Estimating for Additional Circuits

Estimating for Additional Circuits

Every once in a while, we find ourselves with a desire to add an additional circuit to our existing electrical panels. This is often the case when a homeowner wants to acquire a hot tub or build an addition. More increasingly, we find ourselves needing to add a new circuit to support an electric vehicle charging station. If any of these scenarios sound like you, keep reading to find out how you can estimate your existing load and determine the total capacity of additional circuits that may be added.  

A common misconception is that if an electrical panel has space for additional circuit breakers, new circuits can be added to your home without upgrading the panel. Unfortunately, this isn’t always the case. More often than not, a panel has the space for additional breakers but the electrical equipment itself doesn’t have the capacity to support the desired additions.

What does this mean exactly? Electrical equipment has several different ratings: voltage, current, mechanical stress, etc. When we speak of capacity, we are generally referring to the current rating of the equipment. Current ratings are given in amperes, which is abbreviated with a capital ‘A’. Common ratings for residential electrical panels are 60A, 100A, 150A and 200A. This rating is the total amount of current that the equipment can conduct safely. There are additional restraints, such as all equipment must be rated to 125% of all continuous loads. So, for example, a 100A panel cannot have more than 80A of continuous loads connected to it.

Determining how much load is connected to an electrical panel isn’t as elementary as adding up all of the individual circuit breakers. In fact, if one were to do so, they would likely find that the total of all the circuit breakers is greater than the main breaker rating. This is because not all circuits, or more accurately, not all loads, are utilized at 100% capacity or for extended periods of time. 

If you are not planning to supply additional circuits for air-conditioning or space-heating equipment, the total demand load of your current electrical service can be estimated with the procedure detailed in the steps below. This estimation can be used to determine if additional circuits can be added, and a rough idea of the capacity of said circuits.

Please keep in mind that we are providing a ‘watered down’ calculation to help get an idea of the current demand load on your panel to help you get a rough idea of how much, if any, additional load could be added. The full calculation requires detail beyond the scope of this discussion and includes all of the nuances involved for a more precise result. The steps provided below will expose an already heavily burdened panel but will not necessarily provide an accurate estimate of the true capacity of an unladen one. Please consult with your preferred local electrical contractor to help determine if additional circuits can be added to your existing panel.  

We will also note that if your main electrical panel is only rated for 60A or 100A, it is extremely likely that your panel is already operating at or beyond capacity. In this case, we can safely recommend an upgrade without the need of a calculation.

At the time of this writing, the most current version of the National Electrical Code is the 2023 NFPA 70. All estimations stated below are based on this version of the NEC (National Electrical Code).

The first thing we need to do is determine the total habitable square footage of your home, as defined by the NEC. This is the total square footage as measured from the exterior of the structure and includes all habitable space. The square footage of uninhabitable spaces such as garages, open porches or unused and unfinished spaces such as basements should not be included in this measurement.

Now that we have the total square footage of the home, we calculate the general lighting and receptacle load by simply multiplying it by 3VA (volt-amperes):

We can’t stop there, however. Next, we will add 4500VA to cover the two small appliance branch circuits and the laundry branch circuit as required by the NEC. We’ll call this the appliance & laundry circuits. 

The next step is to determine how much load your household appliances are contributing. For this, the nameplate rating of the appliances will be used. These appliances and equipment include dishwashers, garbage disposals, microwaves, cooktop ranges, ovens, water heaters, HVAC systems, baseboard heaters and electric clothes dryers.

This calculation is not as straight forward. Most appliances provide ratings in units of kilowatts (kW) or kilovolt-amperes (kVA).The first step is to convert this to VA. Thankfully, kW and kVA are used interchangeably and converting them to VA is straightforward. Simply multiply the given kW or kVA rating by 1000. That’s it! So 1kW, or 1kVA, is the same as 1000VA.

We will note here also that some appliances are rated in amperes only and to determine the total VA, this value must be multiplied by the operating voltage of the appliance.

Next, we will add all of our loads together to determine the total demand load connected to the existing electrical panel:

Once we have the total load, we can reduce it by applying additional demand factors. The NEC states that we must calculate the first 8000VA at 100% demand, but any load beyond 8000VA may be calculated at only 40% demand.

Once we’ve calculated the total demand load in VA, we will convert it to amperes (A) and compare the total amperes calculated to the total ampere rating of the electrical service equipment of your home.

That seems like a lot so let’s take a look at an example.

Let’s assume that we have a 2250 SF home, but that 750 SF is made up of the garage and front and back porch. Therefore, the total habitable square footage of the home can be reduced to 1500 SF.

Our home will also have a 5.5kW electric clothes dryer, a 1500W hot water heater, a 2.4kVA dishwasher, a 1000VA garbage disposal, a 7.5kW range/oven combo and a 1500W microwave. The home also has 3kW of space heating equipment.

General Lighting & Receptacle Load          1500SF x 3VA = 4500VA
Appliance & Laundry Circuits:                                                  4500VA
Dryer:                                                                                         5500VA
Hot Water Heater:                                                                    1500VA
Space Heater:                                                                            3000VA
Dishwasher:                                                                               2400VA
Garbage Disposal:                                                                    1000VA
Range/Oven Combo:                                                                  7500VA
Microwave:                                                                                 1500VA

Total:                                                                                         31,400VA

So now we may apply the additional demand factors for the first 8000VA and the remaining 23,400VA.

8000VA x 100% = 8000VA
23,400VA x 40% = 9360VA

Total Existing Demand: 17,360VA

The final step is to convert the total volt-ampere demand into amperes and compare this to the total current rating of your electrical service and panel. To do this, we will simply divide the total existing demand by 240V:

In this example, if the homeowner has a 150A service and electrical panel, it would not be possible to add more than 47A of additional load (150A*80% – 72.3A = 47.7A). If they wanted to add a 50A circuit for a new hot tub, they would be forced to upgrade to a 200A electrical service. If the electrical service was already rated for 200A then the homeowner should be able to add a new 50A circuit for their hot tub without upgrading their service and equipment.

Please keep in mind that these simplified calculations are provided to allow a homeowner to estimate if they may need to update their current electrical service. There are additional factors to consider, such as circuit voltages, continuous and noncontinuous loading and feeder sizing requirements. These basic calculations will help to determine if your panel is already overloaded or at maximum capacity, which is likely if you have less than 150A service but have all of the modern-day appliances. Your electrical contractor will be able to complete the correct calculations for your home and confirm if a service upgrade is needed based on your current equipment and existing load. They will also be able to explain whether they can install the circuit from your existing panel, or if they will need to add a subpanel or any other additional equipment.

At Mountain Electric, we encourage learning because we believe homeowners should be empowered with an understanding of their own home. We are always happy to answer any questions you may have and help point you in the right direction. Please remember that there is a difference between learning a subject and the practical application of that knowledge. We wouldn’t read an article about how a brain tumor is removed and assume that we can safely operate on our loved ones. Instead, we feel less timid about an upcoming procedure because we’ve taken the time to understand what is involved, how it is typically accomplished and what we can expect afterwards. Learning is exciting to us, and we hope it is to you as well. All we ask is that you allow properly trained professionals the opportunity to shine as only they can while you remain safe…with an ever-expanding brain.