size

 

Equip Grnding Cond.  Size

Equip Grnding Cond.  Size

Data

Conductor Derated
ampacity

Terminal
Ampacity

AC or MC Cable Derating Factor (if applicable)

Ambient
Correction Factor
Continuous Load Multiplier

Total Load
Terminal Load (total load)
Conductor Derating Factor for more than three current carrying conductors
Ampacity from Table 310.16
Conductor circular mil area
Total Derating Factor

Voltage Drop for above Conductor

Select voltage and phase, Enter Distance, Then Click Calculate Button
(This calculator uses k=12.9 for copper or k=21.2 for aluminum)
Select Voltage
and Phase

Enter Distance in
feet from supply to load (one way)

Input from above program:  (The Load can be changed by using the keyboard)
Material conductor Size CMA of conductor Load k (circular mil-ohm ft.
     

OCPD Minimum Size

OCPD Maximum Size
      Equip Grounding Conductor Size
  See Note 8.
Equip Grounding
Conductor Size

  See Note 8.
Voltage drop
Voltage at load end of circuit
Per Cent voltage drop
 
Copyright Notice
Notes:
1.  Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.
2.  422.13 Storage-Type Water Heaters.
A fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load.
3.  424.3(B) Branch-Circuit Sizing.
Fixed electric space heating equipment shall be considered a continuous load.
4.  426.4 Continuous Load.
Fixed outdoor electric deicing and snow-melting equipment shall be considered a continuous load.
5.  427.4 Continuous Load.
Fixed electric heating equipment for pipelines and vessels shall be considered a continuous load.
6.  310.15(B)(2)(3)(a)Exception No, 5  Circuit is Type AC cable or to Type MC cable without an overall outer jacket under the following conditions:(1) Each cable has not more than three current-carrying conductors. (2) The conductors are 12 AWG copper.(3) Not more than 20 current-carrying conductors are bundled, stacked, or supported on bridle rings. (Load must be 18.6 amperes or less - ambient multiplier 1.04 x .6 x 30))
7.  240.4(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met: (1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord and- plug-connected portable loads.  Also see Table 210.24 and Section 210.23.
8.  Equipment Grounding Conductor for each OCPD device is increased proportionally per 250.122 (A)and (B) when conductor size is increased.
9. 
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 as a current carrying conductor.
(b) In a 3-wire circuit consisting of two phase wires and the neutral 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 as a current carrying conductor.
(c) On a 4-wire, 3-phase wye circuit where the major portion (over 50 per cent) of the load consists of nonlinear loads such as electric discharge lighting and computer loads, harmonic currents are present in the neutral conductor and the neutral shall be considered a current carrying conductor.
(d) Each current-carrying conductor of a paralleled set of conductors shall be counted as a current carrying conductor.
10.  
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:
(1) Conductors rated 60C (140F).
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60C (140F) ampacity of the conductor size used.
(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75C (167F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75C (167F) ampacity.
Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75C (167F)
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75C (167F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors.
 

Author's Note:  In general most terminals are rated for 75 degrees C.  Conductors with 90 degree C insulations can be used on these terminals provided they are not used at an ampacity higher than the 75 degree C. ampacity.   However, the 90 degree insulation ampacity can be used for derating purposes.    For example a No. 12 AWG copper 90 degree C. THHN rated at 30 amperes can be used in a raceway with up to 9 current carrying conductors on a 20 ampere circuit breaker with 75 degree C terminal or 60 degree C terminal since the  derated  90 degree C ampacity is 0.7 x 30 = 21 amperes and the 75 degree C ampacity is 25 amperes and the 60 degree C ampacity is also 25 amperes. 

75 degree terminals may be stamped  AL7CU.  This means the terminal is rated for aluminum or copper at 75 degrees C.

11. Small Conductors. This calculator follows the general rule for small conductors as given in NEC Section 240.4(D).  This rule states that unless specifically permitted in 240.4(E) that covers tap conductors or 240.4(G), that covers specific circuits such as motor circuits the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

12.  The ampacity of Types NM, NMC, andNMS cable shall be determined in accordance with 310.15.  The ampacity shall be in accordance with the 60C (140F) conductor temperature rating. The 90C (194F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60C (140F) rated conductor.  

 Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing that is to be fire- or draftstopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).  See example 6.

 

Examples

 

Example 1 from NEC Annex D Example D3(a)

Continuous Load= 56,600 VA
Non Continuous Load = 42,400 VA

Supply is 480/277 volt
Use XHHW-2 conductors rated at 90 degrees C
Use 75 degree C Terminals

8 current carrying conductors in raceway

Ambient Temperature is 35 degrees C.

Cont load  Amperes = 56,600 VA/ (480V * 1.732) = 68.2 amperes
Non Cont Load Amperes = 42,400 VA /(480V*1.732) = 51.1 Amperes

Answer:

2/0 AWG conductors on a 150 ampere circuit breaker

 

 


Example 1 from NEC Handbook Section 220.40

Given:

Feeder Circuit with following parameters.


3 phase 4 wire feeder  full size Neutral
125 ampere noncontinuous load
200 ampere continuous load
75 Degree C terminal temperature rating
Type THHN insulated conductors
(THWN was given in the original problem, but was changed to THHN in the solution)
Four current carrying copper conductors in a raceway
Majority of load is nonlinear

Solution :

600 kcmil conductors protected by a 400 ampere overcurrent protective device (OCPD)

 


Example 2 from NEC Handbook Section 210.20(A)

Given:

Branch Circuit with following parameters.

25 ampere continuous load
60 Degree C terminal temperature rating
Type THWN insulated conductors
Four current carrying copper conductors in a raceway
Majority of load is nonlinear

Solution:

No. 8 AWG conductors protected by a 35 ampere overcurrent protective device (OCPD)

 


Example 3 from Mike Holt online examples

Given:

Branch Circuit with following parameters.

53 ampere continuous load
75 Degree C terminal temperature rating
Type THHN insulated conductors
Four current carrying copper conductors in a raceway
Majority of load is nonlinear

Solution :

No. 4 AWG conductors protected by a 70 ampere overcurrent protective device (OCPD)

 


Example 4 from Mike Holt online Examples

Given:

Feeder Circuit with following parameters.


3 phase 4 wire feeder  full size Neutral
184 ampere continuous load
75 Degree C terminal temperature rating
Type THHN insulated conductors
Four current carrying copper conductors in a raceway

Solution:

250 kcmil conductors protected by a 250 ampere overcurrent protective device (OCPD)

 


Example 5 from McGraw-Hill's NEC Handbook

Given:

Branch Circuit with following parameters.

100 ampere continuous load
75 Degree C terminal temperature rating
Type THHN and THW insulated conductors
Four current carrying copper conductors in a raceway
Majority of load is nonlinear

Solution :

No. 1 AWG THHN or No. 1/0 AWG THW conductors protected by a 125 ampere overcurrent protective device (OCPD)


Example 6 Nonmetallic Cable

Given:

Branch Circuit with following parameters.

16 ampere continuous load
Four each 2-conductor w/ground NM cables in a 36 inch long  raceway used for firestopping.
Conductors are copper

Analysis:

Per Section 334.80   The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15.  The ampacity shall be in accordance with the 60C  conductor temperature rating.  The 90C rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60C rated conductor.

 To apply these rules select the conductor as "NM Cable - 90 C" and the terminal temperature as "NM - 60 C."    There are 8 current carrying conductors.

Solution :

No. 12 conductors protected by a 20 ampere overcurrent protective device (OCPD)