Nec Rule Continuous and Noncontinuous Loads
Click to Enlarge Installing 90°C rated power distribution blocks and a gutter at each end of a long run of branch circuit conductors just before terminating them in the panelboard or load equipment at each end can create an allowance where smaller size 90°C rated wire can be run between gutter boxes.
Code Change Summary: Revised code language and a new exception to align with a similar exception permitted for feeders.
When sizing an ungrounded branch circuit conductor, section 210.19(A)(1) allows the larger of the following two values to be used for the final selection of the conductor:
- Either 125% of the continuous load without any additional adjustment or correction factors,
OR
- 100% of the load (not 125% of it) after applying adjustment and correction factors.
In the 2020 NEC, this remains unchanged. What has changed is the addition of new exception 2. This new exception requires a very careful read in order to understand it. The new exception allows a portion of a branch circuit connected between pressure connectors (such as power distribution blocks) complying with 110.14(C)(2) to be sized based on the continuous load plus the noncontinuous load rather than 125% of the continuous load plus the noncontinuous load.
The idea is this: Most equipment and circuit breaker terminals are rated at 75°C. Even if a conductor is rated at 90°C, it must be considered as a 75°C conductor if terminating to a 75°C terminal. This means that a 1 AWG copper type XHHW-2 conductor (rated for 145 amps in the 90°C column of Table 310.16) is really only rated for 130 amps (from the 75°C column) if terminating on 75°C rated terminals at each end (see illustration).
Section 110.14(C)(2) covers separately installed pressure connectors such as power distribution blocks and recognizes the fact that many of them differ from circuit breaker terminals in that they are often rated higher than 75°C.
According to the new exception, a run of 90°C rated branch circuit conductors installed between 90°C rated power distribution blocks can be used at the maximum ampacity based on 90°C.
This principle would allow a pull box to be installed at each end of a branch circuit just before the conductors enter the overcurrent device enclosure or load equipment. If 90°C rated power distribution blocks are used at each end of the branch circuit inside each pull box, then technically, a smaller, less expensive conductor such as a 2 AWG copper type XHHW-2 (rated for 130 amps in the 90°C column of Table 310.16) can be used at its full value of 130 amps but only for the portion of the branch circuit between the 90°C rated pressure connectors or power distribution blocks.
From the power distribution blocks down to the termination of the overcurrent device or load equipment, a larger conductor would be used after it is sized based on 125% of the continuous load or 100% of the continuous load after applying correction factors based on conditions of use (whichever is larger). This is because it will be directly connected to either the overcurrent device or the equipment where the load is applied. In most cases, these will each have 75°C rated terminals.
The weakest point along a 90°C rated conductor is where it connects to the 75°C rated terminal. The new exception recognizes the fact that the middle of a conductor run between 90°C rated terminals can be sized smaller than the ends which land on a lesser rated terminal.
The last sentence of exception 2 basically states that if this method is used, the smaller, middle run of branch circuit is not permitted to extend into an enclosure containing either the branch-circuit supply or the branch-circuit load terminations (see image).
Below is a preview of the NEC. See the actual NEC text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC edition of NFPA 70.
2017 Code Language:
210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Conductors shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b).
(a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
(b) The minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.
Exception: If the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
2020 Code Language:
210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the larger of 210.19(A)(1)(a) or (A)(1)(b) and comply with 110.14(C) for equipment terminations.
(a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load in accordance with 310.14.
(b) The minimum branch-circuit conductor size shall have an ampacity not less than the maximum load to be served after the application of any adjustment or correction factors in accordance with 310.15.
Exception No. 1 to (1)(a): If the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load in accordance with 110.14(C).
Exception No. 2 to (1)(a) and (1)(b): Where a portion of a branch circuit is connected at both its supply and load ends to separately installed pressure connections as covered in 110.14(C)(2), it shall be permitted to have an allowable ampacity, in accordance with 310.15, not less than the sum of the continuous load plus the noncontinuous load. No portion of a branch circuit installed under this exception shall extend into an enclosure containing either the branch-circuit supply or the branch-circuit load terminations.
Source: https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=818.0
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