Don't use a $5 range outlet for EV charging
nerdralph.blogspot.comI hope I'm not too jaded, but this reads really weird to me. The author disses a "$5 range outlet" (even though I don't think they're usually that cheap), then talks about spending several hundred bucks on better outlets for "evaluation"... and the evaluation never comes. We just get a paragraph of innuendo without any real-world testing or even a thorough teardown.
Do the plugs have any real-world design flaws, or is this just gear snobbery? Can the author show that they're heating up too much? Given that there must be hundreds of thousands of home chargers, what's the incidence of problems caused by cheap 240 V plugs?
> Do the plugs have any real-world design flaws, or is this just gear snobbery?
Design flaws? No. Limitations, yes.
A $5 NEMA 14-50R is designed for maybe a few dozen to a few hundred insertion cycles. They’re typically used for ranges (and other fixed equipment) that are plugged in once, and then unplugged once more when you replace the appliance. When the contacts wear out, gaps appear and arcs occur inside the device and it melts. Internal shorts can also happen inside the device. Here’s a tear down video of a residential NEMA 5-15R and a commercial NEMA 5-15R that illustrates some of the shortcomings of cheap wiring devices and shows how more expensive devices have a more robust design: https://youtu.be/kX6xnOksQTc
It could also be caused by not using a torque screwdriver to tighten the terminal screws, which is required by the NEC. You must tighten the terminal screws to the torque value listed in the wiring device instructions, and hold it there for 5 seconds. A loose termination can cause all sorts of nasty problems.
There are many grades of wiring devices meant for different applications. More $$ = more durable, better contacts, a full brass strap inside the device, etc.
Most people are unaware that multiple grades of wiring devices exist, and also unaware that it can be very important to use the correct one and also install it correctly which requires a torque screwdriver. This is why it’s a good idea to just pay an electrician to do it correctly.
Leviton sells a heavy duty NEMA 14-50R specifically designed for plug in EV chargers that are designed to handle many insertion cycles. It costs about $67, but that’s the wiring device you buy if you’re going to be doing a lot of insertion cycles.
https://store.leviton.com/products/50-amp-ev-charging-recept...
There’s no such thing as a NEMA 14-50R that isn’t rated for a 50A continuous load. It wouldn’t be UL listed if that was the case.
Premise challenge: I’ve plugged my EV charger in… three times, maybe? The high cycle part is the connection to the car. The connection to the outlet stays plugged in for literally years at a time.
Probably some people have abnormal use patterns that have them moving a 220v charger around all the time, but I’d bet it’s highly unusual amongst EV owners who charge at home regularly.
It's not hard to find pictures online of melted NEMA 14-50 outlets. Design flaw or loose connection? Some will definitely be the latter, but all? Who knows? I'd spend the extra $50.
Or set your car to only pull 24A from the outlet rather than full speed. Still more than fast enough to full charge overnight.
This blog seems to be one person's passion project. They can write whatever they want to in whatever manner they want to.
They're not required to weather your scrutiny or to pass your bar exam to be allowed to post whatever they want.
Aside from that, you raise good points, but you can very well ask your questions directly to the author using the comment section of their blog, you're somewhat more likely to get a satisfying answer there than here.
I need to see evidence of this "overheating and melting the outlet". Electrical code requires downrating of actual capacity to prevent things like this.
You're far more likely to see damage due to loose connections - either the wires are not fully secured to the outlet, or the plug is not fully seated & snug, both of which cause overheating.
Read any EV forum. Plenty of people with melted Leviton 14-50 outlets, despite having fully secure and tightened wires.
Fully secured and tightened isn’t good enough. Per code, you need to use a torque screwdriver (or other approved means) to tighten the terminal screws to the torque value listed in the wiring device instructions.
> NEC 110.14 (D)
Tightening torque values for terminal connections shall be as indicated on equipment or in installation instructions provided by the manufacturer. An approved means shall be used to achieve the indicated torque value.
I can assure you this often has nothing to do with torque. The Leviton outlet lists a torque of 40 inch pounds. You can hand tighten much tighter than that relatively easy with a reasonable screw driver.
There’s some instances of undertorqued outlets, sure, but that isn’t the predominant factor here. The crap outlets are the issue.
True, you can easily hand tighten to 40 inch pounds.
After looking at some more evidence, I agree that it’s probably just a poorly designed receptacle, probably on par with a residential 5-15R which are made to a much lower standard than a comparable commercial 5-15R.
It looks like Leviton may have discontinued the receptacle that lots of people had melt on them so that probably is the case.
The 279 outlet is just replaced by the 279-s00. I don’t think there’s much meaningful difference for Leviton outlets. It’s just similar to residential vs commercial ad you state.
I worked at an EV charger manufacturer. This is a known issue. The issue is a 240V plug value engineered for non-continuous use for things like cooking and laundry loads (where amperage spikes but levels off at fairly low amps) versus a continuous 7-10 hour load like an EV/EVSE (where you have high amps continuously and lots of resulting heat transfer).
It's limited to home EV charging. This is one reason why given the choice between a NEMA plug source or a hard wired source (to circuit in Junction box), I would choose the wired source/circuit connection.
The plug manufacturers also know about the melted NEMA connector issue and are designing versions for continuous use at the higher price points mentioned.
Does this distinction also apply at the lower amps of ordinary 12V plugs? I frequently trickle-charge my EV from an ordinary 12V outlet outside on the porch for 14 hours or more, is that going to be a problem?
(EDIT: sorry, yes, I meant 120V outlet)
I assume you mean 120v outlet, but IMO yes.
One good test that I saw somewhere that I do on new outlets I am charging on, is leave the car charging for ~30 minutes and then quickly unplug the charger from the wall. Touch the prongs to your hands. If they are very warm or hot, you are likely over-drawing that outlet and need to ramp down your amperage draw (or better yet, find a new outlet).
This article focuses on outlets but the wiring is just as (or more) important and outlets can very easily be installed with under-code wiring.
Yes - likely moreso. (I'm assuming you meant 120v plugs, like US residential.)
The issue in the circuit ampacity for standard residential outlets - typically, a 120v 15 amp plug will be wired in 14 gauge NM-B wire (romex), which already de-rates the maximum temperature the wire jacketing is allowed to reach over higher ampacity wire (THHN) which is typically run in conduit. For a given wire gauge, THHN has more ampacity and the jacket is rated to a higher temperature.
for a 20amp 120v circuit (not common unless you install intentionally for that purpose) you need 12 gauge wire.
But, as mentioned in the article, continuous loads need to be up-sized because of temperature, and even standard outlets for the correct ampacity circuit don't often anticipate 100% duty cycle.
BUT, the real question is for your trickle charger, how many amps does it draw? if it tries to draw 12 amps on a 15amp circuit, everything is fine. If it tries to draw 15-20 amps on a 15 amp circuit, there is some danger. Many breakers (esp older ones) will not actually pop if the amp demand ramps up slowly.
Ampacity Charts: https://www.cerrowire.com/products/resources/tables-calculat...
Source: I've wired a LOT of power tools with big loads, but I am not an electrician. Take this advise as indicative and decide whether it warrants you contacting an electrician to validate your situation.
Car chargers only pull 12A from a 15A circuit.
Yeah - you'll see that I mentioned that - if that is true, then great. But, IMO, it's worth verifying. Even more worth double checking if the building is old, or an extension cord is involved.
It's definitely true on a Tesla using their mobile charger. The car will report on its screen exactly what it's charging at. When I charged at 120v (before getting a hardwire charger installed), the car reported charging at 12 amps.
Yes, the one time I used an extension cord to charge the car, I set the max amperage to 8 even though it was a 15Amp rated cord.
> Even though the 14-50R outlet is technically a 50A outlet, when used as a kitchen range plug, they are often wired on a 40A circuit. This is permitted by the Canadian Electrical Code rule 26-744 5).
I wonder if the US National Electric Code allows this.
The “50” in 14-50R refers to the maximum "rated" amperage allowed through the receptacle. Load-wise, to avoid melted connector, you don't plug something with load capacity of more than 37-ish ampere thru this 14-50R receptacle.
The OP is confusing load rating vs. capacity rating.
However, computed loads are always adequately under the breaker size, and beneath wire capacity, and below the receptacle rating, all three and each components of a circuit.
Many urban building codes often demand the 14-50R receptacle to match the 6/3 COPPER (not Romex) wire and 50A breaker size, often as a response to mass introduction of EV.
NEC 2018/2022 is unchanged about the permissible used of 50A-rated receptacle with smaller 40A breaker but NEC code firmly states that a breaker cannot exceed 8/3 AWG wire rated capacity and many types of 8/3 sheathing barely supports 50A; that's because NEC is all about load-rating, well beneath its rated capacity.
Naturally, in light of those NEC "restrictions", if an EV charger calls for a 50A RATED circuit (because charger performs at nominal load operation at 37A-ish, you'll want all three components of the circuit to each carry 50A. This is the part where the former electric stove/range 8/3 wire failed to meet the new EV charger's demand.
If the EV charger is rated for 60A but operates nominally at 46A, DO NOT go the 50A installaion route. Go with 60A. Operating your load capacity near its rated capacity is potential meltdown situation, especially if weather turns hot, Hot, HOT!
Also for 50A rated EV charger in the high-temperature area (garage), 8 AWG copper wire and 6 AWG aluminum wire are not NEC-code recommended despite having the 50 AMP to 60 AMP ampacity rating (NEC code); go with 6/3 copper.
For most residential garage having 50A-rated EV charger, that means 6/3 (non-Romex) copper AWG, 50A breaker, and 14-50R receptacle. Open-air carport and year-round cooler weather region can NEC-wise get away with (but local/urban electrical code may prevent) the use of 8/3 AWG copper (but no aluminum) wires.
What's your math on this? I don't understand how you're getting to 37A.
A 50 amp rated outlet (backed by a 50 amp rated breaker, and wire capable of handling 50 amps) is fully NEC compliant if you run it for 40 amps continuously (80% derating) or 50 amps temporarily (which NEC defines as a load expected to continue for 3 hours or more). EV chargers fall under the 80% derating. I'm not aware of any EV charger that uses 37A. They either support 40A (80% of 50A), 32A (80% of 40A), or 48A (80% of 60A).
I did take liberty here in saying 37A-ish (40A is code) because that's what some national fire marshalls are reporting about hot garage and reuse of range-type wiring for new purpose of EV charging.
It's the wire material and size issue in hot area.
But, if it supports 40A charging it'll charge at 40 amps unless you back off the charging rate (which nobody does, typically). You'd always assume 40 amps for circuit purposes.
See @quickthrowman's comment for detailed breakdown on high-temperature and copper wire.
> What's your math on this? I don't understand how you're getting to 37A
You have to derate conductors installed in ambient temperatures greater than 86F. I would definitely use NM-B 6/2G (or 6/2G MC cable) for a NEMA 14-50R in a garage, just to account for derating since you get to start at 75A for #6 rated for 90C.
The first derating factor is .94 for ambient temps between 87-95F, .94 times 40A is 37.6A if you use the 75C column. I think NM cable is rated for 90C, so if you derate #8 from 55A at .67 to account for a much higher temp of 120-130F, you get 36.85A.
310.15 (B)(1)
Ambient Temperature Correction Factors Based on 30°C (86°F)
For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities specified in the ampacity tables by the appropriate correction factor shown below.
I’m not disputing that you have to derate conductors based on excess temperature according to NEC. I’m saying that no EVSE exists that will pull 37A. It’ll pull either 32A or 40A and needs wiring and a breaker backing it that either is 40 or 50 amp rated, respectively.
Also, Romex (NM-B) is always considered to be 60C rated, and you cannot assume 90C rated conductors (in the case of a properly rated 90C conductor like THHN) because the outlet and breaker must also be matched and capable of 90C. No 40-50A breaker I’m aware of is 90C rated. They’re typically 75C, and sometimes 60. Likewise, no NEMA 14-50 outlet I’ve ever seen is 90C rated. They’re all 75 or 60C. The crappy Leviton one mentioned in the article is 60C. That means everything is essentially treated as 60C for derating purposes besides derating factors for things like bundled conductors.
In comparison, Bryant and Hubbell outlets (the more expensive ones EV forums tend to recommend) are 75C rated, so you can usually use the 75C derating tables for everything, since breakers are THHN are typically capable of 75C. MC cable would also be 75C rated, as well.
Thanks! The further clarification and use of terminology is much needed here. Plus I needed that citation!
Yes, but only for circuits with multiple outlets, an individual branch circuit must use a wiring device not less than the ampacity of the branch circuit.
You are allowed to use 40A and 50A receptacles on a multi-outlet 40A circuit. You’re also allowed to use 15A and 20A receptacles on a multi-outlet 20A circuit.
Table 210.21(B)(3) of the NEC
You have it backwards. If you have a single receptacle on a branch circuit then the OCPD (breaker) must be sized at or under the receptacle. I.e., a 50 amp outlet can be backed with a 50 amp breaker or anything smaller. You could even have a 20 amp breaker (and 12 gauge wire) backing a 50 amp outlet, which is common for welding applications where peak currents are extremely high on arc start but continously are much lower. There's a specific carve out for welders as well, but it applies to any outlet.
What makes a continuous load outlet so expensive? If they were mass produced couldn't they be pretty much the same price?