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I’m planning to add a new dedicated 240v/20A circuit for a window AC unit. The route from the main electrical box to the outlet will require about 65-70 ft of wire. The wire will also travel about 10-12 ft through conduit on the exterior to enter the breaker box. Since we’re planning to run the AC at times for several hours consecutively, I believe this qualifies as a “continuous load”. Given the length, conduit and continuous load, should I use a 10 gauge conductor for this circuit? Or will 12 gauge suffice?

The current AC unit is rated for 15 amps, but we may use something bigger in the future. Will 10 gauge wire pose any problems running the current 15A device?

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    If you looked once look again, I edited my response to include the edits in your question.
    – KMJ
    Commented Jun 17 at 16:03
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    I'm curious what kind of upgrade you are preparing for? Is there a specific kind of unit you have in mind? I've never seen a 30A window unit.
    – jay613
    Commented Jun 17 at 16:56
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    If you install a 20A circuit you are not restricted to 15A, you're restricted to 20A.
    – KMJ
    Commented Jun 17 at 17:28
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    I asked because I'm not familiar with any window A/C requiring > 20A circuit. #10 wire would be only for length, and at around 80 feet ish you don't need it for that either. I don't think there is a practical upgrade scenario you need to worry about. I also don't think you need >4kW A/C for any one room. If you don't think the current unit will meet your needs you should be thinking ahead to mini splits or central. That means spending as little as possible on the current install, not overspending for upgrades.
    – jay613
    Commented Jun 17 at 17:35
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    Please take the tour. You haven't resolved any of your previous questions.
    – isherwood
    Commented Jun 17 at 19:12

2 Answers 2

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Good news - it's not a continuous load. From the 2023 NEC (highlight is mine):

Continuous Load: A load where the maximum current is expected to continue for 3 hours or more.

First off, it's a 20A circuit and a 15A A/C. On top of that, the A/C will use more current at startup than while it is running, so even if you run it 24x7 it doesn't count as a continuous load. The only reasons to upsize from #12 to #10 are derating conditions or length. Unless you have conditions in the conduit which will require you to derate the wire, you can use #12 and be totally fine in this situation.

If you want to have the capacity to turn it in to a 30A circuit in the future, and you don't mind the additional cost of wire, feel free to upside the conductors. Just keep an eye on the devices you use with the larger conductors. Not all receptacles will accept all wire sizes, so you might have to pigtail with smaller wires.

Also: running NM/B through conduit is a bear. Not the end of the world, just keep it in mind when you're planning the path.

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  • Thanks for your feedback. I’m curious to know what conduit conditions would force you to derate the wire?
    – Justin
    Commented Jun 17 at 16:09
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    Common ones are continuous loads like EV charging or pool pumps, temperatures higher than 86F - see table 310.15(B)(1) - or multiple current carrying conductors - see table 310.15(C)(1). However you almost never need to think about these with a single 15A or 20A circuit, as the wire already has ampacity to spare. Even in the 60C column you get 25A on a #12 wire, so most of the time you can absorb the derate without increasing the wire size.
    – KMJ
    Commented Jun 17 at 16:22
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    Absorbing those derates in common installations is part of the reason behind the small conductor rule 240.4(D), so far as I know. It means you don't risk problems installing a rooftop A/C unit where the conduit can get very hot, or when you have a bunch of NM-B going through a single hole in the framing.
    – KMJ
    Commented Jun 17 at 16:25
  • Thanks again. This conduit will almost always be in shade (north side of the building). But it gets very hot here in the summer, with regular temperatures above 95, and multi-day triple digit heat waves. It looks like the heat could be a factor, based on the table, but I don’t quite understand how to adjust for it.
    – Justin
    Commented Jun 17 at 16:47
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    Unless it's on a roof or you expect it to get hotter than 122F ambient you can still use #12 NM-B in your conduit for a 20A circuit. If you live somewhere where it is expected to get over 122F like Death Valley then you should size up to #10. Otherwise even in Phoenix you'll be fine.
    – KMJ
    Commented Jun 17 at 17:27
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There are several websites that have calculators for voltage drop caused by length, most electricians recognize that under 100' the minimum size allowed by Code has already been accounted for. You can use larger size, but may have issues terminating oversized wire without additional splices that provide additional failure points.

Your biggest concern for proper circuit size is to comply with the instructions that come with the air conditioner as instructed in NEC 110.3(B)/110.3(C).

110.3(B) Installation and Use. Equipment that is listed, labeled, or both, or identified for a use shall be installed and used in accordance with any instructions included in the listing, labeling, or identification.

Informational Note: The installation and use instructions may be provided in the form of printed material, quick response (QR) code, or the address on the internet where users can download the required instructions.

(C) Listing. Product testing, evaluation, and listing (product certification) shall be performed by recognized qualified electrical testing laboratories and shall be in accordance with applicable product standards recognized as achieving equivalent and effective safety for equipment installed to comply with this Code.

Informational Note: The Occupational Safety and Health Administration (OSHA) recognizes qualified electrical testing laboratories that perform evaluations, testing, and certification of certain products to ensure that they meet the requirements of both the construction and general industry OSHA electrical standards. If the listing (product certification) is done under a qualified electrical testing laboratory program, this listing mark signifies that the tested and certified product complies with the requirements of one or more appropriate product safety test standards.

(Informational notes are not considered directly enforceable, but direct you to related enforceable other codes or regulations)

If the unit needs continuous rating or unusual startup considerations are necessary the testing lab (UL/CSA/ETL) will have identified that and it will be addressed in the instructions.

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  • Thanks for your response. Based on all the feedback, it sounds like 12 gauge will be sufficient. Thank you all
    – Justin
    Commented Jun 18 at 2:26

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