Is it helpful to use a thicker gauge wire for only part of a long circuit run that could have higher loads?

Question

A new circuit is going in to provide 20A power to a residential garage, mainly for power tools, but also for the garage door motor. All of that and a couple of overhead lights are currently drawing off a 15A circuit that powers various other rooms in this house (a DIY'd home from 1960, where circuits are based more on efficient wire paths than separating uses).

This new circuit will be pretty long, 75-100 ft, going from one end of this ranch house with the panel to the opposite end where the semi-attached garage is. The goal is to have a safer circuit for higher loads and reduce/separate those higher loads from the garage lighting circuit and other stuff on the 15A circuit around the house.

Given the new circuit is almost 100ft long and could be used with high loads (power tools, battery chargers, trickle charging EVs in the future), it made sense to me that the 50 ft stretch of cable from the panel to garage could be a thicker gauge to reduce resistance / voltage drop / heat buildup on wires. For example, running 10/2 from the panel to the edge of the basement wall (where the garage begins), then splicing in 12/2 to go from basement to garage, then wrap 12/2 in conduit around garage for 20A garage receptacles. A local hardware store had a nice sale, so I went ahead and got 50 ft of 10/2 for this reason.

An electrician said that using 10/2 is technically fine, but it's unnecessary / overkill. Is using 10/2 actually helpful in terms of improving circuit safety and performance, or would it contribute nothing (compared to 12/2)? I realize it's helpful to minimize splices for safety (possible loose connections) and voltage drop reasons, but going from basement to garage would likely call for a junction anyway, going from NM along the basement ceiling edge, to conduit in the garage.

Answer 1

It's madness to add electrical circuits to a garage without thinking about EV charging. Because that is a real thing that IS coming. I realize there were EV attempts in the 90s and 00s that went nowhere, and a person might say "well it died the last 2 times". Nope, it's the real deal this time. The X-factors are a) battery tech has arrived, and b) the rest of the world is all-in. America will not be an ICE-only bastion, and enough homebuyers will want to see EV charging capability that it will hurt your resale not to have it.

Your idea of stepping up to #10 isn't wrong, since about 75' will put you over 3% on a 120V circuit simply due to the lower voltage. I would carry it the whole run rather than have an intermediate splice. I would also run 10/3+gnd as a MWBC. But that's besides the point, for wire cost reasons I would do no such thing at all! I would run the very affordable 2-2-2-4 aluminum (safe at these large sizes) as a feeder to a subpanel. Subpanels have terminals rated for aluminum (made of aluminum) and #2 provides 90A to the garage, which is all you'll ever need for EV charging multiple cars. Don't listen to those novices who say you need 48A per car, that's nuts.

This fat wire will completely remove any issues with voltage drop.

Then you install your circuit(s) out of the subpanel.

Now I recommend conduit the entire run, and I recommend at least 1-1/4" preferably 1-1/2". That's to hedge your bets on emerging tech called V2X - Vehicle to Home during outages, Vehicle to Grid if you want to buy up cheap midnight power and flip it back to the utility during peak hours for much more money. The conduit will also provide a route/way for data cables if you need a Dynamic Load Management system to fit EV charging inside the existing home panel capacity.

Answer 2

For a 100' run, the voltage drop will be negligible no matter the wire size - I'd have done it with one piece, but that would be just my preference. 10/2 would let you go up to 30A of current, but you're still limited to the minimum capacity of the circuit overall, so if you have any 12 or 14 gauge wire, that's the max you can have for your breaker (20/15A, depending on minimum gauge). 10/2 won't really get you anything above what 12/2 would do for standard outlets since you can't have standard 120V outlets on anything more than a 20A circuit anyway.

If it were me, I'd have, at least, run it as a /3 wire which would have given a lot more options for 240V expansion in the future, even if not currently set up that way. Trying to save a bit of money now on relatively cheap wire may come back to bite you later if you could have planned for future additional needs rather than just what you currently anticipate needing, especially if you're thinking of being able to charge an EV in some manner.

Answer 3

The #10 wire will give you nothing. On the first fifty feet at full 20A load you'll see a 1.8% voltage drop instead of 2.8% with #12. Plus whatever you lose on the rest of the run ... it's nothing.

It's also harder to pull when installing and you'll have a useless junction box just inside the house.

Judging by prices at a large orange retailer today, that #10 wire is a HUGE waste of money. I hope you bought a whole 50ft package and can return it! I agree with @Milwrdfan, run 12/3 and put in a pair of handle-tied breakers. This will be a LOT more useful.

Thinking about capacity: "power tools and the door motor". Most large power tools use 12 or 13A. They are designed that way in order to be powerful while functioning on a 15A breaker. A new DC garage motor uses about 4A, an older one 10 to 12. So ... if you only have a new DC door motor and a modest tool collection, one circuit will be fine. If you have an old door motor, you're already overloaded, even with a new 20A circuit. If you add ANYTHING else ... a heater (so you can work in the garage in winter), a large standup/box fan (so you can work in the garage in summer or evacuate toxic fumes from welding/grinding/painting), a vacuum, any tool with a motor that will cause a large startup current surge, a small slow EV charger, a new set of fast chargers for a new brand of power tools, or if you have two people using two tools at once .... 20A is not enough.

Now is your chance. Run a 12/3. Put the door and most outlets on one circuit, and put 2 or 3 clearly marked outlets on the other one for high-powered equipment. Where the two circuits diverge you'll have 240V@20A if you ever need that.

Answer 4

Starting the circuit run from the panel with the 10 AWG wire could be misconstrued in the future as being a circuit capable of handling the current capacity of 10 AWG. It would be very important to mark such circuit that reduced down to 12 AWG somewhere downstream as limited to the capacity of that smaller diameter wire. This would be best done in the panel at the start of the circuit and make sure that the proper 20A circuit breaker is used.

Answer 5

While in true voltage drop situations (> 200 feet), using larger wire for most of the run and smaller wire for the ends can help a lot, with a short run voltage drop just isn't a real concern. If voltage drop isn't a concern then neither is excess heating as long as you stick to the rated minimums - e.g., 12 AWG for 20A, 10 AWG for 30A.

So really the question becomes: How do I plan for future use by spending a little bit now to save a lot of time/money/hassle later?

There are a few options. A key is that most options require multiple circuits, which means at a minimum a multi-wire branch circuit and very possibly a true subpanel. Either of those requires 3 wires (plus ground). 12/3 will get you 2 x 20A 120V circuits or 1 x 20A 240V circuit. 10/3 will get you 2 x 30A (or downgrade to 20A, that's OK) 120V circuits or 1 x 30A 240V circuit.

But to really get more you need a subpanel. If you go up just one more size to 8 AWG 3-wires then you get 40A @ 240V. Which by using a subpanel gives you 2 x 20A x 120V (one for tools, one for lights and garage door opener, or whatever) and 1 x 20A x 240V which gives you reasonable baseline EV charging.

And once you are in subpanel mode, if you increase wire size and switch to aluminum then you can do a lot more, dependent primarily on how much power you can get from your main panel, which depends on your utility feed and a proper Load Calculation. The typical recommended cable is 2/2/2/4 aluminum - that's 2 AWG for hots and neutral, 4 AWG for ground. That actually compares well in cost (but varies of course by vendor) with 8/3 copper (the /3 really including a 4th wire as ground as well). Which means that for around the same cost as a 40A max. subpanel feed you can actually set things up for 90A, with the actual capacity depending on the amount you can feed from your main panel based on a Load Calculation.

The other alternative is to run conduit large enough for larger and/or more wires in the future. I suspect in this case that simply running cable will be easier, so run cable that has room for the future.

A subpanel costs very little. In fact, a "main panel", but configured as a subpanel (ground and neutral separate is the main thing) doesn't cost much. A quick search finds an Eaton BR 100A panel with some extra breakers included, 20 spaces/"40 circuits", for less than $100. A subpanel in the same building doesn't need a disconnect, but it can be handy, and a "main" panel main breaker works as a disconnect when used as a subpanel but can be larger than the panel feed (so 100A breaker in subpanel with a 30A or 90A or anything in between feed is just fine).

Answer 6

You haven't said where you are (US or Canada), so I'll answer you both ways.

Under the National Electrical Code (NEC) in the USA (NFPA 70) and the Canadian Electrical Code (CEC) in Canada (CSA C22.1), 120 V 15 A circuits are not permitted to have a voltage drop of more than 5% when under full load. A general rule is that a circuit should never be intentionally loaded to over 80% of the nominal full load. So:

15 A x 0.8 = 12 A

120 V x 0.05 = 6 V drop = 114 V minimum on the circuit at 12 A

120 V x 12 A = 1440 VA 1440/114 = 12.6 A, rounding up to 13 A (still within the nominal rating of the breaker.)

120 V/12 A = 10 Ω circuit impedance (load + conductor resistance)

114 V/13 A = 8.8 Ω circuit impedance (load + conductor resistance)

14 AWG wire has 2.58 Ω/km at 77 ºF (25 ºC) and 2.97 Ω/km at 149 ºF (65 ºC), which matters as the hottest a wire should ever run is in that range. NMD 7 cable (the typical 14/2 or 14/3 cable used in home construction) has a temperature rating of 90 °C. If you compare the two resistances, you will note that copper wire has a positive temperature coefficient of resistance, meaning that as the temperature increases, so does the resistance.

2.97 Ω/km = 0.000905255971 Ω/foot, so 100' of 14/2 cable is going to have 2 x (0.000905255971 x 100) = 0.0905255971 Ω.

0.0905255971 Ω x 15 A (nominal max) = 1.4 V drop which is << 6 V drop allowed. At 77 ºF (25 ºC), the drop will be less, of course.

[1] https://www.engineeringtoolbox.com/copper-wire-d_1429.html