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I have 125amps coming into my house from PGE and my panel is 125amps and has a few free breaker slots. Currently I have a 240v oven, 24v clothes drier, 240v floor heating and 240v EV car charger (plus the usual 110 appliances: laundry, dishwasher, garbage disposal etc). Now I want to add an AC unit which they tell me will require 30amps. Now based on some load calculation table that the city permit folks want me to fill it seems I will go over the 125amps. They give you demand-factor reduction, take into account that you will not use AC and floor heating at same time etc - but taking all that into account and I still go over 125 amps by a little - the total is 135amps.

One solution is to use a DPDT switch that switches between either my EV car charger or my AC. Note I am okay with using 2 breakers in my main panel - one for AC and one for EV car. Alternatively I can use a generator interlock to solve my problem - I don't fully get how that works and would need some guidance.

Would either of the above two solutions pass code? Can EV charging 24v outlet into which I plug my car go through a DPDT switch? Or is there a rule against it like needing a dedicated circuit?

If legal could someone please draw a diagram for the DPDT switch and also for the generator interlock + subpanel. The DPDT switch seems simpler.

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  • It's not uncommon for the total rating of all the breakers in the box to exceed the total on the main breaker. Is the city telling you that you CANNOT exceed the limit or are they just asking? It doesn't seem to me that totaling 135A on a 125A box is out of line. – jwh20 Aug 13 at 12:50
  • You don’t add the circuit breaker values, you add the KW of the device, you will find out quickly that that 30 amp air conditioner may draw 24 amps except startup. But the wattages are used in the actual calculations not the breaker sizes. Of all the homes I have measured most are usually only consuming 50% of their panel value even heavily loaded panels on the west coast. – Ed Beal Aug 13 at 14:56
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    What is your panel make and breaker width? – Harper - Reinstate Monica Aug 13 at 15:09
  • We need to know your panel type to figure out what your cheapest/simplest options will be. – Harper - Reinstate Monica Aug 13 at 19:56
  • How many square feet is your house, and how many breakers/circuits do you have feeding kitchen counter outlets? – ThreePhaseEel Aug 13 at 23:11
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Assuming that your load calcs were based on actual load and not breaker sizes, the DPDT switch idea has merit. What I would look at though is the physical location of the loads. For example is your 240V dryer in the garage? If so, then I would put the DPDT switch in there to feed either the dryer or the car charger (assuming the sizes match), because you are going to manually connect the car anyway, so you throw the switch when you make the connection. It does mean not running the dryer and the car charger at the same time, but you are going to be giving up one load in any case.

What I would get is called a "Double Throw Safety Switch", like a disconnect switch except the handle is On-Off-On, as in Dryer up, Off in the center, Charger down. This is a picture of a 3 pole version, you only need a 2 pole, but it gives you the idea.

enter image description here

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If only there was a gadget that mounted on two adjacent breakers so only one could be on at a time!

Then, you would simply place the two breakers next to each other, mount this device, and then would be able to turn one off and the other on. Easy peasy.

There is.

  • For Siemens/Murray it's called an ECSBPK01.
  • For Square D "QO" it's called a QO2DTI.
  • For Eaton CH it's called a CHML.
  • For Eaton BR it's called a BRPMIKBR .

You get the idea.

You can call it a "Generator Interlock" if you really want to... but the manufacturers don't.

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Since your ac and heat are not used at the same time you can ignore the smaller load when doing load calculations. So your 30 amp ac will be fine. Even with your total breaker count most panels do exceed the main rating. If you want to get exact article 220.80 has a optional load calculations it is easier than the standard method, there are also load calculators on line. If close get an amp clamp and measure your draw , in almost every case you will ding you have room to add the 30 amp ac.

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