Code Change Summary: The exceptions for feeder sizing were reorganized and a new exception was added.

When sizing an ungrounded feeder conductor, section 215.2(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 2017 NEC^®, this remains unchanged. What has changed is the location of the exceptions. They have been relocated to just behind 215.2(A)(1)(a) to make it clear that they do not apply to 215.2(A)(1)(b) at all.

More importantly, new exception 2 was added. This new exception requires a very careful read in order to understand it. The new exception allows a portion of a feeder 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 310.15(B)(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 feeder conductors installed between 90°C rated power distribution blocks can be used at its maximum ampacity based on 90°C.

This principle would allow a pull box to be installed at each end of a feeder just before the conductors enter the overcurrent device enclosure or equipment. If 90°C rated power distribution blocks are used at each end of the feeder 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 310.15(B)(16)) can be used at its full value of 130 amps but only for the portion of the feeder 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 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 states that if this method is used, the smaller, middle run of feeder is not permitted to extend beyond the pull boxes into the associated equipment (see image).

Below is a preview of Article 215. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.