310.15(B)(2)(a) Exception 5

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iwire

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Then when could you ever use the Ex, 310.15(B)(2)(a) ? It is simply not applicable then.

310.15(B)(2)(a) Exception 5 allows us to stack up to 20 current carrying conductors together under certain conditions without any derating at all.

The 60% derating only applies after we have more then 20 current carrying conductors.

But either way the normal rules of Article 240 apply. :)
 

e57

Senior Member
This is an example from the '02 handbook - and has always been my understanding of it.... Coincidentally it meshes with this topic rather well....

Example
A commercial office space will require fourteen 277-volt fluorescent lighting circuits to serve a single open office area. The office area lighting is assumed to be a continuous load, and the office ambient temperature will not exceed 30?C (86?F). Each circuit will be arranged so that it has a computed load not exceeding 16 amperes. The selected wiring method is Type MC cable, 3-conductor (with an additional equipment grounding conductor), 12 AWG THHN copper. Each individual MC cable will contain a 3-wire multiwire branch circuit. To serve the entire area, this arrangement will require a total of seven Type MC cables bundled for a distance of about 25 ft, without maintaining spacing between them where they leave the electrical room and enter the office area.
Determine the ampacity of each circuit conductor in accordance with 310.15, applying Exception No. 5 to 310.15(B)(2)(a) to account for the bundled cables. Then determine the maximum branch-circuit overcurrent protection permitted for these bundled MC cables.
Solution
Step 1.
To apply Exception No. 5, first determine the quantity of current-carrying conductors. According to 310.15(B)(5), equipment grounding conductors are not counted as current-carrying conductors. According to 310.15(B)(4)(c), fluorescent lighting is considered a nonlinear load, so the grounded conductor of each Type MC cable must be counted as a current-carrying conductor.

7 cables x 3 cables each = 21 conductors

Because the quantity of current-carrying conductors exceeds 20, a 60 percent adjustment factor is required by 310.15(B)(2)(a), Exception No. 5.
Step 2.
Determine the ampacity of each current-carrying conductor due to these MC cables with more than 20 current-carrying conductors being bundled.

From Table 310.16 12 AWG THHN = 30 amps
30 amps X 0.06 = 18 amps

Because the actual computed load is 16 amperes of continuous load, 210.19(A)(1) is applicable. The conductors must have an ampacity equal to or greater than the load before the adjustment factor is applied. Because the ampacity of the conductors after the adjustment factor is applied is 18 amperes, no further adjustment is necessary and the conductors are suitable for this installation.
Step 3.
Finally, determine the maximum size of overcurrent protection device permitted for these bundled MC cable branch circuits. Section 240.4(B) permits the use of the next higher standard rating of overcurrent protection device. Therefore, although the conductors have a computed ampacity of 18 amperes, a 20-ampere overcurrent protective device is permitted. In addition, and of significance, the 20-ampere overcurrent protective device is in compliance with 210.20(A), given that the actual 16-ampere continuous load would require a 20-ampere overcurrent protective device, based on the listing of the overcurrent device.
 

iwire

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Massachusetts
Step 3.
Finally, determine the maximum size of overcurrent protection device permitted for these bundled MC cable branch circuits. Section 240.4(B) permits the use of the next higher standard rating of overcurrent protection device. Therefore, although the conductors have a computed ampacity of 18 amperes, a 20-ampere overcurrent protective device is permitted. In addition, and of significance, the 20-ampere overcurrent protective device is in compliance with 210.20(A), given that the actual 16-ampere continuous load would require a 20-ampere overcurrent protective device, based on the listing of the overcurrent device.

Section 240.4(B) permits the use of the next higher standard rating of overcurrent protection device.

Yes, 240.4(B) will allow that for the handbook example as it is lighting circuits.

But if the circuit supplies multiple recptacle outlets 240.4(B) can not be used.
 

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
310.15(B)(2)(a) Exception 5 allows us to stack up to 20 current carrying conductors together under certain conditions without any derating at all.

The 60% derating only applies after we have more then 20 current carrying conductors.

But either way the normal rules of Article 240 apply. :)


That was my point. When you do reach the 60% threshold then your #12 conductors are limited to an ampacity of 18 amps. Depending on the loads you may be able to go up to the next standard size which would be 20 amps. This almost makes derating of huge bundles #12 AC and MC cables non existent, except when those cables are feeding multi-receptacle circuits.

For example:

For (30) bundled 2-wire #12 MC cables feeding lighting loads I would have 60 CCC's. When applying the 60% derating I would have 18 amps as an adjusted ampacity on each cable. 240.4(B)(1) allows the OCPD to be the next standard size which is 20 amps. Change the word lighting to receptacles and you're stuck with a 15 amp OCPD since all of the conditions of 240.4(B) are not met.
 

e57

Senior Member
Section 240.4(B) permits the use of the next higher standard rating of overcurrent protection device.

Yes, 240.4(B) will allow that for the handbook example as it is lighting circuits.

But if the circuit supplies multiple recptacle outlets 240.4(B) can not be used.
True that - yes there are conditions.

(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.
(2) The ampacity of the 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).
(3) The next higher standard rating selected does not exceed 800 amperes.
 
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