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BUILDING A NEMA 14-30P TO NEMA 10-30R ADAPTER

(originally written Apr 25, 2021)

Please read this page thoroughly before building the adapter as there are a few safety precautions to be aware of. Remember that a constant draw on an outlet should not exceed 80% of that circuit's amperage. In most cases, the Tesla chargers step down to the correct amperage automatically. So Tesla's Gen 1 Mobile Connector will draw 24 amperes from a 30 ampere circuit, and 40A from a 50A circuit. Keep in mind that the Gen 2 Mobile Connector maxes out at a draw of 32A - it will still plug into a 50A circuit, but it will always step it down. Mileage recharge rates mentioned below are based on the Model S.

Why? In 1996, the housing code deemed NEMA 10-30 circuits dangerous and obsolete. The 14-30 became the new 30A standard due to the increased safety of the added ground wire. The older receptacles are grandfathered into code, but it is now against code to install new ones. Tesla currently only sells the 10-30 adapter for the Gen 1 Mobile Connector (MC). So what if you want to install a new 30A outlet but have the original spec MC?

Circuit Amperage
A decision has to be made to either install a 30A outlet or a 50A outlet. One might choose the 30A if they are concerned about overloading their breaker box. Boxes are either rated to 150A or 200A, with the larger ones being more commonly found in newer construction. An electrician can calculate whether or not your 150A breaker box can safely handle the addition of another 50A circuit. Otherwise, the do-it-yourselfer with a 150A box is wise to simply settle on a 30A receptacle. For the Model S, the 30A option will charge at a rate of about 17 miles per hour, while the 50A will charge at 29 miles per hour. Big difference, but is that difference necessary? Let's say you need to charge 100miles to reach 90% capacity: 30A will take less than six hours, while 50A will take less than four hours. However, most people need to sleep at least six hours, so 30A should work just fine in this scenario.*

* To put this into perspective: if you only had four hours to charge, 50A would get you 116miles while a 30A outlet would only get you 68miles. That's a significant difference, but it likely becomes negligible if you're able to charge eight hours the next night, and so on. I managed fine on a 15A circuit for two months, so 30A should be no problem for me. Plus, slower charging generates less heat, which helps reduce the possibility of battery degradation ...although you could manually limit the Charge Current if using a 50A circuit.

One workaround on a smaller breaker panel is to program the Tesla to charge when the other appliances are not being used. For instance, the breaker for a typical kitchen range is 50A. Pretty good chance the cooking will end at bedtime, so click the lightning bolt on the Infotainment and set the car to start charging at 10pm. Doing so will allow plenty of headroom to supply the Tesla's outlet. This should give most users eight hours to charge (136miles on 30A; 232miles on 50A) with minimal risk of tripping the main breaker.

Note: I am not going to go into how to install an additional receptacle as there are plenty of videos on YouTube to walk you through the process. Keep in mind that it can be dangerous.

Be aware that NEMA outlets of 30A or more are not designed to be plugged and unplugged as frequently as a common household outlet. Doing so may cause the connections within these higher amperage receptacles to lose tension over time, thus wearing them out prematurely. So, if there's ever an intermittent charging issue, this should be the first suspect for replacement.

Adapter Wiring
My next decision was choosing the configuration to connect a 30A outlet to the car's charging port. The outlet would have to be the newer, code-legal NEMA 14-30R (R stands for female receptacle; P stands for male plug) to power Tesla's NEMA 10-30P adapter on the end of my Gen 1 MC. Again, I had to go with the 10-30 adapter because there wasn't a 14-30 available for my older version MC. I decided to make this adapter myself, so opted to also make it an extension cord ...and give it the ability to plug into 50A and 60A receptacles as well.

My new garage outlet became the NEMA 14-30R with four leads: two hot, one ground, and one neutral. My Gen 1 MC has the NEMA 10-30P adapter with three leads: two hot and one neutral. Not having a ground can be dangerous in many instances, but the Tesla charge port only has two power leads: one hot and one ground, with no neutral. This can be verified by looking at the internal wiring of the Tesla Wall Connector. The MC combines the two 120V hot leads into one 240V hot and if the outlet only has a neutral, it use it as the ground.* Therefore, we can safely omit the neutral lead of the 14-30, which can again be verified via the Tesla Wall Connector wiring. To make the 14-30 to 10-30 adapter, connect each of the two hot wires (black and red) of the male side of the NEMA 14-30 cord to each of the two hot posts of the female 10-30, and the ground wire (green or bare) from the 14-30 to the neutral post of the 10-30. The neutral wire (white) of the 14-30 is not connected to anything - cut off the unused terminal connector and wrap this clipped end with wire tape.

* If the circuit is unsafe and/or unusable due to a discrepancy in the wiring, this should be detected by the Tesla software and will prevent the car from damaging itself.

Caution: Use a multimeter to check continuity between the posts of each wire to ensure correct placement before using the homemade adapter. For example, placing the probes on both "X" positions will produce an audible beep from the meter if it is correctly wired. It should not beep if one probe is moved to a differently marked lead.
Caution: Since the neutral is being eliminated, this wiring configuration makes the homemade adapter unsafe in every circumstance except when used to charge a Tesla.

IMPORTANT! I would advise adding a label to the receptacle end of the new adapter for two reasons: the label should remind the user that it is to be used for Teslas only; the label should remind the user to set the maximum Charge Current to 24A.

PARTS LIST

- NEMA 14-30P with 10-foot Cord
- NEMA 10-30R Dryer Receptacle
- Clamp-on 3/4" Conduit Fitting

At the time of this writing, the cost of those three items was $36, then adding the NEMA 10-30 Tesla adapter is an extra $45. There is this option that costs $50-75, but you're not getting the additional ten feet of extension nor the ability to also plug into an older NEMA 10-30R within 18ft of the car's port.

Modify for Versatility
To allow the male NEMA 14-30P of the new adapter to also conveniently plug into a NEMA 14-50R or NEMA 14-60R outlet, simply cut the neutral prong from the NEMA 14-30P. Teslas come with a NEMA 14-50 adapter, but this will add a ten foot extension if a 50A receptacle is just out of reach. In doing so, and this is very important, remember to always manually set the max amperage on the Infotainment to 24A* to prevent it from drawing the full amperage from the 50A and 60A larger outlets.

* Once plugged in, click the lightning icon in the top left of the Infotainment screen and adjust the Charge Current amperage in the bottom left to 24A. This must be done when utilizing a 50A or 60A circuit since the wires in the extension cord are only rated to safely handle a 30A current draw.

Battery Stress
Among the many factors that affect battery degradation, one of them is "state of charge" - this is how full the battery is. To help maximize battery life, one could hover around 40-60% to reduce battery stress. Tesla Service Centers seem to recommend a 90% state of charge limit as this might be the best compromise between battery health and range readiness. With the ability to recover 17miles in an hour with this 30A outlet, I opted to reduce my charge limit from 90% to 80-85%. This will relieve a little more stress on the battery, and I can simply use my phone to raise the limit back to 90% an hour or two before I know I have to leave.

 

 

CrankyGypsy (established 2001)