A "Quick 220" clone

Use two 120V sockets to get 208V/240V power

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In North America, the power grid provides residences and small businesses with "split phase" 240 VAC power. Normally, one of the two hot lines and a neutral is used to make 120 volt circuits throughout the building that are evenly split between the two phases to balance the load. Dedicated 240 volt circuits can be provisioned by using both hot lines. But what if you need a temporary 240 volt outlet? If you're cautious, you can combine two 120 volt outlets that draw from opposite phase hot lines to obtain 208/240 volt power.

Note that "Quick 220" is a trademark of these folks: . I have no affiliation with them whatsoever. I have not ever used or handled one of their devices and have no idea what their design is. This device is my own attempt at fulfilling the same objective.

In North America, power is usually provisioned to a premises as either split phase or three-phase power. In split phase, there are two hot lines and a neutral line, which is connected to an earthed ground at the service entrance. The two hot lines are each 120VAC RMS relative to the neutral, and 180 degrees out of phase with each other. This allows 120 volt circuits to be provisioned with one of the two hot lines, the neutral and the ground, or 240 volt circuits to be provisioned with both of the two hot lines and a ground (and optionally the neutral as well). Split phase service is the norm for residences and small businesses. Larger businesses get three phase power, which is exactly the same, except that there are three hot lines, each 120 degrees apart in phase. Any two hot lines will yield a 208 volt circuit. Most devices that require 240 volts will work acceptably well with 208 volts.

Sometimes you need to temporarily power a 240 volt device. We have this issue because we occasionally visit our parent's house with our electric vehicle, and desire to charge it. While we can charge with a single 120 volt outlet, it takes far longer than if we can obtain 240 volt power. This device allows us to do exactly that.

I must pause at this point and raise the very serious safety concerns that this project brings with it. In general, you cannot expect to find two outlets on different circuits (which must also happen to be from opposite phases) right next to each other on the wall. That means you're likely going to be using extension cords to do this. You must use heavy duty models for this purpose. You must not exceed the rating of the lightest-duty component in the entire system. Failure to heed this warning will very likely start an electrical fire. In addition, the concept of attempting to combine two separate circuits back into one is fraught with all sorts of dangerous traps. Every portion of this design was chosen to ameliorate a particular safety hazard. Do not attempt to construct this device until and unless you understand what the design is attempting to achieve and do not attempt to make any changes to the design without fully understanding the ramifications.

In general, doing this is actually a really bad idea. I can't recommend anyone attempt to do this without thinking it through. The only reason I am documenting this project is in the hopes that anyone who does try to achieve this won't fall into some of the very dangerous traps that are endemic to the concept.

The design

Follow along with the schematic as you read this design description. Note that the schematic does not completely represent how the circuit is actually constructed, as the high-current path must be made with large gauge wire while the relay coils and pilot lamps can use smaller wire.

There are three sections of the design, each centered around a DPST relay. Two of the relays have 120VAC coils and the third has a 208/240VAC coil. Each of the first two is connected to a 120V plug, such that the hot and neutral directly activate the coil. The hot line of each plug goes into one pole of the relay. The two relays form a sort of "X" figure. The idea is that each hot line must be switched on by both relays. If either relay turns off, then both hot lines will be disconnected from what follows. So one of the hot lines is switched by the first relay, and the output of that switch goes to the opposite relay where it is switched again. The other relay has the exact same setup - its hot line is switched by one of its own poles, and then fed through the unused pole on the opposite relay. Along the way, the two primary relays have neon indicators across their coils. This allows you to see at a glance if the device is functioning and if not, why not.

The purpose of these two relays is to insure that nothing is connected to the output unless both source circuits are energized. One of the hazards of this concept is that if you yank one of the 120 volt plugs of a naively designed...

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  • 2 × 120V plugs Get heavy duty ones at Home Depot
  • 1 × L6-20 receptacle Available at Home Depot
  • 1 × 6' length of 12 gauge 3 wire SOOW cable Available at Home Depot
  • 1 × a few feet of 22 gauge stranded wire
  • 4 × 22 gauge .25" QD crimp terminals

View all 14 components

  • On voltage, current and power

    Nick Sayer03/21/2015 at 17:11 0 comments

    Some people have asked if this allows you to pull 30A from a 15A circuit. The short answer is "no." What confuses them is the conflation of units.

    There are three units of interest for this discussion:

    1. Volts. Volts measure the electrical potential between two points.
    2. Amperes, or amps for short. Amps measure the current. Current is the amount of electricity that flows in a circuit.
    3. Watts. Watts measure the flow of power. Power can be thought of simply as voltage times current (for loads with complex impedance it's not that simple, but that's beyond the scope of this discussion). Power is the rate of flow of energy.

    So the question remains - how is it that two circuit breakers rated for 15A cannot be combined to make 30A? Why bother with this if you don't get more power?

    And that question is its own answer. By combining the two circuits, you do get more power. Twice as much (assuming split phase rather than three phase), in fact. But that's because the voltage doubles, not the current.

    To understand how 240V power works here in North America, think of two lumberjacks working a two-man saw. When one of the lumberjacks is pushing the saw forward, the other is pulling it backward. At the end of the stroke, the two reverse. Each man is contributing one lumberjack's worth of power. The net result is 2 LP (lumberjack-power - compare to horsepower, another unit of power). But crucially, the saw doesn't move any faster than it would if only one lumberjack was working it.

View project log

  • 1
    Step 1

    Prepare the chassis.

    Drill a hole centered in each of the two "short" sides of the case sized for the CG-16 cable gland. Drill a hole sized for the L6-20 receptacle. The hole should be centered horizontally on the panel, but the top edge should be as close as possible to the top of the chassis - 1/4" would be ideal. Once the hole is drilled and sized properly, remove the two mounting bolts from the receptacle's mounting wings and discard them. Hold the receptacle up to the outside of the chassis and mark the holes in the wings on the chassis. Drill 9/64" holes for the mounting hardware.

    Drill holes sized for the pilot lamps in three of the four corners of the lid.

  • 2
    Step 2

    Prepare the mounting plate.

    Lay the plate on your work surface and position the relays as close as possible to one of the long edges. The three relays should have about a half inch gap between them for air circulation, and the middle relay should be centered on the panel horizontally. Mark the 6 mounting holes (2 for each relay) and drill them with a 9/64" drill bit. Mount the relays using 6-32 x 1/2" bolts, lockwashers and nuts. The 240 volt relay should be on the right side, the two 120 volt relays on the left and in the center.

    In the bottom left corner, drill a 1/8" hole for the self-tapping screw that comes with the ground bar. Make sure there's enough room around the ground bar so that it doesn't impinge on where the 220 receptacle will be, nor the incoming cable (through its gland) on that side of the chassis. Mount the ground bar in the hole and screw it down tightly.

    Install the mounting plate in the bottom of the chassis with its supplied hardware.

  • 3
    Step 3

    Cut two 2 foot lengths of the SOOW cable. Prepare each by stripping 6 inches of the outer jacket on one end and 2 inches on the other end. On the short end, install a 120 volt plug. Be sure to connect the black wire to the hot prong, the white wire to the neutral prong and the green wire to the ground prong. Run each cable through a cable gland, making sure the loose 4 inches of wires are on the inside and the plug is on the outside. Attach each gland to the chassis.

    Strip the outer jacket off the remaining 2 feet of SOOW cable. Cut a 4 inch length of each of the white, black and green wires. Attach them to their respective terminals on the L6-20 receptacle. The green wire goes to the ground terminal (which has a green screw), and it doesn't matter which of the other two is black or white. Install the L6-20 receptacle in its hole in the chassis. Make absolutely sure that none of the wires or any metallic parts of the receptacle contact the mounting plate!

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Enjoy this project?



Adam L. Humphreys wrote 02/03/2017 at 22:24 point

Though I've often thought of something like this, I haven't had the need yet. However, concerning the neutral, have you considered using the neutral form just one outlet since most current draw would be realized by each 120V leg? You could always add an extra neon between ground and neutral to make sure it isn't hot, (and possibly a switch between the two, though that could lead to user issues).

  Are you sure? yes | no

Nick Sayer wrote 02/03/2017 at 22:33 point

The code says you're not supposed to intentionally send current to ground, although apparently there are exceptions (for example, EVSEs intentionally leak test current to ground as a ground continuity test), but then the code also says you're not supposed to have neutral-hot reversals.

In principle, you could draw a neutral from just one of the 120 legs, because (again, in principle) both 120v leg neutrals are supposed to be identical, but doing so is unsafe if you don't independently verify that the neutral you pick is at ground potential.

It's just safer - and it met my needs - to just declare that the output was going to be hot-hot-ground only and not offer a solution for an output neutral.

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Omar wrote 04/24/2015 at 21:10 point

Nick, did you use a resistor inline with the led indicators? I am not sure if I should put in a diode and resistor or just run it straight across.

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Nick Sayer wrote 04/24/2015 at 21:22 point

I bought indicators that were rated for 120/240 VAC:

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Dave Blundell wrote 03/22/2015 at 01:58 point

Maybe I'm missing something obvious, but I can't figure out from looking at your schematic how you're able to guarantee that the two 120V circuits you chose to power the converter are indeed 180deg out of phase rather than being synchronous?

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Nick Sayer wrote 03/22/2015 at 02:13 point

If they're not 180 degrees (or 120 degrees) out of phase, the third relay coil will not get 240V (or 208V) and will not close.

You wind up having to do a sort of scavenger hunt looking for two outlets that make all three lights turn on. But at no point is the output socket ever in other than one of two states: either completely dead or properly powered.

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Dave Blundell wrote 03/22/2015 at 03:41 point

duh.  Totally see it now.  Thanks.

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Dave Blundell wrote 03/22/2015 at 15:29 point

So is it a matter of plugging it in to different outlets until you find a combination that works or is there a more methodological  approach?

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Nick Sayer wrote 03/23/2015 at 01:17 point

That really is the easiest way for most folks.

If you know the circuit layout of your house - that is, which circuit breakers control which outlets - then what you can do is go to the breaker panel and try and identify an odd numbered and an even numbered breaker that map to outlets of interest. Usually circuit breaker panels alternate phases as you go so that ganged breakers can make 240 volt circuits that have a common trip bar.

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Duane Degn wrote 03/18/2015 at 03:38 point

This sure looks like an interesting project. I'm following with interest. Thanks for documenting this.

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Nick Sayer wrote 03/18/2015 at 03:42 point

You're welcome. I can't stress enough that folks need to be very cautious if they wish to attempt it. That said, I've built more than one of these with this design, and they've performed their function perfectly.

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