Electrical – How much extra wire should I have in the electrical panel

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Question:

What are some rules of thumb for leaving extra wire within an electrical panel (if any) and routing the extra wire (e.g. create a single loop from the extra wire) to maintain balance between considering potential future wiring needs and also keeping the panel from turning into a complete rats' nest. I am looking for recommendations on additional wiring methods for the wire that feeds the panel as well as the wire that feeds the branch circuits.

Also, with the answers, I am looking for additional reasons why you might need to have extra wire in a circuit panel. For example, I can see leaving some extra wire in the electrical panel for the branch circuits so that you have potential to rework the positions of the circuit breakers in the future without having to run new wire. The only reason I can think of for having extra wire for the main lines feeding the panel is in case I wish to swap out for a different panel in the future which would require the main lugs/breaker in a different spot.

Specifics for My Situation:

I have one 200 A main panel (3/0 AWG wire feeding the panel, spaces for 40 breakers) feeding two other 100 A subpanels (2 AWG feeding these panels) which then feed branch circuits for receptacles (6 AWG to 12 AWG wire feed these circuits). The 200 A panel has spaces for 40 single breakers (~40 inches tall) and the 100 A panels have spaces for 30 single breakers (~30 inches tall) — I presently plan to have lots of unused breaker space in each panel. I mention this because I figure the size of the panel might also influence the amount of extra wire desired (e.g. in the future, you might want to move a circuit from the top of the panel to the bottom) and the amount of extra wire that the box can accommodate (e.g. if all 40 breakers are used, it doesn't seem like there would be a clean way to leave much extra wire at all).

Best Answer

Leave just enough "extra" wire to route the wire along the edge of the board so it looks clean like the image below. Otherwise, do not bother leaving extra wire. You can splice more wire within the panel if you need to move things around later. Leaving a lot of additional coiled wire to try to account for the future could result in excess heat build-up and potentially an electrical fire. Note: Some excess wire is harmless, but if the panel begins to look like a mess of coiled wires, then you are likely creating the following situations the NEC tries to protect against in several areas of code (though not necessarily specific to the panels) to make sure the wire will not exceed the heat rating of its insulation at the expected current load:

  • Current running through a wire generates heat and relies on air to carry that heat away from the conductor. Therefore, the NEC protects against too many current-carrying conductors in close proximity for other areas of wiring:

    310.15(B)(3)(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.

    314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies. Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as calculated in 314.16(B). The minimum volume for conduit bodies shall be as calculated in 314.16(C).

  • When wire is coiled or there are more than one conductor, there is potential to induce additional currents in the wires due to the additional electromagnetic fields not otherwise present with a single conductor. The NEC protects against this related to ferrous metal enclosures and raceways:

    300.20(A) Conductors Grouped Together. Where conductors carrying alternating current are installed in ferrous metal enclosures or ferrous metal raceways, they shall be arranged so as to avoid heating the surrounding ferrous metal by induction. To accomplish this, all phase conductors and, where used, the grounded conductor and all equipment grounding conductors shall be grouped together.

    Exception No. 1: Equipment grounding conductors for certain existing installations shall be permitted to be installed separate from their associated circuit conductors where run in accordance with the provisions of 250.130(C).

    Exception No. 2: A single conductor shall be permitted to be installed in a ferromagnetic enclosure and used for skineffect heating in accordance with the provisions of 426.42 and 427.47.

enter image description here Image obtained from https://en.wikipedia.org/wiki/File:EEUU%28GE%29BreakerpanelInnards.jpg

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