What kind of extension cord can be used for charging my 2017 Prius Prime?

Toy instructs to not use an extension cord for that purpose. I would speculate this may be because most owners would use the cheap #16 or #14 extension cords that they have used for years for other applications. In time the extension cord end receptacle gets loosed and looser on the device cable plug prongs. This contributes to a higher resistance connection at that point, causing the cord end to heat up.
I use a #12 AWG three copper wire extension cord to charge my new '24 PP. Its only about 25 ft long. For anything longer that that, I have and use a #10 AWG three copper wire extension cord, both cords help reduce voltage drop and wire heating along the length of the extension cord. I check the ends of the cord in use by touch, after being used for a full charge, feeling for hot plug body. This works for me. You will have to make your own assesment and do what is best for you. I would recommend a #10 3wire copper extension cord up to 50' long max. But do understand thet Toy does not recommend this.

 

Short answer, If the cord is rated for it, there should be no problem. Most aren't.

Look at the current ratings on the extension cords at the store. Most of them are cheap and use only #14, or #16 wires (like @RandyPete pointed out). The small wires have more of an effect on longer extension cords than short ones too. If you want to run 12 amps through a 100 foot extension cord, look one that is rated for that. It will be listed on the cable. It will also be way more expensive, thick, and harder to find in the store cable too. Which is probably why Toyota says don't use an extension cord.

For example, here's a 50 ft one rated for 15 amps: https://www.amazon.com/Clear-Power-Extension-Resistant-CP10145/dp/B084RYCWY4/ref=asc_df_B084RYCWY4?

 

If the cord is #12 AWG copper wire, it has wire rated for 20A current. The Plug/Receptacle NEMA class ends are rated at 15A/20A. Good cord choice for short extension to Prius Prime 2023-24 power charging cable device.

 

The thinner the wire(higher gauge number), the higher internal resistance, the more heat that gets generated as the amps go up. Then the longer the cord, the more resistance and potential heat generated. That's why thicker wire is needed for longer extension cords.

The current isn't just traveling along the extension cord, but also the cable between outlet and panel. The cord can be safe for charging an EV, but it might lead to the cable in the wall overheating.

 

From above
"The cord can be safe for charging an EV, but it might lead to the cable in the wall overheating."

This is absolutely untrue. The Plug on the end of the extension cord inserted into the receptacle at the house wiring box device may heat up if the plug and receptacel are not fully engaged or either plug or receptacle internal/external parts are worn out. But the extension cord use will not increase the overheating of the cable in the wall !

 

@Joe Wall You'll also want to look at securing the charger to the vehicle as it can cost over $800 to replace (unless it got cheaper from the Gen4 days).

I use a narrow hasp Masterlock and a plastic covered steel cable that is looped on both ends. You run the cable through the rear left wheel and then secure the lock onto both looped cable ends and on the charger cable between the charging end and the power "brick". Unless the thieves have a bolt cutter, they would need to destroy the brick to steal it.

Personally, I would not lock the charger to the Prime using the lock button. I would rather thieves destroy the charger than the charging port on the Prime.

 

There is supposed to be a breaker or a fuse down in the panel to prevent exactly that. If there's a 20 amp breaker it means there's 12 gauge wire in there and you can safely run a twenty amp load as long as you like. If there is only 14 gauge wire in the wall then there should be a 15 amp breaker in the box, and you can run a 15 amp load as long as you like. There are could be older properties with 16 gauge wire, but they would have a 10 amp fuse and thus you'd blow a fuse but not hurt anything.

Do we know how many amps a 2017 Prius will pull? If OP were doing a little welding with an ordinary outlet, that might worry me. My dearly beloved 2012 PIP pulled something like 13 or 15 amps. I gave it a 12 gauge cord and didn't worry about it.

 

Technically, Toyota does not allow extensions cords.

Nevertheless, any three-prong (grounded) extension cord that is rated 15 A will work. Prius Prime uses 12 A @ 120 V. For outdoors, use an outdoor-certified extension cord. The length is not really issue, as it will only result in a lower voltage and a longer charging time. Follow the safety instructions with any extension cord, such as not coiling them, which raises their temperature by reducing the area exposed to cool air, etc.

 

Then why are thicker gauge wires needed for longer extension cords?

 

A 12 gauge extension cord up to like 30 ft would be fine. The cord might get stolen though. I got a pack of two at Costco for $30 I think it was, for outdoor tools. I think Toyota is being careful because the public may use any old extension cord. The Toyota supplied cable is 14 gauge.

 

Simple: The total resistance R is proportional to the the length of the wire and inversely proportional to the cross-sectional area of the wire. For a given current I, the voltage drop ΔV along the wire is proportional to the resistance R due to Ohm's law ΔV = I R. Therefore, to reduce ΔV and keep the voltage V close to 120 V, you need a thicker gauge for longer extension cables.

This is not only true for extension cords but also house wiring. For longer wires, you need a thicker gauge to keep ΔV small.

 

If the appliance is calling for a set amount off watts, and the volts are dropping due to cable length, the amp draw will increase?

 

The current drawn will typically decrease actually because the voltage is lower unless the appliance can control the current. For simple appliances like a hair dryer, the current will certainly decrease when there is a voltage drop due to the length of the wiring. That's because the hair dryer is primarily a resistor; so, Ohm's law applies. For my Gen 4 Prius Prime, the current stays constant precisely at 12.00 A irrespective of the voltage due to a controlling mechanism. My voltage at my garage is typically well below 120 V due to the length of the wiring.

Incidentally, I learned about the wiring gauge that needs to be chosen based on the wire length and current drawn when I took a course named illumination during my EE undergraduate studies.

 

So using an extension cord could lead to longer charging times.

 

I think there are higher heat rated cables. The Chrysler evse I am using has #16 wire size. On the jacket it says for EV. It is a bit lighter and easier to store than the toyota #14. Hot plates, tI have seen 16 with a better jacket, substitute for 14.

 

Only for appliances that are effectively negative resistance, such as those internally powered by SMPS (switch mode power supplies) set for constant power. The cars certainly have SMPSs internally in their battery chargers, but are regulated to constant input current mode (not constant power) according to what the outside EVSE declares as its current limit.

I understand my Toyota EVSE (RAV4, not Prius, but should be the same unit) is capped at 12 amps, in order to remain code complaint on common 15-amp household circuits (NEC allows such long-term loads to draw no more than 80% of the breaker rating). When used off-label at 240V, it doesn't drop to 6 amps as a constant power appliance would, but still draws 12 amps, cutting the charging time to slightly under half.

The RAV4 can be set from the driver's seat to be capped at 8, 16, or Max amps (~28 for this car, or ~14 for some older lower trim RAVs, and probably the same ~14 for Prius Primes).

I'm remembering earlier Prius Primes also having a 8-amps-max setting. This could be useful for people having cords or plugs getting too warm, or having difficulty from long or lighter duty extension cords. This will necessarily lengthen the charging time, but could be useful for drivers having problems at 12 amps.

 

Yes, it will lead to slightly longer charging times. Probably negligible like a few percent for reasonable lengths and 12 AWG. For 25 ft and 12 AWG, the voltage only drops to from 120 V to about 119 V @ 12 A. You need to know the temperature of the wire for a precise calculation. So, that's only about a one-percent-longer charging time in that case.

For 25 ft and 16 AWG, the round-trip (line + neutral wires) voltage drop along the extension cord is about 2.7 V for 12 A. So, in that case, the voltage drops from 120.0 V to 117.3 V, and the charging time increases by about 2.3%, which increases the Level 1 (120 V) charging time for Gen 5 Prius Prime from 11 hours to 11 hours and 15 minutes.

 

I thought the onboard charger did the charging and the charging device is basically a controlled switch?
With the watt meter I see 120 voltage outlets dropping by 10 volts or more sometimes. One was 17 volts drop. So if it is 100 volts and the watts are 1200, the amps are still 12. As I understand it that means a reduction in charging time. 1.2 kw at 100 v and 1.44kw at 120 v. These sketchy outlets are in public areas, at home it’s much better.
I am thinking 3.3 kw from the G4 charger on 240 v. overloads the 12 amp evse, but since it’s high quality it can take the 13.75 amps. On the G5 maybe not, as it charges at 3.5 kw, or 14.6 amps. It could be that close from frying.

 

Are those higher powers ever reached on Toyota's 12 amp EVSE?

Or only on higher current EVSEs such as public charging stations? I would hope they occur only with the later.