Extension cord wire gauge for Prime

Yea, just the title may legitimately excite some folk and the Toyota manual says not to use extension cords, but...

In a few weeks I will be going on my first multi-thousand mile road trip in my new PPPXP (Pretty Prius Prime XSE Premium). I've realized that using public charging stations isn't practical unless they are a VERY short walking distance from the hotel. Checking Chargepoint and Plugshare doesn't look promising. I don't have 4 hours to waste sitting at a charging station when I'm only resting for 12-ish hours in each town. This isn't a complaint, it is exactly why I like wanted a PHEV, electric for around town and gas for road trips. Charging on a road trip is purely opportunistic.

So, the only practical way I feel is if I can sweet talk the hotel to letting me plug into a 110V outlet overnight just like I do at home. At least for my residential rates in Colorado, it is a whopping $2-ish to fully charge the battery, so hopefully I'll have some success in some places. If not, no big deal.

All that is the lead-in to the question, what gauge extension cord to get. If I go by the NEA rules I used for all my house wiring projects, 14 gauge is good for 15 amps and 12 gauge for 20 amps, so at least in theory 14 gauge is OK and 12 gauge would give some comfortable margin. The cord that comes with the car is pretty long, so I don't think I need too long of an extension cord. Any folk have some strong suggestions? Extension cords get really expensive really quickly above 12 gauge, so there is some pressure to not go overkill.

I'm pretty sure the discouraging of extension cords is because it is way too easy to pick a bad one and that would truly be catastrophic, so easier to "just say no". I'm trying to be smart about it. Hence if there is a good technical reason to still say no even with a well-rated cord, interested in hearing that too.

Side note: For EV folk, they like to measure charging rate in miles of range per hour of charging (another MPH). For my Prime at home it is pretty slow at roughly 4MPH (10 hours of charging for 40 miles of range). A level two charger should get about 10MPH. Good EVs can usually get 10x at a DCFC station I believe. I like to point out that on road trips though the Prime can get about 9,000MPH (450 miles for a 3-minute gas stop).

will

 

BTW, this is a good video from Technology Connections on extension cords. I have an IR camera, so will be looking at the cord when testing it ahead of the trip.

will

 

HacksawMark: OK on 12 gauge. With it, do you notice any temperature rise in the extension cord at all?

will

 

vviloovv: Yes, I know i'm asking something subjective, just looking for any kind of even qualitative info ahead of trying my first cord. Pretty sure HacksawMark knows the difference between solar and "IR" heating.

I also fully agree there are probably hundreds of boundary cases besides the ones you mention that could be an issue, but that is the mentality that has already gone into the Toyota users manual flat out ban on extension cords. That's the 100% solution they need to give. I'm going for the 99% case where it is a correctly wired, well maintained plug that meets code, and using a high quality heavy gauge extension cord. Just looking for what gauge should be my first attempt. I'll measure it and share that info with all the caveats I'm expecting from others.

will

 

Thanks HacksawMark, I think I am going to go the same as you, a nice name brand 25 foot, 12 gauge cord. I'll plan on instrumenting it as much as I can too, looking at voltage drop as well as a gander with my consumer grade IR camera to see what it looks like. Nice and easy way to look at the whole cable for hot spots. I'll be sure to include the adapter box thingy too for comparison.

Biomed01: Yep, agree too on longer needing a lower gauge, hence 25 foot seems long enough without going crazy. If the Prius cord is only 14 gauge (is that documented somewhere?), then feeling better about 12 gauge for my extension cord. I have a kill-a-watt and nice fluke multi-meter so can measure fairly accurately. Good point on someone stealing the cord! Hopefully most folk are honest though, so willing to take that risk.

I'm also going to get one of those extension cord plug protectors. They are a bit pricey, and the one I'm looking at may not be big enough to cover the "bigger than average" plug on the Prius cord, but I figure worth a try.

I'm a bit worried this is not going to be as useful as I hope. At the hotel I stayed at earlier this week I tried sweet talking the hotel staff into plugging in somewhere and struck out, so 0 for 1 so far. Hopefully I'll have better luck on my upcoming 5,000 mile trip. I did visit a public charger and while it worked, clearly wasn't worth it. It was far enough away from anything I just sat in the car for two hours of charging doing some reading and it netted me only 15 minutes of driving. Not a great return. I wanted to try it, but no surprise that for a road trip, level 1/2 charging is only going to make sense if I can plug in overnight at a hotel I'm staying at.

will

 

OK, I did go out and get an extension cord and protector and gathered a round of data. This post will be the numbers then a series of posts with the IR camera pictures. Mostly expected, a few surprises, and some fun distractions... The numbers and pictures were all taken after an hour of charging, so hopefully "soaked" when it comes to temperature.

The numbers:
Voltage at the outlet with no load: 122.6V
Voltage at the outlet under load: 117.6V (well, voltage at the Kill-a-Watt, more on that in a bit)
Voltage at the end of the extension cord: 116.7V
Current under load 11.66A

So, between house wiring, the outlet, and the kill-a-watt there is a 5V or 58W drop!! That is a lot. I'm using the kill-a-watt to measure KWh synchronized to my electric bill, so don't want to unplug it just yet, so I can't separate its loss vs the plug, I'll do that around the 22nd which is my billing cycle. As you will see later, the hottest thing between the outlet and the car is the kill-a-watt, so suspect it is contributing a lot to that 5V drop.

The extension cord was a 0.9V drop, so roughly an 8 watt loss. Not too bad. Given the extension cord is 12 gauge and HacksawMark says the Prius cord is 14 gauge I was expecting the extension cord to be cooler and it was.

So, overall, feel the extension cord losses (and heat) are not significant compared to other losses.

BTW, the plug protector I found at Lowes was big enough for anyone looking for one, this is it:
https://www.lowes.com/pd/Twist-and-Seal-0-8-ft-Plastic-Cord-Organizer/1000132805

Now for the fun pictures...

will

 

The Wall and Kill-a-watt:

By way of context, I'm using a consumer grade IR camera. It is decent, but probably not industrial-grade accurate. The garage and floor were around 15C.

I'm sure there are pretty decent IR losses in the house wiring, but clearly the kill-a-watt is dissapating a lot of power too. I look forward to separating them. One additional item of note here and in a later picture is the extension cord has neon bulbs in them, so you can clearly see that contributes to the power loss as the plugs have their own heat source. Data for the cords is next...

 

The Wall and Kill-a-watt:

By way of context, I'm using a consumer grade IR camera. It is decent, but probably not industrial-grade accurate. The garage and floor were around 15C.

I'm sure there are pretty decent IR losses in the house wiring, but clearly the kill-a-watt is dissapating a lot of power too. I look forward to separating them. One additional item of note here and in a later picture is the extension cord has neon bulbs in them, so you can clearly see that contributes to the power loss as the plugs have their own heat source. Data for the cords is next...

View attachment 248594

 

The Extension cord, Prius Cable, and Charge Controller:

No surprise that the charge controller is the hottest thing. Not posted here, but I took some subsequent pictures with that little cross pointing at the two cords to get their temperatures and they were 16 degrees for the extension cord and 21 degrees for the Prius cord.

Also of note again is the socket on the extension cord is warm too. I'm assuming this is due to the resistor for the neon bulb, but some heat could be coming from the contacts too.

One fun observation is that for the Prius cable, the conductors are clearly twisted inside the cable. It is not quite so obvious on the pictures but much more so in motion, but you can mostly make out the "dashes" on the Prius cord. The heat spirals around the cords length.

 

Car receptical:

The cord is a bit warmer here (maybe because it wasn't resting on the cold garage floor?), but nice to see no real heat around the plug itself.

 

Now or some fun pictures of the car itself. Again, this is after about an hour of charging.

Here is the back and passenger side. Clearly more heat inside than out.

 

Driver side:

Clearly here is where the fan is blowing the heat out from the internal charging circuitry. That temperature delta is more than enough to notice if you put you hand on the side of the car.

 

...and finally the inside:

Just confirmation where the fan is blowing the hot air, both inside and outside. It is tempting to pull the carpet and foam to look at the charger directly, but maybe I'll do that another day.

 

Anyway, distractions aside, I'm feeling pretty OK about the 25 foot 12-gauge extension cord. The plug protector gives a bit of extra peace of mind against water if being left overnight.

As vvillovv correctly points out and the Technology Connections video shows (and I agree), if going down this path there are plenty of wrong choices to make, but these look OK. Toyota's easy answer is to say "don't do it" so you don't blame them if you make some poor choices. That is what they should do.

I'll separate the outlet losses from the kill-a-watt losses when my billing cycle comes along and also I'd like to get the plug/receptical temperatures under no-load conditions to see if it is the connector or neon bulb resistor losses that are the major source of heat for the plug and receptical.

will