Adding more tandems to a crowded panel

Question

I am in the process of wiring a hot tub which requires a new 50a breaker in the panel. Unfortunately, our panel is full. It already has a couple tandems, so I know that the panel accepts them. However, the panel appears to be very overcrowded already and is an older model, so I am concerned about adding two more.

Would two additional 20a (40a total) tandems be a reasonable approach here?

One alternative is to use our subpanel, however, it is in the garage and would require a lot of fishing and potentially drywall removal to reach.

Any other recommendations are welcome.

Answer 1

Not without a Load Calculation

I agree with crip659, you should presume that you can't add another 50A load to this panel. Based on the number of breakers, and particularly, 240V breakers - this panel is likely already near or past limits.

The way to resolve this is to do a NEC Article 220 Load Calculation on the service. You must use the NEC procedure there described, or a municipal worksheet derived from same. Don't even bother trying to contrive some fake way to do a Load Calculation that by wild coincidence works in your favor - everyone tries that.

Here is a typical city-supplied Load calculation worksheet, which is based on NEC Article 220. Don't bother with commerical websites as many of them put up fake clickbait articles that "make stuff up that sounds cool" (understandably since the NEC calculation is pretty dense). Any genuine Load Calc starts with "# of square feet x 3".

https://www.cityofsacramento.org/-/media/Corporate/Files/CDD/Building/Forms/CDD-0213_Electrical-Load-Calculation-Worksheet.pdf?la=en

If your Load Calculation checks out with the hot tub added (which I really doubt) then you can just go ahead and merge circuits onto tandem breakers. Cutler Hammer "CH" is an absolutely first-rate panel, one of the finest industrial panels made, and you need not worry about its age.

"The Load Calculation gave me a hard no"

I'm pretty much expecting this. Don't give up yet, though.

Technology Connections just dropped a video last week about this exact problem, so I'll go ahead and link that here. But the upshot is that you can use energy management systems to exclude loads from the Load Calculation. This technology is only starting to arrive on the market, which is why the $7000 SPAN panel is the only consumer-ready offering. But both Square D and Eaton are spinning up smart-breaker retrofit schemes for their panels. Your Eaton CH will lag a couple years behind BR (the breakers are smaller and it's hard to fit the tech inside) but you'll get it.

But it can be a lot simpler than that.

At its simplest, if two loads cannot be on at the same time, only one counts for the Load Calculation. So this could be as easy as an interlock between two 240V breakers so they can't be on at once - the Eaton CHML interlock does that nicely. E.G. Dryer, or hot tub. Moving breakers around isn't a big deal.

If you need to interlock the hot tub with two appliances, then you can use Eaton CHMLs to interlock the breaker with the one above and below.

Other things are possible - for instance, an older A/C system only runs when a 24-volt signal comes from the thermostat. That signal could also be used to interrupt power to the hot tub, so when the A/C is running, the hot tub is not.

For EVs we get a special treat - actual 21st century tech. (well, 1980s tech really, but nevermind that). EV charging can be taken off the Load Calculation entirely by using a Current Transformer sensor on the supply wires, to automatically slow down EV charging when the house would otherwise be overloaded. This is implemented by Britain's "Myenergi Zappi" and by the USA's Emporia Vue home energy monitor paired with the Emporia EVSE - and that's one of the most affordable EVSEs.

Many ways to solve it but hard to speculate without knowing more about your panel.

A 400A service upgrade could be done as well - this would actually preserve your CH panel - no need to mess with that - and add another 200A panel. You might as well get another CH, they're fantastic panels.

Answer 2

As others have mentioned (and as I previously described over here), you really need to do a load calculation before anything else. With an overstuffed panel (a big panel like this doesn't start out with tandems), you may already have exceeded your service and/or panel capacity based on a load calculation. Even if you haven't, adding another 50A very possibly will. So before figuring out how to connect the hot tub, you need to find out if you can connect the hot tub. If you don't have capacity then a heavy-up or some reworking of other loads will be needed.

There are two other possibilities:

• Combine circuits to free up spaces.

This is somewhat limited. Generally only works with 120V 15A and 20A lighting and receptacle circuits. Can combine kitchen countertop circuits but only if you have more than two as two is the minimum (and more is generally better). Can combine bathroom receptacle circuits between bathrooms, but if you have frequent heavy users (hair dryer, curling iron, etc.) then that may not be a good idea. If you have multiple lighting circuits (most people don't - circuits are mixed receptacles and lighting) then those are a good choice to combine because with LED lighting you can light a lot of rooms safely on one circuit. Can't combine the main laundry circuit (if you have one). Can't combine any large dedicated appliances (dishwasher, disposal, etc.). So there may not be much you can do, but that might get you a space or two.

• A new subpanel next to this one.

Get the same type of panel (Eaton CH) so you can move breakers. Put a large feed (e.g., 60A) double breaker in the old panel and move several circuits to the new panel. Ideally the panels should be less than 2 feet apart so that you can link them with multiple pieces of metal conduit. By keeping them less than 2 feet apart, you only have to worry about conduit fill but don't have to derate for multiple circuits.

Answer 3

I am not going to into load calc, or load shedding issues as they have been covered well by others.

Not seeing the labels on the inside of the panel we can't determine if Tandems are acceptable or not, but I will describe what is likely an issue.

Installation instructions and labelling are part of the Listing (UL/ETL/CSA), and they will specify the types of breakers allowed and locations in the panel they are allowed. Your panel was made during an era that the NEC limited "lighting" panels to 42 switches. The code has changed, but old panels are limited to the Listing conditions when built. To prevent tandems from creating more than 42 switches from being installed the bussing was typically built differently in specific locations and tandems had rejection features that allowed them to only be installed in those spaces.

Since your tandems are randomly spaced I would suspect some type of illegal field modification to the breakers or panel has been performed and at very least just to bring your current situation up to code even without adding anything you would need to combine some circuits or add a subpanel and move a few loads to the subpanel.