Does NEC Table 310.15 Apply to SubPanel Feeders?

[Joe592]

Hi, I am trying to figure out if it is acceptable to use wire sizes in NEC Table 310.15 (Single Family Dwelling Services) to go from the Main Panel to a Subpanel, or if you have to go by the standard ampacities in table 310.16.

I have read through the section a few times and although the NEC does not use the term subpanel but it does state that you are not required to upsize feeders (towards bottom of 310.15), and the definition of feeders states that it includes all wires up to their final branch circuit OCPD, so it sounds like it is acceptable but I would really like some clarification as I find the NEC rather confusing to understand, and have received many different answers from different electricians.

Thanks, Joe.

[Jerry Peck - Codeman]

Hi Joe,

The NEC is confusing in that section as it not only uses the term "feeders" but interchangeably uses the term "main power feeders"

(underlining and bold are mine)
- 310.15 Ampacities for Conductors Rated 0–2000 Volts.
- - (B)
- - - (6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

Okay, the "feeders" the above is referring to are *only* the "main power feeders" which run from the service equipment and main service disconnect enclosure to a panel which "supplies ... all loads" "that are part or associated with the dwelling unit". I separated that single quote into two section to highlight the "supplies ... all loads" part.

Let's first take an example where the service equipment is a main service disconnect only, possibly located outside the dwelling (this could be located inside the dwelling, but this setup would be quite rare in a dwelling unit), the feeders from that service equipment to the panel it supplies will carry "all loads" of the dwelling unit. Those "feeders" are the "main power feeders" referred to in the code above. Table 310.15(B)(6) may be used for these "main power feeders". You could also use Table 310.16, but are not required to.

Now let's take an example where the service equipment and main service disconnect is located in an enclosure which also contains a panel, as is typical of installations where the service equipment and panel are located inside the dwelling (but could be located outside the dwelling where a service change included relocating the panel to the outside in retrofits), and then has feeders which supply another panel (which may possibly be the original panel if that panel was not 'suitable for use as service equipment only' which it may likely have been - but that creates other problems). The "feeders" from the service equipment panel to the remote panel are not "main power feeders" and do not carry "all loads" of the dwelling unit. Table 310.16 must be used for these feeders, not Table 310.15(B)(6) as these feeders are not the "main power feeders" and do not carry "all loads".

Using the terms 'subpanel', 'sub panel' or "sub-panel" are pet peeves of mine as there are no such things, unless you are in a submarine and are referring to those panels. The panels being reference are more accurately described simply as "panels", "not service equipment panels", or even "remote panels" to indicate the panel is "remote" from the service equipment. Likewise, "main panel" does not exist either, and while many use that as meaning "the service equipment panel", others use it for the first ("main") panel which comes after the service equipment.

The neutrals are bonded to ground at the service equipment/service equipment panel, the neutral is isolated from ground at all other panels downstream of the service equipment/service equipment panel. Thus it is logical to think of a panel as either a "service equipment" panel or a "not service equipment" panel, and in acknowledging that "not service equipment" panel is rather long and cumbersome to use, "remote" panel works, is descriptive, and is shorter, however, simply using "panel" is even shorter. Thus one could think of, and use, "service equipment panel" and "panel" to differentiate between the two panels. Off that soap box now.

[Joe592]

Thank you Jerry for the quick response that was extremely helpful.


Thanks again, Joe.