How to do a load calculation for adding a Tesla wall charger circuit to my sub panel

[mongo]

What about using the formula sqrt(0.707) * breaker amp = load ? Will that work?

No? Isn't that attempting to convert peak to RMS (of a sinusoid) when the value is already RMS?

I suppose if you had something tryingto simulate a constant current load vs constant resistance, something like that might come into play; but the power company might not like you.

 

[cmaster]

No? Isn't that attempting to convert peak to RMS (of a sinusoid) when the value is already RMS?

I suppose if you had something tryingto simulate a constant current load vs constant resistance, something like that might come into play; but the power company might not like you.

What about this, if I want a 32 amp Level Charger install, I would need a 40amp breaker, no? Or, a 72 amp Level 2 charger, I would need a 90amp breaker? You don't want to have the breaker run at 100% duty cycle of 80a load on a 80a circuit breaker, right? Using the sqrt(0.707) * breaker = load simplifies the math, I think it works.

 

[mongo]

What about this, if I want a 32 amp Level Charger install, I would need a 40amp breaker, no? Or, a 72 amp Level 2 charger, I would need a 80amp breaker? You don't want to have the breaker run at 100% duty cycle of 80a load on a 80a circuit breaker, right?

80A breaker gives 64A charge. Continuous loads are uprated 125% (breaker/ circuit derated to 80%).

 

[cmaster]

80A breaker gives 64A charge. Continuous loads are uprated 125% (breaker/ circuit derated to 80%).

I miscalculated, but, you see the point of using the sqrt(0.707) * breaker = load? Obviously, you need to buy standard breakers at the home depot or Lowes or whichever electrical supplier near the home. But, using their inventory, use the math to get the result wanted. I think it's fair to do so.

 

[mongo]

I miscalculated, but, you see the point of using the sqrt(0.707) * breaker = load? Obviously, you need to buy standard breakers at the home depot or Lowes or whichever electrical supplier near the home. But, using their inventory, use the math to get the result wanted. I think it's fair to do so.

Can't say I follow.
0.707 * peakV = RMS V
0.707 = sqrt(2)/2 = 1/sqrt(2)
0.84 = sqrt(0.707)

Completely seperately:
0.8 = derating factor

 

[cmaster]

Can't say I follow.
0.707 * peakV = RMS V
0.707 = sqrt(2)/2 = 1/sqrt(2)
0.84 = sqrt(0.707)

Completely seperately:
0.8 = derating factor

Vrms is done in a lot of places. 0.8 derating factor are in actually are just averages of a specific duty cycle for use. So, you can use 80% or Vrms. The electrical wire rating will be the same for both types of calculations.

 

[mongo]

Vrms is done in a lot of places. 0.8 derating factor are in actually are just averages of a specific duty cycle for use. So, you can use 80% or Vrms. The electrical wire rating will be the same for both types of calculations.

I don't follow. 120V is RMS and a 20 Amp breaker is 20A RMS. Peak voltage is 170 or so 120*sqrt(2)=120/0.707.

 

[Cosmacelf]

I thought I was done requesting advice on my project and ready to execute, but here I am back again.
I hired an electrician for the project and he came up with an interesting advice, which I tried to capture in this enclosed line diagram.
He reconfirmed the calculation that you kindly did for me for the AC circuit and came up with the load of 66 amp for these two existing ACs on 100 amp feed. But when he did the load calculation again with new 32 amp Charger load added to the AC Circuit, he computed the load for all the three loads at name plate rating ( RLA 28.1 amp + FLA 1.4 amp at 230V ) for condensers and configured load of 32 amp / 240V for the charger and then applied 25% demand factor to the largest load in the group , which is the charger. This way, the total load is 98 amp and so can be supported on 100 amp AC feed from the main panel without altering anything on that panel.

This option involves installing a sub panel , but avoids any service shutdown or main panel upgrade or involve Tesla Solar - which is a big positive. Additionally, oversizing the panel and the wire for the charger provides scope for future upgrade.

Can I request you to take a look at this diagram and validate if it complies with the code ? Thanks

Looks like it would work, but I don’t follow that upsizing the feed to the new 225A panel would help with future expansion since it is still limited by a 100A breaker.

 

[mongo]

Looks like it would work, but I don’t follow that upsizing the feed to the new 225A panel would help with future expansion since it is still limited by a 100A breaker.

Right, but if the main panel/ seevice entrance were upgraded, say to a split output 400A meter or 200A with feed through lugs, nothing in that branch would need redone.

 

[wwhitney]

But when he did the load calculation again with new 32 amp Charger load added to the AC Circuit, he computed the load for all the three loads at name plate rating ( RLA 28.1 amp + FLA 1.4 amp at 230V ) for condensers and configured load of 32 amp / 240V for the charger and then applied 25% demand factor to the largest load in the group , which is the charger.

That is not the correct computational method. For the two identical A/C units, each with two motors of size RLA > FLA, and an EVSE of continuous rating X amps, the load is (1.25 * RLA + FLA) + (RLA + FLA) + 1.25 * X. The rules that require 25% extra for the largest motor, and that require a 125% factor on the EVSE, are two separate rules that both apply; they do not combine to a single extra 25% for the largest load. [Also, the motors (A/C units) need to be calculated at the nominal system voltage of 240V, not the nameplate rating of 230V.]

Having said that, in all likelihood the arrangement shown with a 32A continuous EVSE will work fine and the 100A breaker will never trip. In theory it could trip if the EVSE has been running several hours at a full 32A current, and one A/C unit is running at its full current, and the other A/C unit is off and tries to start up. [And while the service panel is in full sun on a hot summer day, so the ambient temperature is elevated.] Be sure to get an EVSE that is configurable, and if you find that happening, you can downsize the EVSE breaker to 30A and dial back the EVSE to 24A continuous.

Cheers, Wayne

 

[wwhitney]

0.8 derating factor are in actually are just averages of a specific duty cycle for use.

No. The 125% continuous use factor for load calculations and conductor/OCPD sizing is not related to duty cycle. It is a semi-arbitrary factor chosen to reflect the fact that normal OCPD are only tested to hold their rated current in open air, but they are used inside enclosures that contain other OCPD, creating a thermal environment in which heat rejection is worse than open air conditions.

Cheers, Wayne

 

[holeydonut]

One of these days, I think we're going to learn that wwhitney is actually Jim Pauley who heads up the NFPA.

 

[dafish]

....

3VA is for general usage including all non-major appliances. LED lighting and LCD TVs are less power hungry, but gaming computers are more.

Guy, regarding non-maor appliances allowed within the 3va "general lighting" category, which of these fall within that?

Hair dryer

Iron

Humidifier / window AC

Coffee Pot

Counter Broiler

Fryer/Pressure Pot

Fridge/Freezer #1

Fridge/Freezer #2

Fridge/Freezer #3

Sump pump

Thanks!

-d

 

[Sophias_dad]

Hair dryer

Iron

Humidifier / window AC

Coffee Pot

Counter Broiler

Fryer/Pressure Pot

Fridge/Freezer #1

Fridge/Freezer #2

Fridge/Freezer #3

Sump pump

Hair dryer, iron, coffee pot, counter broiler(barely), fryer/Pressure Pot, Sump pump are non-major

AC(even window units) and fridge/freezers need somewhat special treatment.

These are only my opinion.

 

[mongo]

Guy, regarding non-maor appliances allowed within the 3va "general lighting" category, which of these fall within that?

Hair dryer

Iron

Humidifier / window AC

Coffee Pot

Counter Broiler

Fryer/Pressure Pot

Fridge/Freezer #1

Fridge/Freezer #2

Fridge/Freezer #3

Sump pump

Thanks!

-d

Mongo only pawn in game of life, not electrician, but he'll give his fifth of a dime.

All of them, those are all plug in versus hardwired.
Interestingly, many of the items you list have circuits specifically assigned to them, bathroom, kitchen appliance, fridge. If planned ahead of time, you may end up with more than the base 3VA/ sqft of general circuits.

 

[dafish]

Thanks guys. It seems to come down to:

General Use Lighting and Receptacles
Vs.
Small Appliances

In looking further at 210.52 it seems like refridgerator and kitchen circuits for coffee, etc are considered "Small Appliance" while the rest are seemingly "General Puporse" and it seems to be circuits that get counted, not devices or receptacles.

I can't find a thing for the sump pump, so I just called it a small appliance and moved on.

What's interesting is the huge difference in results. I get 272VA using one method and 191VA using the optional "Home Dwelling" method.

Most of that variance is due to the exceedingly generous allowance of EV circuits being included in the 40% multiplier of the optional Home Dwelling method. If I pull out the two WC's from I get 152 vs 143 as preliminary numbers.

Thanks again,
-d

 

[mongo]

Thanks guys. It seems to come down to:

General Use Lighting and Receptacles
Vs.
Small Appliances

In looking further at 210.52 it seems like refridgerator and kitchen circuits for coffee, etc are considered "Small Appliance" while the rest are seemingly "General Puporse" and it seems to be circuits that get counted, not devices or receptacles.

I can't find a thing for the sump pump, so I just called it a small appliance and moved on.

What's interesting is the huge difference in results. I get 272VA using one method and 191VA using the optional "Home Dwelling" method.

Most of that variance is due to the exceedingly generous allowance of EV circuits being included in the 40% multiplier of the optional Home Dwelling method. If I pull out the two WC's from I get 152 vs 143 as preliminary numbers.

Thanks again,
-d

Your post made me realize I misspoke, the appliance circuits add to the 3VA/ sq ft total and result in a combined general circuit total. You'll always have more than 3VA/sqft of general 'lighting circuits.

 

[mtmra70]

Food for thought, it might be worth considering just redoing your entire setup. Our home needed a main panel rework and the Tesla gave us the excuse to do it. Long story short, we ended up rerouting our main feed, got a 200A panel in the garage, Tesla charger installed, etc. It was a win-win-win all araound, and I qualified for some tax credits for installing a home EV charger.

 

[Eric33432]

Wonder what the OP ended up doing? This is like reading a mystery novel and finding the last few pages were ripped out of the book!

I would have listened to his last electrician, and installed the 100 amp sub panel for the AC and 40 amp (32 amp actual charging) EVSE. But depending on how long the wiring is to the AC units, I might get rid of the feeder to the junction box and install separate breakers in the new sub panel for each AC unit.

In my experience, most people overestimate their total demand. Put a clamp on current meter on that 100 amp feeder to your two existing AC units and I bet you will find the actual current is much less than the full load amps.