What Type of Transformer will 'Defeat' a GFCI for EV Charging? [Solved] - Page 1
Forum section: electronics / general

author: kevinds
author_rank: Newbie
timestamp: August 01, 2025, 05:43:09 am
content: Hello,When I am 'out-and-about' (Canada/US) and parked, I will often plug in my car.. When I am able to, I use a Y cable plugged into two different outlets (5-15) in the parking lots to get 240v (or 208v), using the 'hot' from each outlet into my EVSE. 240v 15 amps, 12 usable instead of 120v 15 amps, 12 usable. This provides me 2x the power (80% more if 208v).As the electrical code has changed, GFCIs are appearing in a lot of places which simply put, breaks my Y cable.. Line and neutral not balanced (line 12 amps neutral 0 amps) so it trips.I know I need a 1:1 transformer to keep balanced power on the Line and Neutral to keep the GFCI 'happy' and keep the voltage the same.. But what kind of transformer do I need that will keep hot-to-ground at 120v and neutral-to-ground at 0v? - Yes, I will need two of them.. Is this the job for the standard 'isolation transformer' or do I need to be looking at a different type?TLDR: I want to use a quick220 box without tripping GFCIs

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2025, 06:46:16 am
content: Two 120 volt AC isolation transformers will do what you want.I am surprised that both legs of split phase 240 are available in close proximity.

author: kevinds
author_rank: Newbie
timestamp: August 01, 2025, 11:28:19 am
content: That is what I thought. Thank you for confirming!That has never been an issue.. If the outlets are on (not simply turned off or temperature controlled), I have have always found outlets close together on the different legs of split phase power. Either the outlet on parking space left/right of where I am parked or the vehicle facing mine.

author: Berni
author_rank: Super Contributor
timestamp: August 01, 2025, 11:47:15 am
content: Yep you will need two separate 1:1 transformers. At these sort of powers they are going to be huge and heavy so it is probably not very practical to do.If you are really serious about sucking every ounce of power from available outlets it is likely more viable to build a portable DC fast charger using a switchmode supply. These can then be paralleled up onto 1 output DC rail to suck power from any number of outlets, no matter what phase configuration they are. Problem is that such switching supplies are not very common, the most common cheap high power tiny form factor PSUs are server PSUs but they usually only output 12V so they are not very practical at these sort of powers. So to get a small lightweight PSU at these output powers you would likely need to instead buy an old 3 phase VFD and repurpose the PFC stage inside it as a DC power supply.

author: nctnico
author_rank: Super Contributor
timestamp: August 03, 2025, 09:36:19 pm
content: Also, when using an isolation transformer, you'll want to include a GFCI for safety reasons.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 09, 2025, 02:11:31 am
content: Quote from: Berni on August 01, 2025, 11:47:15 amJust one will do, connect the secondary in series with the neutral of the other circuit to make a second hot that's 180 degrees out of phase.

author: johansen
author_rank: Super Contributor
timestamp: August 09, 2025, 02:34:07 am
content: Quote from: NiHaoMike on August 09, 2025, 02:11:31 amIm not sure how that would work but what you could do, in theory,Is circulate current in the shared neutral between the two outlets with a much smaller dual secondary 6 volt 20 amp transformer. It gets used as a current mirror to circulate the amps in the hot legs through the neutral of both gfci's.I dont expect it to actually work due to leakage inductance not being able to match the currents within 5mA at 12 amps..

author: johansen
author_rank: Super Contributor
timestamp: August 09, 2025, 02:38:12 am
content: Quote from: Berni on August 01, 2025, 11:47:15 amA dual 120vac primary single secondary transformer will work as well, but it will result in a suicide cord situation where you plug one end in and the other male plug is hot.

author: Berni
author_rank: Super Contributor
timestamp: August 09, 2025, 06:42:10 am
content: Quote from: NiHaoMike on August 09, 2025, 02:11:31 amActually now that i think about it yes that is correct. It should work with just one transformer.The 1:1 transformer is just there to take the 120V from one outlet and make a floating isolated 120V that can be placed in series with the other outlet, summing together to 240V.The secondary outlet can only see the transformer primary so it is fine. The primary outlet is also getting all of the current returned to its own neutral as the only path is trough the transformer secondary, into the car, then back to neutral. The current sharing also works because with no load the primary won't take current, but if the primary outlet is pulling 10A that will also make the transformer pull 10A from the secondary outlet. The GFCI in the primary outlet still works if you have an actual earth fault too.You do get a bit of suicide cord situation in all of these cases tho since transformers work both ways, so it is not safe to touch the other plug when one of the plugs is plugged in. Maybe you could set up some relay logic to first check for both inputs having power before it connects in the rest of the system.