Is it permissible to use #8s to feed a NEMA 14–50 outlet?

Is it permissible to use #8s to feed a NEMA 14–50 outlet?

Let’s look at another cost-saving idea. The price from one size wire to the next largest seems to get progressively more ridiculous the bigger it gets. Even using #6s instead of #8s is between 20-50% more if you are talking all copper conductor types, individual and cables assemblies.

But the answer to the question is: generally, yes (and I mean generally, as in Chapters 1, 2, 3, and 4 of the National Electrical Code - the NEC)... If it’s in a raceway with individual copper wires. And assuming the following manufacturer’s specifications:

• The charger plugged into the outlet will draw 32 amps continuously, as defined in the NEC as a load continuously pulled for three hours or more.

Why? While these aren’t the only rules of course, here’s a quick review of the golden rules for a branch circuit, then the details after:

1. The conductors must serve the load and be sized appropriately (NEC 210.19).

2. The conductors, with a couple exceptions, must have an ampacity of the larger of either:

   1. 100% of the noncontinuous plus 125% of the continuous load per 310.14, or;

   2. Equal or greater than the maximum load after adjustments or corrections in 310.15 (NEC 210.19(A)(1)).

3. The overcurrent protective device (breaker) must protect the conductors (NEC 240.4).

4. and be sized appropriately

Selecting the Wiring Method

We’re taught in the trade that if you are installing a new circuit into an existing panel, you should stick to the 75 degree column of NEC Table 310.16. The Table indicates type THHN copper conductors can safely conduct up to 50 amps if all terminations are rated at 75 degrees. The conductor by itself is rated up to 55 amps per the 90 degree column, but we need to check some other code references to see if that’s even an option.

So let’s start from square one - you need to install a NEMA 14-50 outlet at someone’s house. You don’t have specs on the load that will be plugged into it, so you’re kind of flying blind. If you were going to size the circuit based on the load, NEC 625.42 says that an EVSE must be sized as a continuous load. We are not installing EVSE, so it is up to the installer whether they need to install conductors based on 125% of the continuous load or not.

But back to the task: you’re responsible for completing the project and you need to find the most appropriate wiring method. There are only two options in the residential world: 1) cable assembly, or 2) raceway system with conductors installed as a separate step. If you have to take the circuit across a significant distance (in residential terms), say to the other side of the garage, you might end up staying on the surface of the drywall.

Now your options are even more limited. Technically speaking, you would be permitted the same number of options for raceway but only a handful for the cable assembly. Perhaps only one. In the western US, we like to use MC (metal-clad) cable (2020 NEC Article 334), which is a factory assembly of one or more insulated circuit conductors with or without optical fiber members enclosed in an armor of interlocking metal tape, or a smooth or corrugated metallic sheath (2020 NEC 334.2). In other words, it’s a flexible metal tube with wires already inside it. It’s easy to install, but it’s dirty and has some limits to where it’s aesthetically or legally allowed to be installed. Other jurisdictions may allow other cables to be installed in a home anywhere on the surface of a wall, but that’s the most common option.

A more durable option is to install EMT (2020 NEC 358) and select the desired conductor type for your circuit. Small size EMT (½”-1”) is easy to bend with a hand tool and can be installed in a wide range of environments. There is nothing in NEC Article 358 limiting the use of EMT anywhere in a house. An appropriately sized EMT will be a solid choice, provided the topography where you need to install the system is not too challenging.

Selecting the Conductors

Chapter 1 of the NEC is the first chapter because it applies to every other Chapter in the NEC. Among many broadly-applied requirements such as the Equipment Provisions of section 110.14, Chapter 1 contains other important information like guidance for the amount of clearance needed around and in front of electrical equipment, and the requirement to install product according to the manufacturer’s instructions. Section 110.14(C)(1) is titled Equipment Provisions. It discusses the Temperature Limitations (2020 NEC 110.14(C)) of equipment rated 100 amps or less (a) and over 100 amps (b). If you look at the outer jacket of a type THHN copper conductor, it is rated at 90 degrees C. Furthermore, according to Table 310.4(A) of the 2020 NEC, it also says that THWN is rated only at 75 degrees C “Maximum Operating Temperature”. Often suppliers will sell individual copper wire that is type THHN/THWN-2, both of which are rated to 90 degrees.

So, you’ve got that #8 copper wire in your hot little hands because we use it all the time and you want to follow the rabbit to see if it’s allowed in this application. Back to Section 110.14. First sentence of part C reads: “The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device.” It might help to just look at each of those three things separately.

Reviewing the Terminations Temperature Limitations

I could have put parentheses () around the “s” in “Terminations” but then chose not to since a wire has to be terminated on both ends pretty much every single time. So the terminations will always be plural. Which ones do we need to consider? There are two locations where the wire is terminated: one end of the wire goes on a screw terminal on the receptacle (device) and the other end is terminated on a lug, most likely the breaker itself. For the sake of simplicity, we will assume the wire goes directly from the circuit breaker to the device and no splices are made.

Temperature Limitations of a Residential Breaker Panel/Circuit Breaker

The label from the manufacturer typically limits the use of 90 degree conductors. It might say “use 60 or 75 degree copper or aluminum conductors”. It is, of course, okay to use wires that are rated for a more extreme (hotter) environment in this case. The label therefore is referring to the ampacity of the conductors as determined in the NEC. Also, on the breaker itself is a tiny stamp that lists information such as recommended torque rating and the temperature limitation. It will typically say 60/75 degrees C.

Reading further in Section 110.14(C)(1) of the NEC, under subheading (a)(3), we’re allowed to use provisions for equipment rated 100 amps or less, or 14 through 1 AWG: “conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.” Since we are using a 90 degree rated conductor (THHN), but terminating it on a product rated up to 75 degrees, we have to use the 75 degree column of NEC Table 310.16.

Temperature Limitations of a Receptacle Device

The NEMA 14-50 receptacle has 4 screw terminations: a ground, a neutral, and two hots. The two hots are what we’re most concerned about, although all terminations are important, and all must follow the NEC.

(Note on authenticity - this article is AI-free. I am the only contributor, if you don't count links.)