Why Do Electrical Panels Often Have More Loads Than Supply?

Have you ever added up all the breakers in an electrical panel and realized the total capacity of all those breakers exceeds what the main service can actually supply? If you’re like most people, this raises the question: why would the National Electrical Code allow panels to have more load capacity than they can handle?

It seems counterintuitive – potentially unsafe even – but there is sound reasoning behind this design approach. Understanding it is crucial for anyone working with whole home electrical systems, especially as more loads are being added through electrification.

In this excerpt from HeatSpring’s “Avoiding Main Panel Upgrades” course, instructor Spencer Rosen breaks down exactly why this apparent mismatch exists and introduces the two NEC-approved methods for calculating whether or not you can safely add new loads to an existing panel.

Want to master load calculations and learn proven strategies for adding electrical loads without costly panel upgrades? Sign up for the full “Avoiding Main Panel Upgrades” course and learn what you need to navigate these challenges confidently and safely.

Transcript below.

 If all of these breakers were operating, we’d have more demand than our panel could support. It makes you start to think – why would the National Electric Code allow a panel to have more loads than it supplies, in terms of power? 

There’s really two reasons why the National Electric Code would allow you to have more loads than supply.

The first is that all electrical loads are not on at the same time. If you imagine you had an electric furnace, for example, it’s never going to run at the same time as the air conditioner, right? If you’re charging your electric vehicle, it’s most often going to happen at nighttime while laundry’s going to happen in the daytime. How often is every light bulb on in every single room in the entire house while the laundry’s being done, while every TV’s on, while every circuit is at full blast and everything’s plugged in? It’s just unrealistic. All the electrical loads are not necessarily run – all those breakers are not being drawn from – at the same time. 

And two, the main breaker, we talked about this earlier in case for some reason everything does get turned on, which I said never happens, just for extra double triple safety. If everything does get turned on, the main breaker will protect the system. 

If all the loads are not used at the same time, you start asking some new questions, which are:

• How many are used at the same time?
• Which ones are used at the same time?
• What’s the maximum power they could actually draw?

And so these questions – how many loads are used, which ones are used, and what’s the maximum power – all come together to create an answer for those questions. The answer, according to the National Electric Code, is performing load calculations. 

With load calculation, we really answer those questions.

When we look at load calculations, there’s actually two different methods that are approved by the National Electrical Code. The first is one that’s been used for years and years and years and is the more commonly used version of an approval method. It’s 220.83 and it’s a prescriptive based method, meaning it’s a formula you give the house a specific kind of value of wattage based on the square footage and how many small appliances they have.

We’re going to go through that math in a few minutes. But it’s basically kind of like a model of the home. The second method, two 20.87, is a performance based method method. It actually uses real data from the home, right? So while the first method is a bottom up formula, kind of like, think of it like a calculation.

The second method is a performance based method, which is really using actual data.