Calculating branch circuit load capability, by example

Question

United States here. I have a 150 amp main service panel coming into my home and after spending some time going through the house with a voltage pen/tester, I now have a perfect understanding of which outlets, switches and appliances are powered by which breakers (and yes there were a lot of discrepancies between what I found and what was written on the breaker panel labels...).

I found that several 15- and 20-amp 120VAC breakers seemed to be burdened with a disproportionate amount of the electrical load around my house. Several of these poor suckers seem to be providing branch circuits for a little too many outlets and lightswitches for my (uninformed) liking. Similarly there are several breakers/branch circuits that only provide power to a single (lone!) outlet.

I am wondering if anyone can explain to me the process of calculating whether or not a particular branch circuit (and the breaker running gatekeeper to it) is over- or under-utilized; over- or under-"powered" so to speak.

Obviously, different things that plug into the outlets may draw different amounts of watts, current, voltage, etc. If a particular branch has 100 outlets that are part of it, and nothing is plugged into + running from any of them, I'm sure the breaker won't trip and all will be well. But if you plug 100 washing machines into each of those 100 outlets and run all of them at the same time, something tells me that the breaker will trip.

So to me there is the idea of "average, reasonable" load on the circuit as well as maximal load. The average reasonable load might be the average amount of load/burden on the circuit at any given point of time. Maximum is, well, the maximum load the circuit can handle without tripping the breaker.

So I guess I'm looking to understand this: given a particular breaker (amps and voltage; say, 120VAC at 20-amps), how could I tell how many outlets, lightswitches and other loads (ventilation hoods, bathroom fans, dishwashers, etc.) that breaker/circuit can handle, so that I can "size" each of my branches correctly and determine whether they are over- or under-utilized.

Hint: we've lived in this house for 12 years and haven't had any housefires, so I'm not too concerned about the seemingly-overloaded branch circuits. This question is more about the theoretical calculation of branch circuits and what loads they can or cannot carry. Thanks in advance!

Answer 1

The Code requirement is for 2 kitchen, 1 bathroom, 1 laundry and 1 garage 20A circuit that serve nothing but receptacles in that room. You can have more receptacle circuits, but each one needs a 1500VA allocation in your house's Load Calculation.

Most common household 240v loads demand dedicated circuits (on a case by case basis, that's not a general rule for 240V circuits). Certain kitchen appliances need effectively dedicated circuits, so you can have 7-8 circuits just for the kitchen.

Lighting needs to be provisioned at a certain ampacity depending on square footage - about right for incandescent, excessive for LED), so you have a minimum number of circuits for that.

Other than that, the rest of the house can be on 1 circuit. Really.

Circuits that serve bathroom, kitchen, laundry and garage receptacles cannot serve anything else. So if you are looking greedily at their barely-used capacity, hands off!

Other than that, feel free to rearrange the general-use receptacle circuits any way that pleases you. Circuits which require 15A may be 20A instead (No higher) provided all wire is #12 or larger. I don't even own any #14 wire, I just use #12 for everything. All general-use circuits are covered by the 3 VA per squqre foot "catch-all" part of the Load Calculation, so there is no consequence for having plenty of them.

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You determine the power requirement of a device by looking at its nameplate. Every device has one. It will state the power usage in amps, watts, or VA. VA is volts x amps - so is watts. VA is the full sinewave that must be brought to the appliance. Watts is the part of the sinewave it actually uses, which is the same on a resistive load like a heater.

To get amps, divide VA by volts (120 in most cases).

It's common to have several bedrooms barely loaded while the home office is tripping breakers. That's why I prefer 1 circuit per wall, which in practice gives any room access (not exclusive) to 3-4 circuits. It also saves wire.

On space heaters, they are dangerous, get rid of them. If someone wants supplemental heating, baseboards are far safer, and cheaper too - costing only $35-50 + wiring, but lasting 40 years, and they can be on proper thermostats. Those $20 space heaters barely last a season.

Note that any heat-making appliance will tend to be 1500 watts. A 15A circuit has 1800W max. A 20A circuit has 2400W max. You notice you cannot run two 1500W loads on any receptacle circuit. (Since they max out at 20A).

Answer 2

There are only two levels that matter:

• Total = rated circuit breaker value (typically 15A or 20A)
• 80% continuous = rated circuit breaker value x 0.8 (typically 12A or 16A)

Most circuits, most of the time, are way below the 80% value and simply not a concern. That generally includes things like:

• Kitchen appliances - which by code are supposed to have at least 2 20A circuits to avoid problems
• Bathroom - hair dryer, etc. - and bathrooms are also supposed to have (basically...) their own 20A circuits
• Miscellaneous loads around the house - continuous (TV, computer, lights, fans, clocks, refrigerator, etc.) and short term (vacuum cleaner, tools, laser printer (on all the time but only uses a lot of power when printing), etc.)

Ignoring the big stuff - HVAC, water heater, oven, etc. - as they all require larger dedicated circuits. Also ignoring 15A/20A circuits for things like dishwashers - they should be on dedicated circuits whether plug-in (logically, and usually listed that way in the installation instructions too) or hardwired (required by code to not have any regular receptacles if hardwired loads take > 50% capacity of a circuit).

So a modern house (older houses have a lot more problems because they often don't have dedicated kitchen and bathroom circuits) simply doesn't have much of a problem as long as the circuits and the loads are spread around reasonably. The problems come up when a large appliance

• Window air conditioner - Should be on a dedicated circuit. But plenty of people plug them in "wherever", and the same places that typically need them (older homes) are the places that don't have enough circuits to be able to dedicate a circuit to the a/c.
• Space heaters - these are the worst. They almost always use 12A = 80% of a 15A circuit. OK to have on the same circuit as phone chargers or a few LED lights. But if you add them to a circuit with any serious stuff then you get into the danger zone.
• Lots of computers - normal desktops and laptops are rarely a concern. Get into "mining" and it is trivially easy to overload your circuits if you don't watch what you're doing.
• Grow lights - doesn't matter what you're growing, but if you put 1,800W of lighting on a 15A circuit that's not a good thing.

What's the danger zone? I'd consider it 100% - 125% (arbitrary numbers) of total circuit capacity for an extended period of time. A breaker should trip eventually if > 100%, but that's where things can go wrong relatively easily if any part of the system is marginal. Breakers trip very fast if circuits are loaded to 300% of capacity. They trip very slowly at 110% of capacity, because they have to allow for short term extra current for motor startup and other reasons.

In a practical sense, I would make sure required that appliances that should be on dedicated circuits (dishwasher, washing machine, window air conditioners) are indeed on dedicated circuits, and then for everything else:

• Ban electric space heaters in your home except under extreme circumstances (I have)
• Consider carefully what circuits you use for any device or group of devices that uses > 500W for more than a few minutes at a time.

Everything else is small stuff. Is it possible that you could add up to 2,000W of small stuff together on a single circuit between lighting and TV and computer and other stuff? Yes. Is it likely? No, as long as you take care of the big stuff - a.k.a., low hanging fruit.