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

[aks20000]

I have 200 amp service. The outside main service panel ( 200 amp ) is of the type : meter main MLO Distribution panel with no breaker in between the meter ( supply side ) and the panel itself. I has two breakers on the load side. One 100 amp breaker feeds the two outdoor AC condenser units and another 100 amp breaker feeds a sub-panel in the garage. The panel also has 10 kw solar back feed through a 50 amp OCPD. There is no way a new load can be added to the outside main panel due to its type as mentioned above and due to solar back feed , even though it has physical space. In fact, Tesla solar has put a red sticker in the panel saying "Caution: No new load to be added".

The sub panel has typical electrical loads of a 3375 sq ft home , like a 240V dryer and oven ( the only two 240V loads ) , kitchen circuits , microwave, dishwasher, cloth washer, indoor hvac air handlers ( gas heating ) and a whole house fan. There is no other major electrical load like electric range or water heater etc.

I am looking to add a circuit for Tesla Wall charger to this sub panel and need to do a load calculation to see if this panel can support one more 50-60 amp OCPD for a 40-48amp wall charger circuit. The panel itself is 200 amp capable although feed is 100 amp only from the main panel and has plenty of space for new breakers.

I tried following the guidelines of my city from here to calculate the load, but gotten throughly confused as to how to apply these to my situation. This link explains to add up the load for the entire house as served from the outside main panel and with this approach, I find enough capacity to add 60 amp OCPD to the system.

But the new circuit is being added to the sub panel which has only 100 amp supply. If I apply the above load calculation approach for just the sub panel loads ( i.e. not counting the AC load and treating the supply as 100 amp service ) , then sub panel appears to be already overloaded leaving no scope for adding wall charger circuit.

I have consulted two electricians who will be doing the install after pulling the permit and one says that sub panel may be over loaded without any calculation , while the other did a calculation to confirm that a 50 amp OCPD can be added. This 2nd electrician says that load calculation is done with the main service of 200 amp as total capacity as explained on city's web site in the link above and hence his recommendation.

I can go with the 2nd electrician , but concerned what if the inspection fails . Myself being an electrical engineer, my common sense says that the sub panel with 100 amp feed and its existing load only should be considered for load calculation despite main service being 200 amps. But I also see that the wall charger circuit and other major loads on the panel ( dryer and electric oven ) will never run together.

Requesting experts on the forum to help enlighten me on this issue .

 

[Sophias_dad]

I'm not a professional, but IMHO your subpanel is probably already at its limit if you have an electric oven and electric dryer on it(in addition to a myriad of 120V loads). I can pretty much guarantee that it won't support a 50 amp charging circuit.

The first thing to check is the nameplate ratings for your oven and clothes dryer, because you can't rely on the OCPD to be a real indication of what they'll really use(its a maximum, but you are really looking for worst-normal-usage)

You aren't allowed to consider whether two things will be used together unless its very obvious they won't be used together. An AC unit and electric heat is one example of this. A clothes dryer and an electric oven is NOT.

 

[mongo]

I have 200 amp service. The outside main service panel ( 200 amp ) is of the type : meter main MLO Distribution panel with no breaker in between the meter ( supply side ) and the panel itself. I has two breakers on the load side. One 100 amp breaker feeds the two outdoor AC condenser units and another 100 amp breaker feeds a sub-panel in the garage. The panel also has 10 kw solar back feed through a 50 amp OCPD. There is no way a new load can be added to the outside main panel due to its type as mentioned above and due to solar back feed , even though it has physical space. In fact, Tesla solar has put a red sticker in the panel saying "Caution: No new load to be added".

The sub panel has typical electrical loads of a 3375 sq ft home , like a 240V dryer and oven ( the only two 240V loads ) , kitchen circuits , microwave, dishwasher, cloth washer, indoor hvac air handlers ( gas heating ) and a whole house fan. There is no other major electrical load like electric range or water heater etc.

I am looking to add a circuit for Tesla Wall charger to this sub panel and need to do a load calculation to see if this panel can support one more 50-60 amp OCPD for a 40-48amp wall charger circuit. The panel itself is 200 amp capable although feed is 100 amp only from the main panel and has plenty of space for new breakers.

I tried following the guidelines of my city from here to calculate the load, but gotten throughly confused as to how to apply these to my situation. This link explains to add up the load for the entire house as served from the outside main panel and with this approach, I find enough capacity to add 60 amp OCPD to the system.

But the new circuit is being added to the sub panel which has only 100 amp supply. If I apply the above load calculation approach for just the sub panel loads ( i.e. not counting the AC load and treating the supply as 100 amp service ) , then sub panel appears to be already overloaded leaving no scope for adding wall charger circuit.

I have consulted two electricians who will be doing the install after pulling the permit and one says that sub panel may be over loaded without any calculation , while the other did a calculation to confirm that a 50 amp OCPD can be added. This 2nd electrician says that load calculation is done with the main service of 200 amp as total capacity as explained on city's web site in the link above and hence his recommendation.

I can go with the 2nd electrician , but concerned what if the inspection fails . Myself being an electrical engineer, my common sense says that the sub panel with 100 amp feed and its existing load only should be considered for load calculation despite main service being 200 amps. But I also see that the wall charger circuit and other major loads on the panel ( dryer and electric oven ) will never run together.

Requesting experts on the forum to help enlighten me on this issue .

Not an electrician, but 50 seems off the table.

Your general/ lighting load of 3x3375 is over 40 Amps on its own. Then 23.5 for other loads and circuits putting you at around 65. Leaves 35 since it's a 100 Amp feed. Derate that for continuous load and the max EVSE charge rate is 28 Amps.

Edit: second air handler is another 1.7 Amps of load(1000*40%/240)

 

[Cosmacelf]

The dryer probably has a nameplate of maximum 24A, the oven probably around 32A/240V (assuming a 40A breaker for it, add 10A for a 50A breaker. So that’s 56A. The rest of the 120V loads for 3325 sq ft could indeed be 40A/240V of load. So yeah, no room to maneuver.

The biggest problem is that you have 100A feeding 2 AC units. Look at their nameplates, but I am guessing they draw at most 35A each. That is leaving 30A/240V unused. So, tapping into that 100A circuit somehow is one idea.

Another one is to either convert your dryer to gas (if you have piping), or to load share with it using one of the products listed at the bottom of this page: CarCharging.us. That would give you 24A charging, which is fine for most people.

 

[davewill]

DCC Electric has devices that would let you possibly load share with main panel (or the sub if that really can't work), and maybe get you full amperage charging. They aren't cheap, though, so if there's a solution that gets you at least a 20-30a circuit without such help, I'd probably go that way.

One thought I had was to go with the second electrician's recommendation, and if you fail the inspection, lower the amperage of the breaker to whatever the inspector will pass. I'm just a bit concerned that that may turn out to be too low to be acceptable. I don't know anything about your jurisdiction, but if a permit is pulled, can plans be submitted for approval before the work is done? That might get you an official answer before you have the work performed. If you have documentation from the solar install, that might be a great starting point, especially if they documented their load calculation.

 

[ATPMSD]

Tesla solar has put a red sticker in the panel saying "Caution: No new load to be added".

If this sticker is on the first panel, the one with the two 100-amp breakers, then I do not see how you can add anything. If this is the case you need to speak with an experienced electrician as this is not a DIY project.

 

[ewoodrick]

There's a number of seemingly weird things that occur with electrical loads, but they actually make sense.

But let's get down to facts first.
The panel with the 100 breaker is indeed limited to 100A of service. You can't pull 150A from it, not even 110A and probably shouldn't commonly do 100A.

But the fact is that generally, not all loads are running at the same time. So, dependent on the load, and a few other factors, it is common that the total of the smaller breakers may go above the upstream breaker. You aren't actually pulling the current, that you can't do. But you are basically saying that the odds of everything being on is essentially 0.
The Electrical code has some very strict definitions on exactly what can and cannot be counted, an electrician would probably have a computer program that they enter the loads into to make the determination of what is the excess capacity, if any, of the panel.
So you may have 120A of breakers on a master of 100A, but that assume that maybe that bedroom plug breaker won't ever use 100% of it's 15A breaker capacity.

Now adding a 50A breaker to a 100A panel is a huge ask. I'm a little surprised that the original installer split it evenly into 100A/100A panels. But not too surprised, as that probably meant cost savings over a 100A/150A or 100A/200A panels. (panel capacity not only would include the panel capacity but wiring going to it and the breaker cost, both can get very expensive at 100+A)

If you have a 50A panel that has to be derated 80% for the actual charging to 40A. So if the oven is on, and the dryer is on, the car is charging and a variety of other things are running, the question is will they go above 100A. The answer may very possibly be yes.

The other thing to think about is just what capacity is there on the panel? You don't have to charge at 40A. That can easily be decreased and you will probably never notice a difference.

I've got two Tesla which currently alternates between a single 120V 15A socket. That works for me. That adds about 5 mph. You can see estimated charge rates on the table at https://shop.tesla.com/product/gen-2-nema-adapters

Some things to think about:

• Did either electrician go through each of the existing breakers and make a determination of what they are and add up the expected load? If not, time for another electrician.
• Don't assume that you need 50A. How many miles is your normal daily usage? Do you have Superchargers nearby for "all else fails"?
• Something as simple as going with 20A 240V charging can yield 15 mph and 150 miles in 10 hours. That solves over 90% of people's needs.
• While the actual installation is a trivial piece of work and can be done by an apprentice, the calculations are a little more in-depth.
• While the wall charger is pretty, it is far from the only choice. Just look at all the adapter from the link that I posted earlier. I've always used the Tesla Mobile Connector and the right adapter.

And the most important, do it incorrectly and you can trip breakers or BURN THE HOUSE DOWN.

 

[davewill]

Another thought. Did Tesla use a local subcontractor to do the solar install? Can you try to work with that same contractor for this work?

 

[Cosmacelf]

DCC Electric has devices that would let you possibly load share with main panel (or the sub if that really can't work), and maybe get you full amperage charging. They aren't cheap, though, so if there's a solution that gets you at least a 20-30a circuit without such help, I'd probably go that way.

One thought I had was to go with the second electrician's recommendation, and if you fail the inspection, lower the amperage of the breaker to whatever the inspector will pass. I'm just a bit concerned that that may turn out to be too low to be acceptable. I don't know anything about your jurisdiction, but if a permit is pulled, can plans be submitted for approval before the work is done? That might get you an official answer before you have the work performed. If you have documentation from the solar install, that might be a great starting point, especially if they documented their load calculation.

I like the DCC Electric option - the DCC-10 looks like it could do the trick.

 

[davewill]

I like the DCC Electric option - the DCC-10 looks like it could do the trick.

SInce he has breaker space in the main panel, I was thinking the DCC-12 might be the ticket.

 

[Cosmacelf]

SInce he has breaker space in the main panel, I was thinking the DCC-12 might be the ticket.

Yes, sorry, I don't know my way around the various DCC products that well!

 

[aks20000]

Still grappling with what my options are to install the wall charger.

Garage panel , although rated as 200 amp busbar, has 100 amp feed from the main panel.
The AC loads have 100 amp feed from the main panel too.
Main panel , this way, is fully subscribed to 200 amp and no new load can be added here , this being a MLO distribution panel from 1990s era.

I did my own load calc for the Garage panel and it comes up as 78 amp for the existing load counting first 10000 watts at 100% and remainder at 40% ( guidelines from here ). So, only 22 amps of extra capacity for the new circuit.

For the AC load, it comes up as 65 amp counting RLA + FLA of both AC at 100% and adding 25% of RLA of one AC to it. So, there is extra capacity of 35 amp.

As such, on either circuit I don't see enough capacity to have even a 30 amp wall charger , which needs 125% or 38amp.
But my electrician is very persuasive in saying that load calc is done on the entire service for adding the new load and combines loads on both circuits to come up with a extra capacity of 40 amp. He recommends to put the charger of 40amp on the garage panel. I am not convinced yet.

What is the opinion of you experts on this subject ?

As an alternative, I am thinking to reduce the garage panel OCPD to 80 amps and AC load OCPD to 70 amps at the main panel. This will free up 50 amp capacity at the main panel and we will be able to add one 50 amp OCPD for the charger circuit there. The main panel has the extra space for this breaker.

Do you think this alternative approach will work ?

 

[davewill]

Still grappling with what my options are to install the wall charger.

Garage panel , although rated as 200 amp busbar, has 100 amp feed from the main panel.
The AC loads have 100 amp feed from the main panel too.
Main panel , this way, is fully subscribed to 200 amp and no new load can be added here , this being a MLO distribution panel from 1990s era.

I did my own load calc for the Garage panel and it comes up as 78 amp for the existing load counting first 10000 watts at 100% and remainder at 40% ( guidelines from here ). So, only 22 amps of extra capacity for the new circuit.

For the AC load, it comes up as 65 amp counting RLA + FLA of both AC at 100% and adding 25% of RLA of one AC to it. So, there is extra capacity of 35 amp.

As such, on either circuit I don't see enough capacity to have even a 30 amp wall charger , which needs 125% or 38amp.
But my electrician is very persuasive in saying that load calc is done on the entire service for adding the new load and combines loads on both circuits to come up with a extra capacity of 40 amp. He recommends to put the charger of 40amp on the garage panel. I am not convinced yet.

What is the opinion of you experts on this subject ?

As an alternative, I am thinking to reduce the garage panel OCPD to 80 amps and AC load OCPD to 70 amps at the main panel. This will free up 50 amp capacity at the main panel and we will be able to add one 50 amp OCPD for the charger circuit there. The main panel has the extra space for this breaker.

Do you think this alternative approach will work ?

On the AC panel, you say you have room for a 30a breaker, which would result in 24a charging on either a wall connector or mobile connector. There is no configuration that results in 30a charging. The next step up would be a 40a breaker resulting in 32a charging. Assuming that the run to where you want to charge isn't insanly long or something, a 30a circuit in that panel sounds like a decent choice.

Another thought, how long is the run from the main to the garage panel and what kind of wire is it? Could you upgrade the wiring to the garage panel, if needed, and change the breakers in the main panel to 125a to the garage and 75a to the AC panel? That should give you room for a decent circuit in the garage panel.

 

[Sophias_dad]

Your AC calculation is most likely incorrect. It should be(worst case) FLA for the two ACs combined. In reality, you should really be using RLA for the two AC's combined. At no point is FLA+RLA correct since FLA includes RLA. If I understand correctly, FLA is full load amps while RLA is rated load amps, the latter being what the manufacturer really expects it to use.

Here's a link to a slightly dated load calculation method, complete with an 'optional' method.

Residential Service Calculations in the National Electrical Code

iaeimagazine.org

 

[mongo]

Still grappling with what my options are to install the wall charger.

Garage panel , although rated as 200 amp busbar, has 100 amp feed from the main panel.
The AC loads have 100 amp feed from the main panel too.
Main panel , this way, is fully subscribed to 200 amp and no new load can be added here , this being a MLO distribution panel from 1990s era.

I did my own load calc for the Garage panel and it comes up as 78 amp for the existing load counting first 10000 watts at 100% and remainder at 40% ( guidelines from here ). So, only 22 amps of extra capacity for the new circuit.

For the AC load, it comes up as 65 amp counting RLA + FLA of both AC at 100% and adding 25% of RLA of one AC to it. So, there is extra capacity of 35 amp.

As such, on either circuit I don't see enough capacity to have even a 30 amp wall charger , which needs 125% or 38amp.
But my electrician is very persuasive in saying that load calc is done on the entire service for adding the new load and combines loads on both circuits to come up with a extra capacity of 40 amp. He recommends to put the charger of 40amp on the garage panel. I am not convinced yet.

What is the opinion of you experts on this subject ?

As an alternative, I am thinking to reduce the garage panel OCPD to 80 amps and AC load OCPD to 70 amps at the main panel. This will free up 50 amp capacity at the main panel and we will be able to add one 50 amp OCPD for the charger circuit there. The main panel has the extra space for this breaker.

Do you think this alternative approach will work ?

The house service can do it.
The subpanel's feed cannot do it.

Resizing the main breakers to add an EVSE may be a reasonable approach.
@Vines and @wwhitney are in the trade.

 

[wwhitney]

I did my own load calc for the Garage panel and it comes up as 78 amp for the existing load counting first 10000 watts at 100% and remainder at 40% ( guidelines from here ). So, only 22 amps of extra capacity for the new circuit.

That procedure (called the optional load calc) is only for the entire residence and all its loads. So it would apply to the service conductors and the size of the MLO service panel bus. It does not apply to the 100A feeder to the garage.

For the 100A feeder to the garage, you need to use the standard calculation. It will give you a bigger number. You most likely have zero available capacity on that 100A feeder.

The electrician telling you that you only need to do the load calc for the whole service, and not for the 100A feeder to the garage, is wrong. You need to do both. Every conductor in the electrical system needs to be sized for the downstream load.

As an alternative, I am thinking to reduce the garage panel OCPD to 80 amps and AC load OCPD to 70 amps at the main panel. This will free up 50 amp capacity at the main panel and we will be able to add one 50 amp OCPD for the charger circuit there. The main panel has the extra space for this breaker.

That might have worked prior to January 1, 2023. But then CA moved to the 2020 NEC, which does not allow an MLO service panel in new installations. Each service disconnect is required to be in its own compartment. Therefore you can't add any new service disconnect breakers to your MLO service panel.

Cheers, Wayne

 

[wwhitney]

Your AC calculation is most likely incorrect.

To clarify the OP's information, each of his outdoor units is labeled with compressor RLA and fan FLA. For that type of simple outdoor unit, the minimum circuit ampacity (MCA) for one unit is just 125% * RLA + FLA. [MCA also appears on the nameplate, and for this simple case you don't actually need to look further than that.] And MCA is what you need to use for the standard load calc, which applies to any conductor that isn't carrying all of the household loads.

For two or more motor driven HVAC units, you only need to add 25% of the one of largest motor to the MCA. The largest motor is the compressor's RLA. And the OP's two units are identical. So if RLA and FLA are the nameplate data on one of his units, the load on the feeder supplying two of them is 225% * RLA + 200% * FLA. [Which is why for two or more units, you do need to look at the individual FLA and RLA; if you just use 2*MCA, then you would get 250% * RLA + 200% * FLA, an overestimate. That also shows you could use 2 * MCA - 25%* RLA.]

The OP brought this topic up on a different forum, which is why I have the above info.

Cheers, Wayne

 

[Sophias_dad]

To clarify the OP's information, each of his outdoor units is labeled with compressor RLA and fan FLA. For that type of simple outdoor unit, the minimum circuit ampacity (MCA) for one unit is just 125% * RLA + FLA. [MCA also appears on the nameplate, and for this simple case you don't actually need to look further than that.] And MCA is what you need to use for the standard load calc, which applies to any conductor that isn't carrying all of the household loads.

For two or more motor driven HVAC units, you only need to add 25% of the one of largest motor to the MCA. The largest motor is the compressor's RLA. And the OP's two units are identical. So if RLA and FLA at the nameplate data on one of his units, the load on the feeder supplying two of them is 225% * RLA + 200% * FLA. [Which is why for two or more units, you do need to look at the individual FLA and RLA; if you just use 2*MCA, then you would get 250% * RLA + 200% * FLA, an overestimate. That also shows you could use 2 * MCA - 25%* RLA.]

The OP brought this topic up on a different forum, which is why I have the above info.

Cheers, Wayne

Thanks, I stand corrected!

 

[aks20000]

That procedure (called the optional load calc) is only for the entire residence and all its loads. So it would apply to the service conductors and the size of the MLO service panel bus. It does not apply to the 100A feeder to the garage.

For the 100A feeder to the garage, you need to use the standard calculation. It will give you a bigger number. You most likely have zero available capacity on that 100A feeder.

The electrician telling you that you only need to do the load calc for the whole service, and not for the 100A feeder to the garage, is wrong. You need to do both. Every conductor in the electrical system needs to be sized for the downstream load.


That might have worked prior to January 1, 2023. But then CA moved to the 2020 NEC, which does not allow an MLO service panel in new installations. Each service disconnect is required to be in its own compartment. Therefore you can't add any new service disconnect breakers to your MLO service panel.

Cheers, Wayne

Does that ( 2020 NEC ) mean that my existing MLO Panel distribution setup will be treated as "new installation" if I derate existing breakers and and add a new breaker in that panel to use freed up capacity , as I am planning to do ?
Will this approach be code compliant If I install a new 200 amp panel in the garage and install a new 200 amp OCPD in the outside MLO panel to feed this panel. Remove both existing 100 amp load OCPD which leaves only the solar back feed OCPD in outside MLO panel together with this new 200 amp OCPD. Then, from this new 200 amp panel - feed the garage sub panel on a 90 amp OCPD , AC Circuit on a 70 amp OCPD and wall charger circuit on a 40 amp OCPD.

 

[Cosmacelf]

How much driving do you do? Most people can get by with a 30A/240V breaker for charging. Many could even get by with a 20A/240V if their commute isn’t very long. Yes, it is always nicer to have the fastest charging possible. Indeed the other day, due to a variety of circumstances, I had four hours to juice up from 60% battery to 100% battery, and my 60A breaker EVSE did the job. But that was a once every three years kind of thing, and the backup plan would have been to stop at a supercharger once for a few minutes for that trip. Hardly a disaster or much of an inconvenience.