Question about Voltage Drop on UMC Charging

Question about Voltage Drop on UMC Charging

Hello guys, I have a question that bother me a long time ago.
I was having this UMC came with the vehicle to charge my car on a NEMA 14-50 for years. Previously I didn't noticed my charging voltage which shows on my car is about 205-210 V. I recently went to my friends home and saw his MS is charging at 239 V constant, it brings me the question, why my UMC can only deliever 208 V to my vehicle.

I tried to use meter to measure the voltage from the hot to neutral on my nema 14-50 outlet, the number is about 121 V for both of them, which make me confused more. Previously, my guess was because my house is may be on the end of the power lane which the main cable have a lower voltage than others, but it shows me the voltage on my end is pretty solid and good which is above 120V. So in my mind, 120+120 should at least have 230 V or more.

However, I found something fun, if I turn the current down to about 5 A. the voltage will increase to about 215V-218V.
Please help me, is there possible something wrong with my UMC, are the input voltage which 121V is low. Thank you for any response!

Rocky_H | 04/12/2017

@gongzhen2015, This calls for some explanation of electrical systems:
Most residential buildings have the two voltage levels as 120V and 240V. But most commercial properties, like businesses and industrial buildings have 120V and 208V. So somewhere around 208-210V could be totally normal if that's what your building has.

You are not measuring the part on your 14-50 outlet that is useful to help us figure anything out when you're just getting 120V. Measure between the two hot slots on the left and the right. Those should read somewhere near 240V or 208V, and then we will know kind of a starting point of which starting voltage level you have.

olivier | 04/12/2017

To add to Rocky_H explanation,

It is true that different buildings may output different voltages, however I think there is a better explanation in your case.

First, It is possible that when you measure your voltage with your meter on your 14-50 outlet and get 121V it is because you are using your meter in DC mode, but this outlet is AC.
If you switch to AC on your meter, it should display the correct voltage (210-240).

Also, If the voltage drops while charging at higher current, that means you have too high of a resistance in your cable or an old or inefficient electrical system in your building, or sometimes the cables simply cover a too long distance in the walls.
Maybe try using an outlet closer to the source or changing the cables in the wall.
Also, try to avoid using extensions.

Hope this helps !

Frank99 | 04/12/2017

Actually, if gongzhen2015 is in the USA, it's expected that hot-to-neutral will be 120V. Hot-to-Hot should be 240V or 208V (great catch, and post, by the way, Rocky).

gongzhen2015 | 04/12/2017

I found the reason, as I measer from the panel box, from each bus, the reading is 212V. Which means I am having a 120/208V service. Anybody have any idea why it is a 208V service but not a 240V service, and if I can changed to 240V service? I am in NJ with PSEG.

Rocky_H | 04/12/2017

@gongzhen2015, Quote: "Anybody have any idea why it is a 208V service but not a 240V service,"

Yes, here is a page with a good explanation (with diagrams!) of what split phase versus 3 phase electrical systems are.

Quote: "and if I can changed to 240V service?"

No, there is nothing you can do to change that. It is just how the electrical supply comes from the main utility into your building.

SUN 2 DRV | 04/12/2017

"if I can changed to 240V service? I am in NJ with PSEG."

Ask PSEG if they can provide you with a 240 volt split phase feed? And what the cost would be to reconnect your service that way. Basically you're currently on a three phase system, and they may or may not have a convenient way to connect you to a single phase system.

olivier | 04/12/2017

What about simply calling an electrician to upgrade the wires so that it can take a lot more current, as well as having a bigger breaker, then you will be able to charge faster.
At the end, charge speed will be equivalent to Volts x Amps... If you can't get more volts, try pulling more amps !
If you set higher amps in the car, Voltage will not reduce that much if the wires can handle it.
Not sure but I think that your cable from Tesla can handle up to 80 amps.. | 04/12/2017

208 volts at 40 amps is a little more than 8 kilowatts. Takes a little longer, maybe an hour to go from 20% to 80%. Not a big deal. Make sure the outlet isn't getting hot. Also the wires to the outlet. Should be OK.

gongzhen2015 | 04/12/2017

Replay to olivier
My car only equipped with 48amps regular charger, which the only option for me is to install HPWC to get a little more speed.
Replay to
I feel so bad that I live in a residential apartment but only which 3 phase power, the best volt get is 210V, and I never feel the outlet gets hot.

Rocky_H | 05/12/2017

I don't really see why it's that big a deal with the 210V versus 240V. You still charge overnight within a few hours. You sleep more than that.

Rocky_H | 05/12/2017

I really don't get why people do this. He's carrying on the same question/discussion on both Tesla forums and giving copy/paste answers on both of them.

psusi | 05/12/2017

Wait, so you see 120v from each leg to neutral, but only 212 between the two? I thought all homes in the US got split phase power, not 2/3 phase.

Voltage drop as the current ramps up indicates bad wiring. Last summer I was charging on vacation with a 120 volt extension cord. The voltage dropped several volts as the current ramped up to 13 amps then the car said I had bad wiring or an extension cord and limited the current to 9 amps.

Coming home from vacation we moved into a new home and I was using a 120 volt outlet for a while and it was dropping maybe 6 or 9 volts. I installed a 14-50 outlet with 50 amp cable but only on a 30 amp circuit and charge at 15 amps and now only see 1 volt drop.

Also the UMC is limited to 40 amps, and gets quite warm doing that.

Frank99 | 05/12/2017

psusi -
gongzhen2015 lives in an apartment complex; some of those are served with three-phase 208/120 power and only have 208V across two phases, rather than single-phase 240V/120V that single-family homes are wired with.

Rocky_H | 06/12/2017

@psusi, Quote: " I installed a 14-50 outlet with 50 amp cable but only on a 30 amp circuit"

Augh!!! DON'T do that! That violates electric code. You need to match 30 to 30 or 50 to 50. You can't mix those two.

Frank99 | 06/12/2017

Well, you CAN mix them, as psusi did.
However, it's really not a good idea, unless you're not fond of your house and you'd prefer that it burn to the ground, and you have good smoke detectors to give your family a good chance to get out of the house when it does, and you don't have any neighbors who'd object to their house catching fire after yours.

If the wires from the circuit breaker box to the outlet aren't rated for 50 amps, put in a 30 amp dryer outlet - I think it's a NEMA 14-30 outlet. You can get an adapter from Tesla for it, and your family and home insurance company will approve.

psusi | 08/12/2017

@Frank99, you've got it backwards. I have 50 amp wire with a 30 amp breaker, not the other way around. I just could not fit a 50 amp breaker in the box, so I can't charge the Tesla quite as fast. Oh well. The heavier gauge wire keeps the voltage drop down, and the 50 amp plug means I don't have to buy the 30 amp UMC adapter and then be limited to 24 amps.

Rocky_H | 08/12/2017

@psusi, Everything about that is so messed up. If you could not fit a 50A breaker in and only had capacity for a 30A breaker, only have 30A capacity.
"and the 50 amp plug means I don't have to buy the 30 amp UMC adapter and then be limited to 24 amps."

Holy frickin' _____
You NEED to stay limited to 24 amps to stay within your 30 amp circuit rating!

Frank99 | 08/12/2017

At least the only thing he's putting at risk is the 30A breaker - with the 50A rated wire and outlet, at least he's not gonna burn down the building.
He mentioned a few weeks ago why he didn't have room for a 50A breaker - I don't remember precisely, but I think he had to install a tandem breaker because he was out of slots. I have to do the same on my box, but I was able to source a 20A/50A tandem breaker so I'll have a real 50A circuit.

psusi | 08/12/2017

@Rocky_H, last time I checked, 24 amps is less than 30 amps. A 30 amp circuit can handle... guess what... 30 amps!

Rocky_H | 11/12/2017

@psusi, Quote: "@Rocky_H, last time I checked, 24 amps is less than 30 amps. A 30 amp circuit can handle... guess what... 30 amps!"

I'm not at all sorry to say that you are absolutely totally WRONG and clueless about electric code.

READ that section about continuous loads and the ratings requirements for them. It is absolutely clear and required by the National Electric code that you can only have 24A for electric car charging from a 30A circuit.

You can only use the full rating of a circuit for short term loads, so an electric dryer can use up to the full 30A of a 30A rated circuit. Or an electric stove can use the 50A of its 50A rated circuit.

Electric car charging is defined in NEC as always to be considered a continuous load. And the rule for continuous loads is that the circuit rating MUST be 125% of the constant current level. So 24A times 125% means a 30A circuit. 40A draw MUST have a 50A circuit.

DTsea | 11/12/2017

Rocky +240 (split phase) or +208 (3 phase)

DTsea | 11/12/2017

Humorous intent. Rocky great post!!!!

Rocky_H | 11/12/2017

This is why you can't just come up with how you think something should work when dealing with wiring and building electrical systems. NEC has a lot of detailed, nitpicky, obscure stuff, but it's in there for specific reasons.