240 volt charging issue

Where do you live? A comdo, apartment, or other large-scale residential complex?

Electric service in many large-scale commercial installations provides 208VAC because it's provided from a 3-phase electric service, while for most homes it's 240VAC. The lower voltage is the reason that your miles per hour of charging is lower.

I live in a single family house with a dedicated 50A line and I rarely get 240v. It's usually somewhere around 232v or so, although it still charges at 40A unless the garage is very warm (typical this time of year in Arizona). I just kicked off charging again and it's at 31/31A. I'm fairly certain it's due to the heat. It's 94 degrees in there right now according to the car.

Is your parking spot subject to excessive heat? Or perhaps it's just the 3-phase issue FlasherZ mentioned.

So, yes, sounds like you have a type of three phase power supply which means your nominal (no load) voltage would be 208 volts. Seeing 200 volts under load isn't unusual in that case.

stoneskid said:

Funny you say that, because I totally forgot to mention that the power is 480 volt 3 phase that is sent to a transformer that converts it to 220 volt before it hits the panel.

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The transformer likely converts it to 208V, not 220V. Take a look at the voltage when you start charging. It will probably be higher than 200V - closer to 208V. As the charge current ramps up, you'll see the voltage dip a bit down to the 200V you are seeing.

stoneskid said:

Funny you say that, because I totally forgot to mention that the power is 480 volt 3 phase that is sent to a transformer that converts it to 220 volt before it hits the panel.

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Given that, you most likely have 208/120 Volt, 3-phase service. In 3-phase service, the leg to leg Voltage is 2*sin(60) times the leg to neutral Voltage. That gives you 208 Volts leg to leg and 120 Volts leg to neutral. Because most household appliances expect 120 Volts, 208/120 is the pairing for 3-phase. In Single or Split-phase homes, it is 240 Volts and 120 Volts.

Starting at 208 Volts, and adding some line losses due to wire resistance, 200 Volt is quite plausible.

FlasherZ said:

Electric service in many large-scale commercial installations provides 208VAC because it's provided from a 3-phase electric service, while for most homes it's 240VAC. The lower voltage is the reason that your miles per hour of charging is lower.

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Years ago when I worked at Toronto Hydro, we would install 120/208 volt 3 phase 4 wire secondary bus on the street and run 2 phases and a neutral into the home (giving 120/208 volts instead of the more typical 120/240 volts. It's not common, but not unheard of either.

mknox said:

Years ago when I worked at Toronto Hydro, we would install 120/208 volt 3 phase 4 wire secondary bus on the street and run 2 phases and a neutral into the home (giving 120/208 volts instead of the more typical 120/240 volts. It's not common, but not unheard of either.

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That's exactly the setup I had at my Boston house. I had a 30 m run from panel to car, and so I typically was charging at about 200 v, depending on the temperature.

stoneskid said:

Thanks for all your help guys. I guess there is no way to correct it?

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Move somewhere else

Depending on how the outlet is wired, you might be able to get closer to 208V instead of 200V using thicker wire, but it won't make much of a material difference in your charge rate. If you really do need more than 24 mph of charge, your only option would be to get twin chargers installed (if you don't have them already) and a HPWC connected at 80A (or something higher than 40A anyway)

At one of the hospitals where I work, there is a bank of 4 Sema Connect J1772 chargers. The starting voltage is usually about 202 or so, and sags to about 192 or so as more EVs plug in. It is not infrequent that after I begin charging, and others connect, that the Model S safety is triggered to drop the current from 20 to 22 amps. Really makes for a slow charging rate. Infrastructure is everything.

No easy way to fix it. But it really shouldn't matter much. You should still get plenty of charge overnight.

There are a lot of factors, we don't know your power distribution. If it's a high-density housing area, it may not be the case. If it's a single-home case, they might be able to replace your transformer to give you 240V. You could always call them and ask if they can get you 240V instead of 208V.

stoneskid said:

Thanks for all your help guys. I guess there is no way to correct it?

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There is a way to correct it, but it will take a boost transformer that will cost several hundred dollars, an electrician that understands how to connect it, and several hundred dollars of installation cost.

Is speeding you charging by 15% (32V/208V) worth spending many hundreds of dollars; most think no...

Cottonwood said:

There is a way to correct it, but it will take a boost transformer that will cost several hundred dollars, an electrician that understands how to connect it, and several hundred dollars of installation cost.

Is speeding you charging by 15% (32V/208V) worth spending many hundreds of dollars; most think no...

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It depends - they might change / reconfigure the transformer for free, never hurts to ask - depends on the circumstances.

Is this a single-family dwelling? or a commercial setting?

Cottonwood said:

There is a way to correct it, but it will take a boost transformer that will cost several hundred dollars, an electrician that understands how to connect it, and several hundred dollars of installation cost.

Is speeding you charging by 15% (32V/208V) worth spending many hundreds of dollars; most think no...

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Yes, a 10 KVA boost transformer 200/240, plus enclosure/ wire and installation may be more than a few hundred $.

A better option (?) - All distribution transformers come with taps. Commercial models generally have taps at 5% above and below prime input. Spec models have them at 2 1/2% and 5 % above and below. If they are currently on the centre or low value tap, you could squeeze 5-10% higher voltage out of the transformer. You WILL need to hire an electrician to do this, so there is a cost, and your building owner may not be agreeable....

The proper "boost" transformer would be an autotransformer wired in a boost configuration. This means that the primary is wired in series with the secondary. All the current bypasses the primary except for the current required to boost the secondary, and the power through the secondary is measured by the voltage across its windings (32V) multiplied by its current (40A). This all means that you only need an autotransformer rated to carry 1.28 kVA, not 10 kVA. A Square D model 1.5S46F fits the bill, affording you approximately 32V of boosted voltage. The only caveat here is that you could not use a 14-50R anymore, as the neutral would not maintain 120-120. You would need to replace the 14-50R with a 6-50R. For your 6-50R, the circuit would now be protected by a 60A breaker on the input side to the autotransformer; cost ~$16. In order to still meet the specifications of the device connected ("UMC") you will need to also put a secondary overcurrent protection device after the autotransformer. This will be a 50 amp breaker in a 2 space load center; cost ~$33. The autotransformer costs ~$285.

To be clear, while the UMC and Tesla would not care about the neutral, anything else plugged in that did utilize the neutral would now see 152V on one of the lines. This is why it is absolutely necessary to use the 6-50R, or direct connection to a HPWC, in this application.

Total materials cost not including extra wiring for hookup: ~$334. Add an hour of labor and some various odds and ends I'd expect an electrician could get you hooked up for ~$500.

FAQ Explaining Autotransformers wired for Boost/Buck: As an autotransformer, how can a Buck-Boost transformer supply loads significantly higher than its nameplate rating as a low voltage lighting isolation transformer? | HPS | Hammond Power Solutions

FAQ Explaining Overcurrent Protection for Autotransformers: When a Buck-Boost transformer is connected as an autotransformer, what is the procedure for determining the current rating of the over-current protective device, such as the fuse or circuit breaker? | HPS | Hammond Power Solutions

Explanation of why this can only be used with 240V loads and not 120/240V (Page 19 Q3, and Page 20 M1): https://www.automationdirect.com/static/manuals/buckboosttransformer/buckboostmanual.pdf


Zoro listing for example autotransformer pricing: SQUARE D Buck Boost Trans, 120/240, 16/32, 1.5kVA 1.5S46F - G3427907 at Zoro</title><meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1"/><!-- base meta, css, js --><link rel="canonical" href="/square-d-buck-boost-trans-120240-1632-15kva-15s46f/i/G3427907/"><!--base Meta--><meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0"><meta http-equiv="Content-Type" content="text/html;charset=UTF-8"><meta http-equiv="Content-Script-Type" content="text/javascript"><meta http-equiv="Content-Style-Type" content="text/css"><meta http-equiv="Pragma" content="no-cache"><meta http-equiv="Cache-Control" content="no-cache"><meta equiv="Expires" content="0"><title>SQUARE D Buck Boost Trans, 120/240, 16/32, 1.5kVA 1.5S46F - G3427907 at Zoro

Thank you @sefs! That is exactly what I was referring to. Most people don't realize that for boost-buck configurations, the iron/copper of the transformer is only carrying the "difference" kVA, here 32Vx40A or 1.3 kVA.

Still, is it worth $500+ to increase charging speed from 24 to 28 mph?

Cottonwood said:

Thank you @sefs! That is exactly what I was referring to. Most people don't realize that for boost-buck configurations, the iron/copper of the transformer is only carrying the "difference" kVA, here 32Vx40A or 1.3 kVA.

Still, is it worth $500+ to increase charging speed from 24 to 28 mph?

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Whether the price is worth it or not is a valid question, but it's good to know what the options are anyway.