Testing 48A Charging on 40A Circuit

[gavine]

I want to do this test to find out how much time it would take to trip the breaker running at 48A instead of the rated 40A.

A little background. It's a 50A breaker protecting 6AWG wire to a Tesla HPWC. I turned the HPWC up to 48A and did two tests so far, charging with the car set at 48A. Both times, I had to leave before it tripped the breaker. First time was only 30 minutes and second time was a little more than 1 hour. Temperature in the garage was around 40F both times (not sure if that matters).

I periodically felt the breaker and, even after an hour of charging, it felt "warm" at most (and my hands were cold too), It would need to get fairly hot before it trips, right?

Does anyone have any intuition as to how long it should take to trip the breaker? I figured it would not take long at all. Otherwise, I probably wouldn't have bothered doing this test in the first place.

 

[TexasEV]

Are you trying for the Darwin Award?

 

[NeverFollow]

I want to do this test to find out how much time it would take to trip the breaker running at 48A instead of the rated 40A.

Does anyone have any intuition as to how long it should take to trip the breaker?

I figured it would not take long at all. Otherwise, I probably wouldn't have bothered doing this test in the first place.

I am not really knowledgeable on this subject, but I did some researches for finding the correct AWG wires size fore my EVSE.

To give a quick answer, when evaluating the capacity of an electric system,
it's not just the Amperage but the estimated nominal temperature on any point of a line under load that need to be considered.
Typically you try to verify that any components of your wiring will stay up to 60 degree Centigrade under continuous load.

So a system could go up to 75 degree Centigrade to accommodate some burst,
like when an electrical motor will start without triggering a breaker all the time.

Don't take my reply as a technical and scientific answer. Consult with an electrician otherwise.
But I just wanted to give you an overall perception that I have about understanding
why a 40 A breaker don't get triggered when a particular given threshold was reached.

 

[AWDtsla]

I want to do this test to find out how much time it would take to trip the breaker running at 48A instead of the rated 40A.

A little background. It's a 50A breaker protecting 6AWG wire to a Tesla HPWC. I turned the HPWC up to 48A and did two tests so far, charging with the car set at 48A. Both times, I had to leave before it tripped the breaker. First time was only 30 minutes and second time was a little more than 1 hour. Temperature in the garage was around 40F both times (not sure if that matters).

I periodically felt the breaker and, even after an hour of charging, it felt "warm" at most (and my hands were cold too), It would need to get fairly hot before it trips, right?

Does anyone have any intuition as to how long it should take to trip the breaker? I figured it would not take long at all. Otherwise, I probably wouldn't have bothered doing this test in the first place.

The breaker might NEVER trip, and your house could burn down in the process. It is not based on temperature, although it is obviously not immune to temperature affecting the operation of the device.

 

[iluvmacs]

The breaker might NEVER trip, and your house could burn down in the process. It is not based on temperature, although it is obviously not immune to temperature affecting the operation of the device.

?? Any household breaker is designed to work by tripping when it gets hot enough inside. There are fully magnetic breakers, but they are not used residentially (and are relatively rare in commercial/industrial applications anyway).

 

[mongo]

A 50 A breaker with 48A load in a cold environment should not trip and even NM-B (Romex) with 65C 60C insulation is rated for 55 Amps. So, it will be a boring test assuming connections are tight.

Look up your breaker's trip curve to see the specific activation points vs temperature, time, and load.

From the QO data sheet:

Edit: corrected NM-B insulation temp

 

[iluvmacs]

Your "little bit of background" didn't include why you wanted to violate the NEC and the ratings of the wire & device. How about you just put it back to the right setting and back away?

 

[mspohr]

Are you trying for the Darwin Award?

No, he just wants to see how house insurance works.

 

[mongo]

Your "little bit of background" didn't include why you wanted to violate the NEC and the ratings of the wire & device. How about you just put it back to the right setting and back away?

Until the test exceeds 3 hours (NEC definition of continuous load), nothing is being violated. Longest test thus far was slightly over 1 hour.

 

[gavine]

Guys. I was monitoring the situation the entire time. The wire is protected by the 50A breaker. I just wanted to see if charging at 48A would trip the breaker quickly or not. Obviously, it didn't. I don't plan on charging at 48A normally. This was just a test. I figured the breaker and wire would at least get more than warm but they didn't.

 

[gavine]

Until the test exceeds 3 hours (NEC definition of continuous load), nothing is being violated. Longest test thus far was slightly over 1 hour.

Thank you for this. I don't drain my battery enough to allow for 3 hours of 48A charging so this test is complete.

 

[mongo]

Guys. I was monitoring the situation the entire time. The wire is protected by the 50A breaker. I just wanted to see if charging at 48A would trip the breaker quickly or not. Obviously, it didn't. I don't plan on charging at 48A normally. This was just a test. I figured the breaker and wire would at least get more than warm but they didn't.

Wire dissipation is 44% higher at 48A than 40A, but that is still a small number.
6 AWG copper is 0.3951 mOhm per foot. At 48A, that is 0.91W per foot per conductor or 1.82W/ft for the circuit. Normal would be 1.26 W for the pair.

 

[animorph]

48A is not going to quickly trip a 50A breaker. It's below the breaker's rating.

 

[gavine]

48A is not going to quickly trip a 50A breaker. It's below the breaker's rating.

Good point, but, by code is should be a 60A breaker which is why I wanted to test it. I knew it would take a while, but I didn't realize 3+ hours most likely.

 

[mongo]

Good point, but, by code is should be a 60A breaker which is why I wanted to test it. I knew it would take a while, but I didn't realize 3+ hours most likely.

Per specification, it will never trip unless the ambient temp is sufficiently above 40C.

 

[miimura]

Here is a sample thermal trip curve for Square D QO circuit breakers 45-70 amps.

From the catalog.
You can see that at 2X the breaker's rated amperage, it should take 10-60 seconds to trip. At 1.5X it might take 5 minutes.

 

[gavine]

This is turning out to be a good thread. Started off a little shaky though.

 

[gavine]

Are you trying for the Darwin Award?

Dawin is about SURVIVAL of the fittest. So, by definition, winning the Darwin Award would mean I'm the fittest.

 

[mongo]

Dawin is about SURVIVAL of the fittest. So, by definition, winning the Darwin Award would mean I'm the fittest.

It's counter intuitive, but the Darwin Awards are for people removing themselves from the gene pool in interesting fashion.
Darwin Awards. Chlorinating The Gene Pool.

(nothing you did was actually risky (beyond the possibility of finding poor wiring the hard way) people who pop breakers in their kitchen/ bathroom are more at risk).

 

[Evbwcaer]

I have wondered about pulsing a charger. Just a totally random scenario, 40 amps for 1 minute, 50 amps for 30 seconds, repeat. 30 seconds is a lot shorter than 3 hours.....

I know it seems stupid to ponder, but I can imagine a scenario in the future when EV's make up over 50% of cars and renewables can generate over 100% of demand. If you''re looking at millions of cars charging just 5% faster, when the sun is blazing or the wind is blowing, it could real money and/or kWh being saved and/or not wasted.