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https://paloaltoonline.com/blogs/p/print/2021/10/10/your-electric-panel


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By Sherry Listgarten

Your electric panel

Uploaded: Oct 10, 2021

Can you convert your gas furnace to an electric heat pump? How much space is in your panel? There's more than you might think! This is the second in a three-part series on home electricity fundamentals. This part will focus on the electric panel and specifically the answer to the question “Do I have room in my panel?”

As you may remember from last week’s post, 240 volts of electricity are coming into your home from the grid. The electric panel controls how much current (and power) are allowed in. The panel might be rated at 100 amps (which allows 240 volts * 100 amps = 24,000 watts = 24 kilowatts). Or it might be 200 amps (48 kW). Or it might be something like 60, 125, 150, 300 amps, etc. The panel capacity determines the maximum amount of power that your lights and appliances can be drawing at any given time. The panel serves to limit the amount of electricity entering the residence.

But that is not all it does. The panel apportions the electricity into different areas of the home via switches (breakers). A breaker might specify that at most 20 amps at 120 volts (2400 watts) can be used by a bathroom’s outlets. Or at most 40 amps at 240 volts (9600 watts) can be used by an oven. The main reason for creating these various electric gates is to protect the home’s wiring, which can overheat and potentially start a fire if too much current goes through. An appliance can also get damaged if there’s a short circuit. When current exceeds a breaker’s rating, the breaker flips, shutting off electricity to the corresponding area, hopefully preventing damage and/or fires. The breakers in a panel also help to isolate electric faults to small areas. When one breaker flips, the rest of the house stays powered up.

All of the wiring in the house must go through the panel. Any new appliance will either be added onto an existing circuit or get a new breaker. So you might wonder “Do I have room in my panel for this?” That is a question that needs to be answered in two ways, once with regard to physical space and once with regard to electrical space.

Is there physical room in the panel?

Panels come in different sizes. Higher-amp panels are larger, with more room for breakers. The picture below shows a small electric panel (a subpanel) with twelve numbered slots, only two of which are empty (on the bottom right).


A small 80-amp subpanel for a bedroom and laundry area

Taking a close-up look, you can see the slot numbers embossed on the light gray metal, odd numbers down the left side and even numbers down the right. Each breaker is labeled with the maximum amps it allows.


A closeup of an 80-amp subpanel

Most of the breakers in this panel are “single pole” breakers, which allow for only 120 volts. Each is rated for 20 amps or 15 amps, as marked. But the 30-amp breaker taking up slots 2 and 4 in the upper right is a double-pole breaker, which means it allows 240 volts.

This 30-amp 240 volt breaker is labeled for an electric dryer, but the nameplate (label) on the dryer says it uses just 20 amps. A breaker is always sized a little bit larger than the nameplate amperage, but in this case it could probably have been 25 amps. It’s possible the electrician didn’t have a 25-amp double-pole breaker on hand, or maybe he/she sized it a little larger in case the homeowner wanted to install a more powerful dryer at some point. If you size a breaker too large, then you risk having a wire overheat or an appliance getting damaged in case of a surge.

So, when you are considering if there is physical space in your panel, you want to determine (a) if you need one pole (120 volts) or two poles (240 volts), and (b) how many empty slots there are in the panel. If all the slots are taken up, you might consider whether an electrician can combine some of the breakers. For example, if you have replaced all of your lights with LEDs, then you might be able to combine the lights for multiple rooms onto one breaker. (1) The subpanel shown above allocates 2 x 15 amp breakers (9 and 11) for lights for three bedrooms and a hall. If those are all LED lights, they could almost certainly be combined under just one of those breakers (1800 watts is a boatload of LED bulbs), freeing up a slot.

Another technique an electrician can employ to get more space is to use tandem breakers. These are half-width single-pole breakers, two of which can fit into one standard slot. (Quad breakers also exist, which are the double-pole equivalent of tandem breakers.) These 2-for-1 breakers can almost double the physical space of a panel, though not all panels allow them to be used in all slots. A label may indicate where they are allowed. You can see them in use in the photo below.


The top of this 200-amp panel shows tandem breakers used in slots 1-3, a regular breaker used in slot 4, and an empty slot 5.

Finally, an electrician can put in a subpanel to create space for more breakers. A subpanel will take up only two slots in your main panel (using a double-pole breaker), while providing ten or more slots. You can put a subpanel where it will provide convenient electricity to appliances in your home. For example, you might put a subpanel in a garage to make it easier to electrify cars and heaters there, especially if your main panel is not close to the garage.

The 125-amp panel shown below is located in a garage near the kitchen. The kitchen plugs and induction stove, along with an EV charger and heat pump water heater are connected to this panel.


125-amp panel for a 3BR/2BA condominium which is all-electric except for space heating

A 90-amp subpanel is located closer to the bedrooms and living area. It covers the lights and outlets for those areas, the laundry, gas furnace, and a few kitchen appliances (dishwasher, disposal and refrigerator).

Both panels have plenty of physical space for more devices. But is there enough electrical space to replace the remaining gas appliance, a furnace, with a heat pump space heater/cooler?

Is there electrical room (“ampacity”) in the panel?

Unfortunately you don’t just add up the breakers and subtract that sum from the panel rating to determine if there is electrical room in the panel. In fact, the breakers will typically add up to more than the capacity of your panel.

Consider the 125-amp panel above. The panel allows 125 amps * 240 volts = 30 kW of electricity into the house. On it there are seven breakers. The three single-pole (120 volt) breakers total 55 amps, for 6.6 kW. The four double-pole breakers (240 volt) total 180 amps, for 43.2 kW. The 49.8 kW from the breakers greatly exceeds the 30 kW rating of the panel. Why is this allowed? There are a few reasons:

1. The appliance load is less than the breaker load. For example, a dryer in a 30-amp breaker may use only 20 amps. A stove in a 40-amp breaker may use only 30 amps. The electrician uses the “nameplate” load from the appliance documentation, not the breaker rating, when evaluating whether there is room in the panel. (For lighting and outlets there is a rule of thumb to apply.)

2. Not everything in the house operates at the same time. Some appliances run in the background, like a refrigerator, hvac equipment, and a certain amount of lighting and plugged-in devices. But it’s not likely that all outlets will be used at maximum capacity while the dryer, stove, and microwave are also running. This is something the electrician takes into account when determining panel load. Beyond a certain amount, potential loads are assumed to have only a 40% coincidence. (2)

Although that 125-amp panel may seem very “full” based on the breaker labels, in practice it has about 7 kW left, which is more than enough power for a mini-split.

What if the actual load is even lower than the “nameplate” value?

The nameplate load that an electrician uses for load calculations is the maximum that an appliance will use. Your actual load from the appliance may be much lower. You can imagine a heater that has high, medium, and low settings. The nameplate will reflect the high setting, but your power consumption will be lower if you use it on medium.

That difference is pronounced with a heat pump water heater (HPWH). The labeled amperage, often 30 amps (7.2 kW), reflects the power used when the heater’s backup resistance heat is operating. If your HPWH is sized properly, that mode will rarely kick in and your water heater will draw closer to 3 amps (about 700 watts). That is a 10x difference! As another example, efficient mini splits have variable speed compressors that draw only as much power as they need, depending on the temperature and other variables. While the nameplate might say 5 kW, in practice it could use half that on average, or even less in our temperate climate.

If you believe your actual load is much lower than the nameplate load, even with the 40% coincidence assumption, there is an option to use your actual load to evaluate panel capacity. If your power provider has hourly records of your power consumption (e.g., you have a smart meter), an electrician can use your maximum actual hourly or 15-minute load to determine if there is space left in your panel. See footnote 2 for more information.

The real-world load is also useful for estimating an electric bill or getting an idea of how many hours a heat pump will be operating. In the next blog post we’ll go over how to estimate the actual load (and electricity bill) of a new electric appliance.

How do I make more electrical space?

If it turns out that you need more electrical space in your panel, there are a few things you can do. One is to switch to more efficient devices with lower nameplate ratings. That will increase the remaining space in the panel. You can also consider sharing a circuit between two appliances. For example, if you have two high-power appliances that do not need to be used at the same time, you can share a circuit using a device like a SimpleSwitch (for circuits) or a NeoCharge (for outlets). These allow you to specify that one appliance (e.g., a clothes dryer) will get priority over another (e.g., an EV charger) when both are turned on. The EV charger will be paused when the dryer starts and will resume charging when the dryer is finished.

Local electrification enthusiasts Josie Gaillard and Tom Kabat have contributed to some terrific material on how to get the most out of your electric panel space. You can find some information here and on pages 19-24 of this free, comprehensive home electrification guide.

I hope this helps some of you get a better understanding of what your electric panel does and how to determine if there is space for a new device. I’d love to hear your comments or questions below.

Note: You can find the third and final post in this series on home electrification here. It is about how to understand heat pump specifications to answer common sense questions about how they will operate.

Notes and References
0. Thank you to Tom Kabat for his careful review of this blog post.

1. Technically, the wires for the lights would be combined in a junction box, and then a single wire from that box would go to the breaker. It is not electrically safe to put two wires into the same clamp on a breaker in the panel.

2. The electric code allows two ways to determine how much electrical space (“ampacity”) remains in the panel. In one method (National Electric Code section 220.83(B)), all of the loads except HVAC are added up. The first 8 kW are assumed to apply in full, while the remainder are applied at 40%. Then the higher of the heater or air conditioner is added (since these are not used at the same time). In the condominium example above, the breakers add up to about 50 kW but the nameplate load is at most 45 kW. The draw of the gas furnace is negligible and there is no cooling. So 8 kW is the baseload and 45 kW - 8 kW = 37 kW is applied at 40%, or 15 kW. The panel therefore has no more than 8 kW + 15 kW = 23 kW of load, so it has at least 7 kW of headroom. You can find some information and a spreadsheet calculator here.

There is also an optional method (NEC section 220.87) that is based on your actual usage. You can take your highest hourly or 15-minute average watts used in the past year, multiply that by 1.25, and use that as your load. If you subtract that from the panel-rated wattage you find how much power is left for new appliances. For example, if PG&E told the owners of the 125-amp panel above that their highest load last year averaged 14 kW (for the highest hour or quarter hour), then the residence could accommodate another 12 kW of nameplate rated devices (30 kW - 1.25 * 14 kW = 12 kW). That would allow them to get not only a beefy 5 kW mini split but also update their EV charger to 40 or even 50 amps. I believe this method only works with smart meters, though, so Palo Altans are out of luck for another three years, argh. Reader Eric also mentions in a comment below that this option is not available for homes with rooftop solar.

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