“What size breaker do I need for a microwave?” embodies the general confusion that homeowners get when it comes to appliance amps and voltages.
Regarding this question, 15A and 20A breakers tend to suffice. Still, your product’s nameplate is worth checking out. It should display the power rating, which dictates the microwave breaker size.
Table of Contents
- Why It’s Recommended to Follow the Power Rating on the Nameplate
- Other Appliances and Gadgets in the Circuit
- Calculating Based on Circuit Capacity
- Consider Using a Clamp Meter, Too
- Make Sure the Present Wires Match with the Final Breaker Size
- Why Does a Microwave Require a Specific Circuit?
Why It’s Recommended to Follow the Power Rating on the Nameplate
Some homeowners assume that when they’re trying to size breaker for microwave, they’ll only need to look at the wattage it’s rated for. When we browse through online stores, we see products rated at 850W, 900W, and 1200W. These are output ratings.
If we apply the formula watt = amp x volt, the correct breaker size for 1000 watt microwave at 120V should be 10A (since its ampacity is 8.3), right? To know the detailed answers, please check out this article about suitable amp for a microwave.
Well, the thing is, sizing is not as straightforward as that. It might work, but it could lead to a less-than-optimal setup.
At best, we’re only looking at the microwave’s minimum power draw when making these calculations. And if we consider other factors that come into play, we’ll see that it’s rarely advisable to go for minimum breaker size.
Take note of the most important ones as follows.
1. Input Wattage and Output Wattage
The input wattage differs from the output wattage. That latter is the energy used by the microwave to heat its contents. Meanwhile, input wattage refers to the total watts the microwave pulls but does not use.
For example, a 1,100W microwave could, in reality, be drawing more than its rated output wattage. Oftentimes, it’s 1.5x (or even 2x) that amount, so expect this device to actually be consuming up to 1,600W, if not a bit more.
- If we use the 1600W input rating and use the basic formula above, we get:
1600W / 120V = 13.33A
That means this microwave would require at least a 20A circuit breaker when considering the 80% rule.
- If we stick to calculating the breaker size based on the output wattage, we would have got 9.16A. Obviously, that would result in a 12A breaker, but it’s likely that the microwave will experience nuisance trips with this size.
2. Amp rating
If you look at the label displaying the microwave oven power requirements, both input and output wattages are included, along with the amp rating. The amp rating or power rating already makes allowances for the two wattages.
That’s precisely why if the amp is already provided on the label, it’s the best reference to use when figuring out the exact microwave circuit breaker size you need.
Look for this specification inside the microwave or on its side if you can’t see it elsewhere.
Other Appliances and Gadgets in the Circuit
While it’s often recommended for microwaves to be fed by a dedicated circuit, I understand that compact microwaves rated at 600W to 800W can be exceptions. The same goes for some counter-top models.
- If you’re putting other appliances on a circuit for microwave, you’ll have to add the amperages of everything. Ending up with a breaker that doesn’t cover for that total load will lead to frequent tripping.
- Be mindful of appliances with high power draws like refrigerators, heaters, and clothes dryers. The 2020 version of the National Electric Codes explicitly states that microwaves can’t share circuits with a fridge.
Calculating Based on Circuit Capacity
As mentioned before, most homes have a microwave 15 or 20 amp circuit.
Assuming you’re replacing an old microwave with a new one and are wondering whether that same circuit can accommodate it, one alternative route you can take is to calculate the maximum wattage your circuit can handle.
Let’s assume that you have a 20A circuit. To get the wattage, we need to use this formula:
P = I x V
120V is a common voltage so let’s use that.
20A x 120V = 2400W
That means the present circuit can handle up to 2,400W. We have to factor in other elements, though.
As per NEC’s general electric guidelines, you can only load up to 80% of the circuit’s capacity. That means we only really have 1,920W to work with.
- Will that work with a 1,200W microwave with an input rating of 1,800W? Yes, since it’s still well below the limit.
- Will it be able to accommodate a 1,600W microwave that uses up to 2,600W when at maximum capacity? Evidently, no.
Thankfully, microwaves that belong to the latter tend to be the ones solely used in commercial settings.
Consider Using a Clamp Meter, Too
Let’s say you can’t find the specific information about your microwave’s amp rating anywhere.
It could be that the microwave’s old and what’s written on the nameplate has already faded. I’m also well aware that most of us tend to just toss away instruction manuals.
If you ever find yourself stuck in this rut, I have two other suggestions:
- Reach out to the manufacturer and ask for the exact amp rating of your microwave. Check their website because they may have digital resources available, including product’s technical specifications.
- Use a clamp meter to check the microwave’s ampacity. Here’s a tutorial on how to do that:
The second method is useful regardless, since you can confirm the amount of power that the unit is actually pulling. It will help you decide whether the circuit’s ampacity is enough to support the microwave or if you need to run a new circuit for it.
Make Sure the Present Wires Match with the Final Breaker Size
Of course, it’s not enough to match the circuit’s amperage with the microwave’s own rating. You need to make sure that you’re using the right wire, especially if you’re working with a new appliance.
Over the range microwave wiring requirements are no different from any other microwave type. Instead, you need to pay more attention to the amperage.
Since most residential microwaves only need either a 15A or 20A breaker, a 12-gauge wire or 12/2 NM, to be exact, will suffice. Moreover, it’s actually common practice for electricians to use this type of wire when building circuits for home appliances.
Why Does a Microwave Require a Specific Circuit?
Smaller microwaves do not really require a dedicated circuit.
However, if your appliance pulls in over 1,200W and is a fixed microwave, then that’s a different story. Specific or dedicated circuits are needed as per NEC’s recommendations for all fixed equipment.
Basically, any model with a high power pull needs a circuit that can handle that kind of demand to ensure safe and smooth operation. Keep this in mind when considering the safest amp breaker for microwave.
I hope I’ve given you the most complete and reliable answer to “What size breaker do I need for a microwave?” Here’s everything you need to do as a recap:
- Base the breaker’s size on the microwave’s amp rating.
- Factor the input wattage and other appliances on the same circuit into your calculation.
- Calculate the wattage and amperage if the need arises or to double check.
- You can confirm your reasoning further by using a clamp meter to measure the exact ampacity.
I am Edwin Jones, in charge of designing content for Galvinpower. I aspire to use my experiences in marketing to create reliable and necessary information to help our readers. It has been fun to work with Andrew and apply his incredible knowledge to our content.