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NEMA 14-50 vs NEMA 14-30

rsomrek

Member
Jan 1, 2018
64
50
Burr Ridge, IL
It's getting close to delivery and I'm trying to get some estimates for installation of a charging outlet in my garage. The house has 100 amp service and I'm a little concerned that I'll be told that a the "standard" 50 amp circuit (NEMA 14-50) may be too big for my box (even if the car only draws 32 amps max). I wouldn't be surprised that I have to got to a NEMA 14-30. Outside of the slower charging rates, is anyone familiar with any other drawbacks of the smaller circuit?

P.S. And recommendations for an electrician in the southwest Chicago suburbs?
 

DMack

Member
Jan 15, 2018
178
97
Oakland
I was told that I could only go with a 14-30 after hoping i would be running a 14-50.

The obvious drawbacks are that i charge at a rate of 18 miler per hour on 24A versus the ~29 on a 14-50 @ 30A.
Other than that, I'm pretty satisfied with my setup - I haven't' had any issues with tripping breakers, or feeling like charging at a rate of 18mhp is not enough for me.

I commute daily, 27 miles each way and I can top it off every other day (I do off-peak charging from 11pm to 7am) and i'm good to go.
 

KJD

Supporting Member
Supporting Member
Dec 14, 2013
1,371
1,019
SLC, UT
You should be fine with a 30 amp outlet for overnight charging. I have mine set at 30 even though I have a 14-50 outlet. The cable and connector stay cooler with a lower setting.
 

SSedan

Active Member
Jul 24, 2017
2,948
2,327
Greenville Wisconsin
I believe the 18/29miles per hour posted above are for Model S so I think you will do better. That said I am up by Green Bay and use a 14-30, and it works perfectly for me. Even on cold days I run a bunch of errands it still has plenty of time to charge overnight, but I am boring and middleaged with small kids, not staying out late or anything.
I charge soon as I get home in winter so I make use of the heat already in the battery, and warm the car up before I head out in the morning. Battery warming takes a variable amount of time depending on temp so I had little success timing charging to end right before I left.
 
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TexasEV

Well-Known Member
Jun 5, 2013
7,650
8,482
Austin, TX
You should be fine with a 30 amp outlet for overnight charging. I have mine set at 30 even though I have a 14-50 outlet. The cable and connector stay cooler with a lower setting.
Not relevant. You don’t draw 30A from a 30A outlet, you can only draw 24A.
 

NRG4All

Member
Jan 24, 2016
41
27
Show Low, AZ
It's getting close to delivery and I'm trying to get some estimates for installation of a charging outlet in my garage. The house has 100 amp service and I'm a little concerned that I'll be told that a the "standard" 50 amp circuit (NEMA 14-50) may be too big for my box (even if the car only draws 32 amps max). I wouldn't be surprised that I have to got to a NEMA 14-30. Outside of the slower charging rates, is anyone familiar with any other drawbacks of the smaller circuit?

P.S. And recommendations for an electrician in the southwest Chicago suburbs?

I have a 40A 240V circuit connected to an AeroVirnoment EVSE. Its rated output is 30A, thus I'm getting around 7.2 kW which translates to 29 miles per hour of charge. As many have said, that is plenty. I calculated my requirements based on my driving habits. As it turns out 10 hours of charge over night is almost 290 miles. I never drive that much in a day unless I'm on a vacation and then I will be using Superchargers or Destination Chargers. IMHO the extra money for higher rated home charging would only apply to someone with short charging times and long daily distances.
 

linkster

Active Member
Apr 22, 2013
1,128
268
USAX2
DM

14-30 will provide:
~18 on an S
~22 on a 3

rs
5+ year S owner and have charged on a 14-30 the entire time with 80 and 160-mile commutes. I am currently charging my 3 on a 6-20 obtaining a15 mph charge rate.

Good-luck!
 
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eprosenx

Active Member
May 30, 2018
2,074
2,517
Beaverton, OR
It's getting close to delivery and I'm trying to get some estimates for installation of a charging outlet in my garage. The house has 100 amp service and I'm a little concerned that I'll be told that a the "standard" 50 amp circuit (NEMA 14-50) may be too big for my box (even if the car only draws 32 amps max). I wouldn't be surprised that I have to got to a NEMA 14-30. Outside of the slower charging rates, is anyone familiar with any other drawbacks of the smaller circuit?

P.S. And recommendations for an electrician in the southwest Chicago suburbs?

So before you do anything you need to run the load calculations to figure out how much of your 100 amp service is committed and actually used right now. There is a NEC formula (actually a couple optional ones) for doing this. Note that for an existing service there is some different math used to determine if you are allowed to add more to an existing service vs. what you would have to install new (when installing a new service they want you to add some overhead to it for later additions).

With that being said, I might suggest you consider installing a Wall Connector (HPWC is the old name for it). While others may jump out and say "but the UMC can do up to 32 amps so why bother", the trick is that the Wall Connector has a lot more configurable amperage settings. 13 of them to be specific. Options include 12a, 16a. 20a, 24a, 28a, 32a, etc... So you may be able to thread the needle of exactly how much capacity your 100a service has remaining. (yes I know you can crank down amperage in the car settings, but this depends on a human being remembering to do it which does not pass NEC muster - also the Tesla software has been known to reset itself back to full charge rates without folks realizing it)

Also, depending on your whole setup and deal, you might consider say running 6awg wire to it (in conduit so you can use the 75c rating of the wire) which will allow up to a 60a circuit to be used (48a of charging capacity due to continuous load derate). This would accomplish three things. 1. You get the granularity as described above, 2. You could change the setting up in the future if you removed some load from your house (convert something from electric to gas, etc...) by simply setting a rotary dial switch in the HPWC, 3. If you later decided you can't live with the slow charging rate and decide to upgrade your service, everything would be in place to make use of it once your service was upgraded. Just change the rotary dial in the Wall Connector.

For myself, I wanted to always keep the UMC in my car, so I was going to buy a second EVSE anyway, and the Wall Connector just made sense...

Anyway, just food for thought!
 
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eprosenx

Active Member
May 30, 2018
2,074
2,517
Beaverton, OR
Looks like the load calc rule I mentioned above for existing services is 2017 NEC code section 220.83 - Existing Dwelling Unit

It is a seemingly simplified calculation to determine if a "service" (supply from utility) or "feeder" (wire to subpanel) is of sufficient capacity to allow additional loads to be added. It would appear to be more relaxed in capacity compared to what you need to size for when building a new home or putting in a new service or feeder.

Basically they don't want you building undersized stuff, but if you already have something installed they want to be able to let you use it up to the safe maximum limit.

You can get free access to the 2017 NEC via the NFPA web site. You just have to register your email address.

P.S. Also, while this might not help you with the NEC rules, if you get a "Sense" The Sense Home Energy Monitor you could see what your existing load is. I have one and I love it. Would be great for figuring out how high your actual load is. Also, you might be able to get historical usage from your utility company that gave you perhaps 30 minute granularity into peak loads?
 
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rsomrek

Member
Jan 1, 2018
64
50
Burr Ridge, IL
So before you do anything you need to run the load calculations to figure out how much of your 100 amp service is committed and actually used right now. There is a NEC formula (actually a couple optional ones) for doing this. Note that for an existing service there is some different math used to determine if you are allowed to add more to an existing service vs. what you would have to install new (when installing a new service they want you to add some overhead to it for later additions).

With that being said, I might suggest you consider installing a Wall Connector (HPWC is the old name for it). While others may jump out and say "but the UMC can do up to 32 amps so why bother", the trick is that the Wall Connector has a lot more configurable amperage settings. 13 of them to be specific. Options include 12a, 16a. 20a, 24a, 28a, 32a, etc... So you may be able to thread the needle of exactly how much capacity your 100a service has remaining. (yes I know you can crank down amperage in the car settings, but this depends on a human being remembering to do it which does not pass NEC muster - also the Tesla software has been known to reset itself back to full charge rates without folks realizing it)

Also, depending on your whole setup and deal, you might consider say running 6awg wire to it (in conduit so you can use the 75c rating of the wire) which will allow up to a 60a circuit to be used (48a of charging capacity due to continuous load derate). This would accomplish three things. 1. You get the granularity as described above, 2. You could change the setting up in the future if you removed some load from your house (convert something from electric to gas, etc...) by simply setting a rotary dial switch in the HPWC, 3. If you later decided you can't live with the slow charging rate and decide to upgrade your service, everything would be in place to make use of it once your service was upgraded. Just change the rotary dial in the Wall Connector.

For myself, I wanted to always keep the UMC in my car, so I was going to buy a second EVSE anyway, and the Wall Connector just made sense...

Anyway, just food for thought!

Thanks for your thoughts/input.

The short response from the electrician that finally installed the circuit is that given my house setup (A/C, electric oven/range, normal small appliances, LED lights) I would never pull the 100a my box is rated for. The 50a circuit for the car would be okay.

I know now about the NEC formula but I'm assuming that given the electrician's experience he was well aware what that formula would yield. The circuit has been working fine. I charge in the middle of the night (ComEd hourly pricing generally $0.015 - $0.02 /kW - sometime negative so they pay me! - drawing 8kW during charge) and wake up to a fully-charged car. I was very pleased with the quality of the installation - would recommend him to anyone in south/west Chicago area.
 

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