*Conductors & Overcurrent Protection | Electrical System Design*

*Conductor Sizes & Types*

*Conductor Sizes & Types*

* The area of conductor is expressed in terms of Circular Mils. 1 inch = 1,000 mils to find the area in circular mils, the diameter in mils is just squared.*

*Area in sq. in. = (π/4)*(diam. in inches)²*

*Area in mils = (diam. in inches * 1,000)²*

*Solving both equations: Area in sq. in. = Area in mils * (π/4) / 1,000,000*

*Or Area in mils = Area in sq. inches * 1.273 x 1,000,000*

* The resistance of a conductor is a function of the conductor length, cross sectional area, and material resistivity as given by the following:*

*R = ρ . L/ A, where ρ is conductor resistivity, L = conductor length, A = cross sectional area of the conductor.*

*The resistivity of a conductor material varies as a function of temperature.*

*R = R25 [1+α(T2 – T1)],*

*where R = resistance at the new temperature, R25 = resistance at 25˚C, T2 = new temperature, T1 = reference temperature 25˚C, α = temperature co-efficient of resistance, 0.00385 for copper and 0.00395 for aluminium.*

*Continuous and Non-continuous Loads*

*Continuous and Non-continuous Loads*

*To determine the minimum conductor size and overcurrent protection device rating (OCPD) required for a branch circuit or feeder to supply a certain load, it is necessary to determine the loading in amperes on the circuit. Generally, the load on a circuit will be comprised of continuous and non-continuous loads. *

**Article 100 of the NEC** defines a continuous load as a load that is energized for three hours or more. Example of continuous loads are lighting, computer terminal, copy machines, HVAC equipment, and other devices and equipment that found in offices. Most processes in an industrial setting are also considered to be continuous loads. Non-continuous loads are general purpose receptacle outlets, residential lighting outlets, and so on. If there is a doubt as to whether or not a load should be continuous or non-continuous, it is best and recommended to assume that the load is continuous.

*Overcurrent Device Rating Selection*

*Overcurrent Device Rating Selection*

*The overcurrent device selected to protect a feeder or branch circuit must be properly rated to supply the load and protect the circuit conductor. Section 210-20 and section 215.3 of the NEC require that the rating of the overcurrent device must be at least equal to 125% of the continuous load plus 100% of the non-continuous load. This increase in the load current for continuous loads is due to the fact that higher temperature can be expected in the overcurrent device as a result of the continuous load current.*

*Cable Insulation Types*

*Cable Insulation Types*

**THW-2 –** *Thermoplastic insulation (Usually PVC), Heat resistant (90˚C), suitable for Wet locations*

*THWN-2 – **Same as THW except Nylon jacket over reduced insulation thickness. Also rated THHN*

*THHN – **Thermoplastic insulation (Usually PVC), High Heat resistant (90˚C), dry location only, Nylon jacket.*

**XHHW-2 –** *Cross-Linked polyethylene insulation (X), High Heat resistant (90˚C), for wet and dry locations.*

**RHH –** *Rubber insulation, most manufacturers use cross-linked polyethylene because it has the same properties as rubber, High Heat resistant (90˚C), for dry locations only.*

*RHW-2 – **Rubber insulation (cross-linked polyethylene), Heat resistant (90˚C), suitable for Wet locations.*

*USE-2 – **Underground Service Entrance, most utilize XLP (cross-linked polyethylene) for 90˚C in direct burial applications. Product is usually triple rated RHH-RHW-USE.*

*NM-B – **Non-Metallic sheathed cable. The ‘B’ denotes that individual conductor insulation is rated 90˚C, however, ampacity is limited to that of a 60˚C conductor. Thermoplastic (PVC) conductor insulation, nylon jacket, with overall PVC cable jacket.*

**SEU –** *Service Entrance Cable Underground, usually type XHHW insulated conductors with overall PVC jacket. As such, the cable is rated for 90˚C dry, 75˚C wet locations.*

*SER – **Service Entrance Cable, Round. Same material construction as SEU but round construction.*

*Derating Based on Ambient Temperature*

*Derating Based on Ambient Temperature*

*The following formula shall be applied to calculate cable new ampacity while applying ambient temperature derating factor:*

*Derated Ampacity = Table Listed Ampacity x Correction Factor*

*Cable ampacity can be read from Tables 310-16 & 310-17 in the NEC. Derating correction factor due to ambient temperature can be found in Table 3-01 next page.*

*Table 3-01 is just part of Table 310-16, which shows ambient temperature correction factor for cable rated for 90 C only. For complete list, please refer to the NEC Tables 310-16 and 310-17.*

*Derating Based on Number of Current Carrying Conductors*

*Derating Based on Number of Current Carrying Conductors*

*The following formula shall be applied to calculate cable new ampacity while applying number of current carrying conductors derating factor:*

*Derated Ampacity = Table Listed Ampacity x Adjustment Factor*

*Cable ampacity can be read from Tables 310-16 & 310-17 in the NEC. Derating Adjustment factor due to number of current carrying conductors can be found in Table 3-02 below.*

*When derating is required for both ambient temperature and raceway fill, conductor ampacity shall be multiplied by both ambient temperature correction factor and number of current carrying conductors adjustment factor as shown below:*

*Derated Ampacity = Table Listed Ampacity x Correction Factor x Adjustment Factor.*

*Temperature Limitations On Device Termials*

*Temperature Limitations On Device Termials*

*In the previous sections, the ampacity of a given conductor size was determined based on the table listed ampacity for a given insulation temperature rating and device temperature rating. The application of ambient temperature correction and raceway fill adjustment factors was also discussed. The maximum permitted ampacity of a given conductor to protect the conductors and device terminals from overheating was established. It is now necessary to determine the conductor size required to supply a given load.*

*The next step in conductor selection process involves application of section 210-19 and 215-2 of the NEC, which require the branch circuit or feeder conductors supplying a load to have an ampacity equal to or greater than 125% of the continuous load plus 100% of the non-continuous load. This minimum conductor size is determined without the application of ambient temperature correction factor or raceway fill adjustment factors and is based on the 60 ˚C or 75 ˚C table listed ampacity as dictated by the device terminal temperature rating.*

*In general the conductor temperature is dependent on the magnitude of the current flowing in the conductor, the ambient temperature, and the raceway fill. Both derating procedures were necessary due to the restriction of heat dissipation when conductors are placed in elevated temperature or in close proximity to other current carrying conductors. *

*In particular, table 310-16 and 310-17 of the NEC listed the allowable ampacities of various conductor sizes for the various insulation temperature ratings. Specifically, conductor insulation temperature ratings of 60 ˚C, 75 ˚C and 90 ˚Care displayed in the tables. The data contained in both tables 310-16 and 310-17 can also be viewed as an indication of the expected temperature of a conductor for a certain current level. *

*For example, if 75Amp flows in a #6 copper conductor, the temperature of the conductor would be 90 ˚C. If 65Amp were to flow in the same conductor, the temperature would be 75 ˚C. Lastly, if 55Amp were to flow in the conductor, the temperature would be 60 ˚C. Thus, even if the conductor insulation were rated 90 ˚C, the temperature would only be 75 ˚C if 65Amp were flowing, or 60 ˚C if 55Amp were flowing.*

*Branch circuit and feeder conductors will typically be terminated on the supply side by connection to a circuit breaker terminal lug or fuseblock terminal. On the load side, the conductors will typically be terminated on a device terminal such as the screw terminal of a receptacle or switch. By virtue of this connection, the heat produced in the conductor will be transmitted to the device terminals, thereby causing heating of the device terminals themselves. *

*In cases where the conductor temperature is greater than that of the device terminals, the terminals will act as a heat sink. In this case, heat will be transferred from the conductor to the device terminal until an equilibrium condition is reached. It is generally assumed that the temperature of the device terminals will be the same as that of the conductor. If a sufficient amount of heat is transferred to the device, false tripping of circuit breakers, nuisance blowing of fuses, or equipment damage may result.*

*To minimize false tripping, nuisance fuse blowing, and possible damage to equipment and devices, the temperature rating of the device terminals can not be exceeded. Section 110-14 (C)(1) of the NEC discusses the selection of conductor temperature rating as related to the temperature limitation on device terminals. Typically, the maximum temperature rating of device terminals is 60 ˚C for device rated 100Amp or less or designed to accommodate wire sizes #1 and smaller. *

*However, several manufacturers list 75 ˚C terminals for some devices less than 100Amp. For devices rated greater than 100A, or devices having terminals designed to accommodate wire sizes greater that #1, the maximum temperature rating of the device terminals is typically 75 ˚C. Based on these restrictions, the maximum allowable current flow through a conductor can not produce a temperature that exceeds the maximum permitted device terminal temperature rating. Therefore, if a 90 ˚C rated conductor is used, the ampacity can not exceed that shown in either the 60 ˚C or 75 ˚C column of table 310-16 or 310-17 in the NEC.*

*Based on that, one might wonder why it would be advantageous to use 90 ˚C rated conductor insulation. The answer is that the use of higher temperature conductor insulation is desirable when derating is required due to higher ambient temperature or raceway fill. Initially, the ampacity of the conductor can be selected from the 90 ˚C column. *

*The ambient temperature correction factor and/or raceway fill adjustment factors are shown in the 90 ˚C column. The resulting derated ampacity must then be compared to the ampacity listed in the 60 ˚C or 75 ˚C column, depending on device terminal temperature limitation. The lower of the two values becomes the maximum permitted ampacity of the conductor. Using a higher temperature rated conductor when derating is required generally results in selection of a smaller conductor size to supply the load.*

*Example: **A three phase, non-linear load is to be supplied in a 35 ˚C ambient. Determine the maximum allowable ampacity for the following conductors: #6 THW copper (75 ˚C rating), and #6 THW-2 copper (90 ˚C rating).*

**Solution:** #6 THW (75 C rating)

*The adjustment factor for raceway fill is (80%), and the ambient temperature correction is (94%), the table listed ampacity at 75 ˚C is 65Amp. The derated ampacity is then = 65Amm x 0.8 x 0.94 = 48.9Amp.*

*It is now necessary to look up the table listed ampacity of the #6 AWG conductor from the 60 ˚C column to ensure that the device terminal temperature limitation is not exceeded. Table listed ampacity at 60 ˚C is found to be 55Amp. Since the derated ampacity is less than the table listed ampacity at 60 ˚C, the maximum permitted ampacity is 48.9Amp.*

*#6 THW-2 (90 C rating)*

*The adjustment factor for raceway fill is (80%), and the ambient temperature correction is (96%), the table listed ampacity at 90 ˚C is 75Amp. The derated ampacity is then = 75Amm x 0.8 x 0.96 = 57.6Amp.*

*In this case, the derated ampacity of the conductor exceeds the 60 ˚C table listed ampacity of 55Amp. However, due to device terminal temperature limitations, the maximum permitted ampacity is 55Amp.*

*Conductor Size & OCPD Selection Process*

*Conductor Size & OCPD Selection Process*

*The following procedure summarizes the steps necessary to select overcurrent device rating and conductor size:*

*Summary of OCPD & the Conductor Selection Process:*

**Step 1.** Determine the design load current based on 125% of the continuous load plus 100% of the non-continuous load.

**Step 2.** Determine the minimum overcurrent device rating required based on the design load current calculated in step 1.

**Step 3.** Determine minimum required conductor size based on the design load current in step 1 and temperature rating of the overcurrent device terminals. The required minimum conductor size is based on either the 60 ˚C or 75 ˚C ampacity, depending on the temperature rating of the device terminals.

**Step 4.** Determine the minimum required conductor size utilizing the actual load current and taking into account derating due to ambient temperature and raceway fill adjustment as required. (Note that the actual load current does not include the 125% factor applied to the continuous part of the load current for this part of the selection process). This requires the application of equation (minimum table listed ampacity = load current to be supplied/ total derating factor).

**Step 5.** Compare the conductor size from step 3 and step 4. The larger of the two becomes the smallest conductor size permitted.

**Step 6.** Determine the maximum rating of the overcurrent device permitted based on the final selection of conductor size from step 5. The maximum rating so selected must be high enough to supply 125% of the continuous load plus 100% of the non-continuous load as determined in step 2.

**Example: **Determine the required size THW (75 ˚C insulation rating) copper conductor and overcurrent device rating required for a dedicated feeder supplying a 70Amp non-continuous load. The ambient temperature is 35 ˚C, and there are eight current carrying conductors in the raceway.

*Solution:*

**Step 1.** Design load current = (1.0)(70A) = 70A

**Step 2.** Overcurrent device rating = 70A

**Step 3.** The minimum required conductor size must be taken from the 60 ˚C column since the device is rated less than 100A. Therefore, a #4 AWG THW copper conductor, with a 60 ˚C ampacity of 70A, is specified as the required minimum.

**Step 4.** Apply ambient temperature correction factor and raceway fill adjustment factor. The ambient temperature correction factor is 94% and the raceway fill adjustment factor is 70%. Minimum table listed ampacity = 70/ 0.7 x 0.94 = 106.4A.

*Therefore, a #2 AWG THW copper conductor, with a table listed ampacity at 75 ˚C of 115A, is required based on this criterion. The derated ampacity is equal to 0.7 x 0.94 x 115A = 75.7A.*

**Step 5.** Conductor size from step 3 = #4 AWG THW

*Conductor size from step 4 = #2 AWG THW*

*Larger of the two conductors, a #2 AWG THW copper conductor, having a derated ampacity of 75.7A, is required for this load condition.*

*Step 6. The maximum rating must be high enough to supply 125% of the continuous load plus 100% of the non-continuous load. In this case, an 80A overcurrent device is permitted since this is a dedicated feeder, and rounding up to the next higher standard rated device is permitted.*

**Summary:** 80A overcurrent device, #2 AWG THW copper conductor.

**Example:** Repeat the previous example using THHN (90 C insulation rating) copper conductor.

*Solution:*

**Step 1.** Design load current = (1.0)(70A) = 70A

**Step 2.** Overcurrent device rating = 70A

**Step 3.** The minimum required conductor size must be taken from the 60 ˚C column since the device is rated less than 100A. Therefore, a #4 AWG THW copper conductor, with a 60 ˚C ampacity of 70A, is specified as the required minimum.

**Step 4.** Apply ambient temperature correction factor and raceway fill adjustment factor. The ambient temperature correction factor is 96% and the raceway fill adjustment factor is 70%. Minimum table listed ampacity = 70/ 0.7 x 0.96 = 104.2A.

*Therefore, a #3 AWG THHN copper conductor, with a table listed ampacity at 90 ˚C of 110A, is required based on this criterion. The derated ampacity is equal to 0.7 x 0.96 x 110A = 74A.*

**Step 5.** Conductor size from step 3 = #4 AWG THW

*Conductor size from step 4 = #3 AWG THW*

*Larger of the two conductors, a #3 AWG THW copper conductor, having a derated ampacity of 74A, is required for this load condition.*

**Step 6.** The maximum rating must be high enough to supply 125% of the continuous load plus 100% of the non-continuous load. In this case, an 80A overcurrent device is permitted since this is a dedicated feeder, and rounding up to the next higher standard rated device is permitted.

**Summary:** 80A overcurrent device, #3 AWG THW copper conductor.

*Comparing the results of both previous examples, the overcurrent device rating is the same. However, the smaller conductor size is permitted because of the higher insulation temperature rating combined with the use of the derating factors.*

*Parallel Conductors*

*Parallel Conductors*

*Due to reasons of economics and installation considerations, the largest size conductor is limited to 500 kcmil copper for most electrical distribution systems. Cables larger than 500 kcmil become difficult to install and the ampacity rating of conductors larger than 500 kcmil does not increase in direct proportion to the conductor size.*

*In addition, the heat dissipation is more limited with the larger size conductor, since the outside conductor surface area does not increase in the same proportion as the cross-sectional area. Another factor is that the current density in a conductor is not uniform across section of the conductor. This phenomenon is referred to as skin effect in a conductor. Skin effect increases with an increase in frequency. The current density in the conductor tends to be higher toward the outer surface of the conductor. This results in higher power losses, with a corresponding increase in temperature. In this order, parallel conductors method has become a common practice.*

*The rules for parallel installation of conductors are stated in Section 310-4 of the NEC. Generally, conductors must have the same insulation type, temperature rating, conductor material, conductor size, and length. The conductors must be terminated in the same manner. The raceway or conduit system must also be the same for each set of conductors. Also, parallel conductors must be #1/0 AWG or larger.*

**Example:** determine the required THW copper conductor size and overcurrent device rating to supply a three phase, four wire feeder supplying a 700A continuous nonlinear load. The ambient temperature is 35 ˚C.

**Solution:** since this is a continuous load, the factor of 125% must be applied. This results in a design load current of 125% x 700A = 875A. If two conductors per phase are used, the design would have to be based on 875A/2 = 437.5A per conductor. This exceeds the maximum rating permitted for the 500 kcmil copper wire size. If three conductors per phase are used, the design would be based on 291.7A per conductor. Now the previous six steps can be applied and put to the test to prove whether three conductors per phase is acceptable or it should be increased to four or more.

**Step 1.** Design load current = 125% x 700A = 875A.

**Step 2.** Minimum overcurrent device rating = 1,000A.

**Step 3.** Minimum conductor size based on 875A/3 = 291.A per conductor. The minimum conductor size is 350 kcmil THW copper conductor, having an ampacity of 310A as taken from the 75 C column of table 310-16.

**Step 4.** Since this is a continuous nonlinear load, the neutral must be counted as a current carrying conductor, three phase wires and the neutral. The raceway fill adjustment factor is 80%, and the ambient temperature correction factor is 94%. Also, each conductor is expected to carry 700A/3 = 233A. The minimum table listed ampacity is = 233A/ 0.8 x 0.94 = 310A. Therefore, a 350 kcmil THW copper conductor having a table listed ampacity of 310A would be required to meet this criterion. The derated conductor ampacity is 310A x 0.8 x 0.94 = 233.1A.

**Step 5.** Conductor size from step 3 = 350 kcmil THW.

*Conductor size from step 4 = 350 kcmil THW.*

**Step 6.** The overcurrent device rating is based on the ampacity per conductor multiplied by the number of conductors per phase. In this case, the result is 3 x 233.1A = 699.3A. Note that since the minimum required overcurrent device rating 1,000A exceeds 800A, rounding up to the next higher rated device is not permitted. Thus, the 350 kcmil conductor is not large enough to allow protection by the 1,000A OCPD required from step 2.

*To allow use of a 1,000A OCPD rating required to supply the load, the next larger size conductor must be selected. A 400 kcmil THW copper conductor has a table listed ampacity of 335A, as taken from the 75 ˚C column of table 310-16 of the NEC. The derated ampacity is 0.8 x 0.94 x 335A = 252A.*

* The resulting circuit ampacity including all three conductors per phase, is 3 x 252A = 756A. Based on the derated conductor ampacity of 756A, a maximum rating of 800A would be permitted. This is less than the required 1,000A minimum rating as determined from step 2. Selection of the next larger size conductor is required.*

*The next larger conductor size is 500 kcmil, having a listed ampacity of 380A as taken from the 75 ˚C column of table 310-16. The derated ampacity is 0.8 x 0.94 x 380A = 285.8A. The resulting circuit ampacity including all three conductors per phase is 3 x 285.8A = 857.3A. Note that the use of 500 kcmil conductor size is still not adequate for the application.*

*To avoid the use of a conductor size larger than 500 kcmil, consideration will be given to the use of four conductors in parallel per phase. Based on a design load of 875A and four conductors per phase, the current per conductor is based on 875A/4 = 219A per conductor.*

*The six step procedure is repeated.*

**Step 1.** Design load current = 125% x 700A = 875A.

**Step 2.** Minimum overcurrent device rating = 1,000A.

**Step 3.** Minimum conductor size based on 875A/4 = 219A per conductor. The minimum conductor size is #4/0 AWG THW copper, having an ampacity of 230A as taken from 75 ˚C column of table 310-16.

**Step 4.** Since this is a nonlinear load, the neutral must be counted as a current carrying conductor. Therefore, each conduit will contain four current carrying conductors, three phase wires and the neutral. The raceway fill adjustment factor is 80%, and the ambient temperature correction factor is 94%. Also, each conductor is expected to carry 700A/4 = 174A. The minimum table listed ampacity = 175A/ 0.8 x 0.94 = 232.7A.

*Therefore, a 250 kcmil THW copper conductor, having a table listed ampacity of 255A, would be required to meet this criterion. The derated conductor ampacity is 255A x 0.8 x 0.94 = 191.8A.*

**Step 5.** Conductor size from step 3 = #4/0 AWG THW.

*Conductor size from step 4 = 250 kcmil.*

**Step 6.** The overcurrent device rating is based on the ampacity per conductor multiplied by the number of conductors per phase. In this case, the result is 4 x 191.8A = 767.2A. Note that since the minimum required overcurrent device rating (1,000A) exceeds 800A, rounding up to the next higher rated device is not permitted. Thus, the 250 kcmil conductor is not large enough to allow protection by the 1,000A OCPD required from step 2.

*To allow use of a 1,000A OCPD rating required to supply the load, the next larger size conductor must be selected. A 300 kcmil THW copper conductor has a table listed ampacity of 285A, as taken from the 75 ˚C column of table 310-16. *

*The derated ampacity is 0.8 x 0.94 x 285A = 214.3A. The resulting circuit ampacity, including all four conductors per phase, is 4 x 214.3A = 857.3A. Based on the derated conductor ampacity of 857.3A, a maximum of 800A would be permitted. This is less than the required 1,000A minimum rating as determined from step 2. Selection of the next larger size conductor is again reqired.*

*A 350 kcmil THW copper conductor has a table listed ampacity of 310A, as taken from the 75 ˚C column of table 310-16. The derated ampacity is 0.8 x 0.94 x 310A = 233.1A. The resulting circuit ampacity, including all four conductors per phase, is 4 x 233.1A = 932.5A. Based on the derated conductor ampacity of 932.5A, a minimum rating of 800A would be permitted. Selection of the next larger size conductor is again required.*

*A 400 kcmil THW copper conductor has a table listed ampacity of 335A, as taken from the 75 ˚C column of table 310-16. The derated ampacity is 0.8 x 0.94 x 335A = 251.9A. The resulting circuit ampacity, including all four conductors per phase, is 4 x 251.9A = 1,007.7A. Based on the derated conductor ampacity of 1,007.7A, a 1,000A OCPD would be permitted. Note, it is necessary to check on the breaker lug specifications to determine if four conductors per phase can be terminated.*

**Summary:** 1,000A overcurrent device, four 400 kcmil THW copper conductors per phase.