How to Test if an 120V Outlet Is Safe for Charging

[Mach-Lee]

While a dedicated 240V outlet is the safest/fastest for charging, sometimes 120V is the only available option. Not all 120V outlets are suitable for extended duration Level 1 EV charging. Some may have poor wiring or worn contacts, which could potentially cause a fire if used. This guide will explain how to do some simple safety checks to prevent fires, charging errors, and ensure the fastest charging speed. You will need:

• An outlet tester
• An AC voltmeter
• A 1500W space heater

These items are all relatively inexpensive and you may already have them. Follow these steps to test:

1. Visual inspection - Verify the outlet is in decent shape, has a ground prong, and that it isn't wet. Do not use if the outlet has burn marks, cracking, or broken parts. Remove any adapters or splitters, these are not to be used with EV charging for safety. The cord must plug directly into the outlet with nothing in between.
2. Use outlet tester - Verify outlet is wired correctly (see guide on device). Two amber lights on my tester is correct. Do not use an ungrounded outlet for EV charging, ground is required to prevent shock hazards. You may optionally test the GFCI on the circuit by pressing the test button if equipped (afterwards reset the button on the nearby tripped GFCI receptacle). A GFCI-protected outlet is not required since the EVSE has its own GFCI protection built in.
   
3. Measure open circuit voltage - Measure and record the open circuit (no load) voltage with a suitable voltmeter.
   
4. Verify plug insertion/retention force - Plug in the space heater and verify the plug is held tightly. Do not turn it on yet. Gently tug on the cord, it should not move or come out easily. If the plug inserts easily or pops out with less than a 1/2 lb. force applied, the receptacle is worn and should be replaced.
   
5. Measure voltage under load - Turn the space heater on high (1500W setting) and measure the new voltage. The voltage drop should not be more than 8%*, or about 10V for the 12A space heater load. In this example the voltage drop was 7 volts (122-115=7), which is acceptable since it is less than 10V. If the voltage drop is too high, or if the reading under load is less than 108V, use another outlet.
   
6. Check for bootleg ground (optional) - Measure the neutral to ground voltage while the heater is running on high (circuit loaded). Only do this if you are comfortable inserting multimeter leads into outlets, do not let the bare leads touch each other or your fingers (practice on a dead outlet first if you haven't done this before). Neutral is the longer slot if wired correctly. The reading should be roughly half of the total voltage drop from the previous step if a true isolated ground path exists. In this case the 3V reading is about half of the 7V total voltage drop, which is normal/expected for an isolated ground path. If you have a bootleg ground (neutral and ground illegally connected together) you will see little to no N-G voltage difference. Double check your leads are inserted properly if you see a low reading.
   
7. Check for heat buildup - Run the heater on high for 5 minutes, then turn off, unplug, and immediately feel the prongs. They may be warm but should not be too hot to touch.
   
8. Pass/Fail - If the outlet passes all the above tests, it is likely safe to use for extended duration charging and will not produce charge errors. You should always monitor the plug for heat the first time using an outlet for charging (come back and feel it periodically). If the outlet did not pass, repeat the tests on another outlet that is electrically closer to the main circuit panel. Ideally you want to use the first outlet in the circuit chain (typically the GFCI receptacle in garages) which will have the lowest voltage drop. If the outlet does not pass, the issue could be in the outlet or upstream in another outlet or circuit splice. It may take some detective work to find the fault location, it's recommended that you test all the outlets in your garage (including ceiling outlets) if you are looking for a fault. If there is a bad connection you will see a sudden increase in voltage drop from one outlet to the next. This information may be useful for your electrician. Here is an example of a dangerous outlet that should NOT be used for EV charging (extreme 15V/12% voltage drop):
   

*The NEC specifies a 5% max voltage drop for performance rather than safety reasons. However, most houses that are properly wired will have some circuits that test above the 5% NEC guideline, with 2-8% being typical of new construction. There is no official maximum safe voltage drop, although many experts believe a 10% drop at the circuit's rated current is the most that can be considered safe. This is the basis of the 8% threshold figure used here, as a 10% drop on a 15A circuit is equivalent to a 8% drop with a 12A space heater load.

Other loads - Please be careful of other loads on the same circuit while EV charging, as this could cause an overloaded circuit/tripped breaker. The Mach-E mobile charger draws 12 amps on 120V. Avoid running any heaters, A/C units, air compressors, etc on the shared circuit while charging. The amp draw cannot exceed the circuit breaker rating (e.g. you only have 3 additional amps to work with on a 15A circuit). Older houses may have very confusing wiring that has been added onto over the years, as a result you may not know what things in your house share the garage circuit. At one older house I worked on half the kitchen and living area were also on the garage circuit. Charging and running the microwave or vacuum cleaner at the same time would blow the breaker. To understand what's on the same circuit, flip off the garage circuit breaker (verify labeling) and see what's dead in your house, those items share the same circuit. You may be surprised.

Outdoor outlets - Be especially careful using outdoor outlets. It is somewhat common to find forgotten outdoor outlets that are corroded internally due to moisture damage. GFCI outlets may no longer function. Inspection is important. If anything strange happens (including noises), stop using the outlet immediately. Knowing which circuit the outlets belong to is also generally a challenge, as they may share a circuit with the garage, other outdoor outlets, or the room circuit on the inside of the wall. Modern code requires the use of an in-use cover, this larger cover keeps rain out of the connection even while something is plugged in, hence the name.

Receptacle replacement - Most people don't know that receptacles are supposed to be replaced periodically due to wear and age, just like tires on your car. If you are using a 120V outlet as your main charging source and it's older than about 5 years, I recommend you replace the outlet to be safe (they are cheap). This will ensure a good, safe connection. Outlets come in different grades—buy a spec or hospital grade outlet which is designed to be extra-durable (see figure below). Also try to avoid plugging/unpluging the charger frequently as this will wear out the outlet faster. Leave the EVSE/charger plugged in all the time and mount it to the wall using the wall bracket.

Extension cords - Generally you should avoid using an extension cord while EV charging, as this adds additional risk and slows charging. Per Ford's instructions, you should not use an extension cord with the Mobile Charger, it should only be directly plugged into an outlet with no adapters or splitters. However if you just can't get the car close enough to an outlet, an extension cord can be a lifesaver. Use the shortest cord that you can, and make sure it is 14 gauge or larger. 25' should be long enough to get you out of most predicaments (the total length will be 45' when combined with the 20' charger cable). I carry a 14 gauge 25' cord for emergency use. When using an extension cord, always uncoil it fully for heat dissipation. If you leave it in a tight coil, the wires may overheat and cause a short circuit or fire.

Heat buildup - It should be explained that heat buildup is a result of bad connections. The worse the connection and higher the current, the more heat is generated and the higher the voltage drop. Because EVs charge for many hours, this heat can accumulate to the point it causes insulation to melt and wires to short out. Wood may also start on fire one day with enough heat over a long time (pyrolysis causing lowered-temperature ignition). Heat may build up anywhere in the circuit—in the cord, the plug, the outlet, in other outlets, wires in the walls, other junctions, or at the breaker. You may not be able to detect where the heat is building up, which is why checking the voltage drop is important. A low voltage drop means you have a good circuit that won't produce much heat.

Charger derate - The Ford Mobile Charger EVSE includes circuitry that monitors the voltage and temperature of the plug while charging. If low voltage or excessive temperatures are detected, the EVSE will drop the current draw (amber light). This is a last-measure safety feature meant to reduce electrical fires on poor circuits. If you see the flashing amber light, you should stop and move the charger to another outlet that has tested good. An electrical fire is still possible on a poor outlet even with this feature in place, so it's important that you choose a good outlet for safety, and for the fastest charging speed (lower voltage drop outlets charge faster). Note: a number of Ford Mobile Chargers display the amber light due to internal overheating, this also limits charge speed. The outlet being used should be tested per above to make sure it's not the cause of the overheating (the outlet overheating will generally cause a flashing rather than steady amber light).

Disclaimer - Do not perform electrical work if you are untrained. You may be injured or burn down your house. If you find a bad outlet, have an electrician replace it for you.

Someone is going to ask: Hughes Autoformers Dual Color Voltmeter - Useful for monitoring voltage while charging.

 

[RickMachE]

Seems very thorough. But the average consumer is never going to do this, nor even consider doing this.

 

[Mach-Lee]

Seems very thorough. But the average consumer is never going to do this, nor even consider doing this.

If it’s too complicated then folks can just buy the plug-in voltmeter to keep an eye on the voltage while charging. Voltage below 112V = bad outlet is about as simple as I can make it.

 

[RickMachE]

OK. I applaud your process.

At what point should consumers not just plug things in? Is this something that you think people should do if they buy a space heater? Air compressor?

I ask seriously, because 99.9% of people are never going to do this.

 

[P. T. Magoo]

Could well be that most consumers will not want this level of expertise, but I tend to think of this forum as a self-selecting sample of rather geeky types. People that don't want this level of knowledge are free to skip over the thread.

Looking at people's avatar descriptions, I see a lot (a lot) of engineers and healthcare professionals. People who solve problems for a living. People for whom differentially diagnosing and correcting problems, and even troubleshooting potential problems prophylactically, is in their blood.

I'm no electrician but I find this procedure fascinating and may just use it check out all my garage outlets just for fun.

 

[tannerk89]

I am unlikely to ever go into this much detail to check a 120V outlet because it’s a lot of effort for the emergency time I will use an unknown 120V outlet to charge my car. However, one suggestion I would say is that in place of a space heater to put load on the outlet, just use the mach-e, and measure voltage drop/heat build up/etc. That way you don’t have to lug a space heater with you, in addition to an electricians tool box, in the event you might need to test an outlet. Great info on how to check the electrical safety of an outlet though!

 

[Mach-Lee]

I am unlikely to ever go into this much detail to check a 120V outlet because it’s a lot of effort for the emergency time I will use an unknown 120V outlet to charge my car. However, one suggestion I would say is that in place of a space heater to put load on the outlet, just use the mach-e, and measure voltage drop/heat build up/etc. That way you don’t have to lug a space heater with you, in addition to an electricians tool box, in the event you might need to test an outlet. Great info on how to check the electrical safety of an outlet though!

This was intended more for checking your primary at-home charging outlet if you use 120V. But if you’re trying to charge in public then you could just use the small voltmeter and the Mach-E for the load. However the Mach-E is not a constant load, it ramps current up and down so you might be fooled into thinking a circuit is fine when it’s only drawing 8 amps due to heat or voltage drop. Therefore you’d need an additional way to check the current draw (scan tool or Kill A Watt meter). The space heater is a simple resistive load that shouldn’t change, so it’s best for testing purposes. If you’re on a trip you might have a hair dryer packed that you could use too. But you don’t need to carry these around in your car.

I've used these procedures in the past when dealing with bad outlets that won’t start an air compressor. Finding the best outlet at a location ensures the compressor starts reliably.

Ford could help out a lot here by displaying the live voltage and amperage on the screen while charging, just like Tesla does. Then people would be more likely to notice when they are using a bad outlet when they see low voltage and the amps drop.

 

[TonyaE]

Thanks! Sharing knowledge is an awesome thing.

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