Question 1. This inquiry seeks a clarification or description of what actually constitutes the bundling of NM cables as relates to derating. In general, the question relates to NM cable in single-family residences and in accessory structures.In a TJI floor-joist system, the 1-1/2-inch knockouts in the web of the joists can be used for routing the NM cable. If, for example, ten No. 12 with ground NM cables are run through a single knockout in the TJs (which are spaced at 16inches off center) and are supported by the knockout in the TJ — and if the cables are not cinched, stapled, taped or otherwise secured together but are simply fed through the knockouts — would this close proximity and non-continuous contact of the NM cables require derating?

Similarly, if, for example, six No. 12 with ground NM cables are routed through holes bored in the center of wood studs and are supported by the studs — and if the cables are not cinched, stapled, taped or otherwise secured together — would this close proximity and non-continuous contact of the NM cables require derating? 

In the jurisdiction where I work, the building inspectors have differing opinions on what actually constitutes “bundling” of NM for the purposes of derating. Some feel that the mere proximity or contact of the cables qualifies for derating. Others feel that unless the cables are actually cinched tightly for over two feet, then non-continuous contact would not qualify for derating. Is there a standard or acceptable distance or minimum spacing that can be applied?  Rather than simply relying on a consensus among the small group of inspectors (which may or may not be consistent with the NEC), we would like an opinion or perspective from a more authoritative source. Is there a standard, guideline or criteria that can be applied to determining if bundling actually occurs?   — K.C.

Answer 1. This is a question that was brought to Panel 6 in the 1999 code cycle. The panel agreed that most contractors pulled in cables tight, leaving no slack in the cables. This caused the cables to be snug against each other or stacked with no maintained spacing. Additionally, there was a problem with joist gathered or bundled cables every 16 inches where spacing was not maintained. If there was any spacing, it soon disappeared with gravity. The NEC does not address non-continuous or continuous contact but requires maintained spacing. If there was 1/8 or 1/4 in. spacing that could be maintained, we could say it is only bundled or stacked at the joist with no derating required. I don't know of any product that is available and approved to guarantee that cable spacing will be maintained between joists. This is similar to a raceway where the conductors may or may not be in continuous contact; however, derating would be required.

Section 334.80 [NEC 2002], Ampacity, allows the 90-degree column in the ampacity table to be used for derating, and the 60-degree column for maximum ampacity. It also  refers to 310.15(B)(2),  Adjustment Factor, that states  where multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a).

Therefore, if there are ten – No. 12 NM cables pulled through 1½  in. holes in joists without maintained spacing, the ampacity would have to be derated using Table 310.15(B)(2)(a). Ten MN cables would have twenty current-carrying conductors and be required to be derated to 50 percent of the ampacity. Table 310.16 allows 30 amps in the 90-degree column with a derating of 50 percent, equaling fifteen amps allowed on these cables and fifteen amps of overcurrent protection.

If there were six  No. 12 NM cables pulled through studs, the same would apply. There would be twelve current-carrying conductors. Section 310.15(B)(2)(a) would require derating 50 percent of 30, equaling fifteen amps allowable, and fifteen amps off center.

If there were only four No. 12 NM cables, there would be eight current-carrying conductors with a derating of 70 percent of 30 amps, equaling 21 amps. Twenty-amp overcurrent protection is allowed per 240.4(D).

Additional derating may be required for high ambient temperatures in attic spaces and other high temperature areas.

Please also be aware that the holes in a 3½ in. stud could only be 1 in., leaving 1 1/4 in. on each side of the hole. — John Stacey,  Alternate CMP-6    

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Question 2. Twice in the last 20 years I have read a discussion to the effect that I can use the 75°C column in Table 310.16 without derating the wires per 240.4(D) when hooking up motors. In other words, I can hook up motors with 12 AWG wire using the ampacity listed of 25 amps. I can use 14 AWG wires as 20-amp wire and number 10 at 35 amps. I have no memory of how this was justified nor am I able to find anything in the current Code supporting this.

Can you enlighten me as to whether this was ever allowed and, if so, is it still allowed? — K. L. R.

Answer 2. Thank you for your questions. Let us examine the NEC 2002 text to answer them.

Section 240.4(D), Small Conductors, limits the ampacity of 14 AWG, 12 AWG, and 10 AWG, "unless specifically permitted in 240.4(E) through (G), 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."

An exception to the above rule for motor protection is (G), Overcurrent Protection for Specific Conductor Applications. "Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G)."

430.22 Single Motor.

"(A) General. Branch-circuit conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)."

430.6(A)(1) Table Values. "The values given in Table 430.147, Table 430.148, Table 430.149, and Table 430.150, including notes, shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Tables 430.147, 430.148, 430.149, and 430.150, interpolated if necessary."

430.52(C) Rating or Setting for individual motor circuit. (1) In Accordance with Table 430.52. "A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used."

Some examples that might help:

Example 1. A 1-hp 115-volt, single-phase ac motor. Table 430.148.

FLA=16 amps. 16 x 125% = 20 amps. A 14 AWG conductor can be used from Table 310.16, 75°C column to feed this motor and protected in accordance with Table 430.52.

Example 2. A 3-hp, 208-volt, single-phase ac motor. Table 430.148. FLA = 18.7 amps. 18.7 x 125% = 23.4 amps. A 12 AWG conductor can be used from Table 310.16, 75°C column to feed this motor and protected in accordance with Table 430.52.

Example 3. A 20-hp, 460-volt, 3-phase ac motor. Table 430.150. FLA = 27 amps. 27 x 125% = 33.75 amps. A 10 AWG conductor can be used from Table 310.16, 75°C column to feed this motor and protected in accordance with Table 430.52. The above information is what allows the ampacity in the 75°C column in Table 310.16 to be used for motor circuit conductors.

The only other problem would be if the termination temperatures were less than 75°C, then the rules in 110.14(C)(1)(a)(3) and (4) would not apply.

110.14(C) Temperature Limitations. "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to 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.

"(1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(A) or (B). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(1) through (6).

"(a) 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 60°C (140°F)

"(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) 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, D, or E, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity."

240.4(D) Small Conductors. "Unless specifically permitted in 240.4(E) through (G), 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."

The same rules would apply to conductors supplying ac equipment and the protective device sized from 440.22(A). — O. P. Post, CMP-6

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Question 3. In NEC Table 310.16, information is listed for derating conductors due to ambient temperatures. The ampacities listed with the table are based on an ambient temperature of 30°C. Are conductors that are installed where outdoor ambient temperatures can reach the 41–45°C range in the summer months required to be derated using these tables? — B.B.

Answer 3. The answer to this question is, yes, the correction factors would apply. The ampacity of conductors in Table 310.16 is based on an ambient temperature of 30°C (86°F). If these conductors are installed in a higher temperature, in your example 41–45°C, then they must be derated using the correction factors at the bottom of the table. The 90°C column can be used for this calculation if conductors with 90°C insulation are used, which is .87. After the ampacity is calculated, the conductor must be taken from the 75°C column. An example would be 500 kcmil in the 90°C column is 430 amps at 30°C; after derating by .87 for 41–45°C it would be 374 amps, which would still be good for a 400-amp feeder or service conductor. — O.P. Post, CMP-6

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Question 4. My question is concerning derating of conductor in accordance with Section 310.15, Table 310.15(b)(2)(a). It has always been my belief that you could use the 90°C chart of 310.16 when derating a conductor. However, one of the local inspectors states that you must derate using the 60° or 75°C chart, depending on the rating of the termination point of the conductors. Please help clear this up for me. — T. C.

Answer 4. Thank you for giving me an opportunity to talk about a frequently misunderstood subject. I always have questions about this topic in my code class.

Take a closer look at NEC 110.14(C). "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to 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." This means that the derating for adjustment (the number of conductors in a raceway or cable) or correction (ambient temperature other than 30°C (86°F) or both can start with the allowable ampacity for the type of conductor being used.

Let’s try different scenarios. If we are installing two 50-amp, 240-volt circuits in one raceway, what size conductors are required? The receptacle and the breaker are rated for 60°C termination. In Table 310.15(b)(2)(a), Adjustment Factors, there are two columns. The first column is entitled "Number of Current-Carrying Conductors." The second column is titled "Percent of Values in Tables 310.16 through 310.19 as Adjusted for Ambient Temperature if Necessary." There are four current-carrying conductors and one equipment ground. The equipment ground is not counted. The table requires a reduction to 80 percent of the conductor ampacity due to more than three current-carrying conductors in a raceway. If we were using 90°C conductors, we would start the derating in the 90°C column. If we were using 60° or 75°C conductors, we would start the derating in the 60°C or 75°C column accordingly.

Let’s try a 6 AWG 90°C conductor. In Table 310.16, the allowable ampacity is 75 amps x .80 adjustment factor = 60 amps allowable on this conductor. Remember, 110.14(C) would limit the ampacity to the 60°C column due to the devices being rated 60 amps. Therefore, 55 amperes in the 60°C column could not be exceeded for the load. A 50-amp circuit would not exceed, and a 6 AWG, 90°C conductor would work.

Now try a 6 AWG 75°C conductor that has an allowable ampacity of 65 amps x .80 adjustment factor = 52 amps available. A 50-amp circuit would not exceed 52 amps allowing a 6 AWG, 75°C conductor to be used.

To install a 50-amp circuit through an attic where the conductors are exposed to high ambient temperatures around 130°C we look at the bottom of Table 310.16, which is a Correction Factor Table for temperatures other than 30°C (86°F). There is a correction factor of .76 for 6 AWG under the 90°C column. The reason it is under the 90°C column is to allow derating from that column. A 6 AWG 90°C conductor has an allowable ampacity of 75 amperes x .76 correction factor. This will only allow 57 amps on the 90°C conductor. Remember 110.14(C) would limit the ampacity to the 60°C column due to the devices being rated 60 amps. Therefore 55 amperes could not be exceeded. After derating the conductor to 57 amps allowable, a 50-amp circuit would be allowed on 6 AWG.

We are installing a 50-amp circuit. The breaker and the outlet are rated at 75°C. What size conductors are required? Table 310.16 allows 55 amps on an 8 AWG in the 90°C column; however, 110.14(C) limits the ampacity due to the temperature rating of the device. Since the device—both the breaker and the outlet—is only rated for 75°C, Section 110.14(C) would limit the ampacity to the 75°C column of 310.16 due to the temperature rating of the device. An 8 AWG is 50 amperes in the 75°C column. Therefore, an 8 AWG would comply. — John Stacey, CMP-6

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Question 5. I have attached some photographs of NM cable installation in factory-built housing (see photos 1 through 6). In your opinion, does the installation pictured represent bundling (NEC 310.15), therefore requiring ampacity derating? — G. N.

Answer 5. The pictures are not real clear, but it looks like there are at least five two-wire cables and two three-wire cables in the bundle. In the two-wire cable, both wires are current-carrying. The three-wire cable, depending on what it serves, could be two or three current-carrying conductors. The cables are bundled and require derating, according to NEC 310.15(B)(2). If all of the cables are NM-B, you can use the 90-degree column for the ampacity adjustment, there are 14 to 16 current-carrying conductors in this bundle.

Table 310.15(B)(2)(a) requires a 50 percent adjustment for 10–20 current-carrying conductors. Using Table 310.16, the 10 AWG conductors must be on a 20-amp overcurrent device, the 12 AWG conductors must be on a 15-amp overcurrent device, and the 14 AWG conductors can be on a 15-amp overcurrent device.

10 AWG is 40 amps x 50%= 20 amps

12 AWG is 30 amps x 50% = 15 amps

14 AWG is 25 amps x 50% = 12.5 amps rounded up to the next standard overcurrent device = 15 amps. 

— O. P. Post, CMP-6

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Question 6. We are writing to request clarification of Section 310-15(b)(6) [1999 NEC]. We are electrical contractors doing service work in 34 different jurisdictions on the central coast of California. The inspectors in one local jurisdiction have taken this section (per the clarification on page 153 of the IAEI Analysis of the 1999 National Electrical Code) to apply only to the main power feeders (between the main disconnect and the branch-circuit panel board) and not to the service-entrance conductors. We take the section to mean that for single-phase, 120/240-volt, 3-wire residential service, the service-entrance conductors, as well as the service-lateral conductors, as well as the feeder conductors can all be sized according to Table 310-15(b)(6).

By our interpretation, when we install or replace a typical 100-A, single-phase residential service, we will use 4 AWG copper service-entrance conductors. These local inspectors would have us use 3 AWG (which is not generally available, so we have to use 2 AWG). Which interpretation is correct? — V. K.

Answer 6. Section 310-15(b)(6) in NEC- 1999 was previously the text in note 3 to the ampacity tables of 0 to 2000 volts. If a single set of 3-wire, single-phase, service-entrance conductors in a raceway or cable supplies a one-family, two-family, or multifamily dwelling, the reduced conductor size permitted by Section 310-15(b)(6) is applicable to the service-entrance conductors, service-lateral conductors, or any feeder conductors that supply the main power feeder to a dwelling unit. This section permits the main power feeder to a dwelling unit to be sized based on the conductor sizes in table 310-15(b)(6) even if other loads, such as air conditioning units and swimming pools, are fed from the same. The feeder conductors to a dwelling are not required to be larger than its service-entrance conductors. An example of this would be if you installed the service panel outside and from that location fed an air-conditioning unit, then ran a feeder into the dwelling unit to a subpanel, that feeder would be sized from table 310-15(b)(6), the 4 AWG copper would be 100 amps, and 2 AWG aluminum would also be 100 amps.— O. P. Post, CMP-6

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