Last Updated: April 2026
If you’re planning to run a three-phase piece of equipment off a phase converter, there’s one important question you need to answer before anything else: is my single-phase service panel big enough to handle the load?
It’s one of the most common questions we get at NAPCco. If your single-phase panel is not big enough for your application, it may trip and the phase converter will struggle to operate. Addressing this question before purchase helps to avoid headaches down the line.
This post walks you through exactly how to calculate your single-phase input requirement, shows real examples, and covers other important considerations.
Why Single-Phase Input Amps Are Higher Than You’d Expect
When you look at the nameplate or specification sheet of your three-phase equipment, you’ll find the amperage rating, which is sometimes referred to as Full Load Amps (FLA). For example, a motor nameplate may give an amp rating of 10 amps. It’s a common assumption that this is what your single-phase panel would need to supply. However, this is not the case, and a higher input amperage is needed.
This is because a phase converter takes the two lines provided by the single-phase breaker box and distributes them across three output lines to power your load. Because two lines are doing the work of three, your single-phase input has to carry more current.
To determine the single-phase power needed for the three-phase output, the square root of 3 (approximately 1.732) is used as a multiplier.
The Formula: 3 Phase Amps to Single-Phase Amps
The calculation is: Minimum Single-Phase Input Amps = Three-Phase Load Amps x 1.732
A Simple Example
Let’s say you want to power a three-phase motor with a nameplate rating of 10 amps.
- Step 1: Determine the three-phase amp rating based off the motor nameplate → 10 amps
- Step 2: Multiply by 1.732 → 10 x 1.732 = 17.32 amps
- Step 3: Confirm that your single-phase panel can supply a minimum of 17.32 amps continuously
If your breaker panel can’t handle a current draw of 17.32 amps, you will need to address your single-phase service capacity first before adding a phase converter.
An Example With Multiple Pieces of Equipment
It is common for one phase converter to power multiple machines at the same time. In this case, the calculation is based on the combined amperage of all loads. As an example, let’s say you plan on running a 10 amp motor and a 15 amp motor simultaneously.
- Step 1: Add up each load’s amps to get the total amps → 10 amps + 15 amps = 25 amps
- Step 2: Multiply by 1.732 → 25 x 1.732 = 43.3 amps
- Step 3: Confirm your single-phase panel can supply a minimum of 43.3 amps continuously
The total load draw should not be any higher than the rated motor load, assuming the motor is appropriately rated for the work to be performed.
The NEC Article 455 on the Three-Phase Amps to Single-Phase Amps Calculation
The National Electric Code Article 455 covers the installation and use of phase converters. Article 455 uses a different formula, recommending multiplying the three-phase amps by 2.5 to get the required single-phase input amps. The larger multiplier of 2.5 accounts for starting current with a margin of safety built in to cover potential current loss at the phase converter panel. For a more conservative estimate, use this formula:
Minimum Single-phase Input Amps = Three-phase Load Amps x 2.5
Continuing with the previous example of a 10 amp three-phase load, the required single-phase input would be 25 amps (10 x 2.5).
Does the type of phase converter change this calculation?
The answer is no. In fact, the size, type, or style of the phase converter has no impact on the input current required to drive the load. So whether you have a rotary phase converter, a static phase converter, or a digital phase converter, the calculation remains the same. The single-phase power needed is determined entirely by the downstream load.
Common Mistakes to Avoid During Calculation
- Using horsepower to estimate amps instead of reading the nameplate: Horsepower-to-amp conversions vary by voltage and efficiency. Always use the actual FLA from the motor data plate for accuracy. If your nameplate doesn’t have an amp rating, check your equipment manufacturer’s documentation or specification sheets.
- Forgetting to account for multiple motors or loads: If you plan to run more than one three-phase load off a phase converter, you’ll need to calculate the total load requirements and size your service accordingly. Don’t size for just one machine if others will be running simultaneously, as this will lead to issues down the line.
- Not double-checking if you are on the edge of your service capacity: The converter itself has a small overhead draw. If you’re right at the edge of your service capacity, factor it in. If you feel that you may be cutting it close to full capacity, contact us to discuss available options.
- Rounding down to the nearest whole amp instead of rounding up: If your calculation gives you 17.32 amps, round up to 18 amps minimum. It is best to have a margin of safety as rounding down can risk running your service right at or above capacity.
- Not checking that your utility transformer is big enough. The breaker inside of your shop and the transformer maintained by the utility company are two different things. Even if your single-phase breaker is big enough based on the above calculations, the utility transformer feeding your shop might not be. You need to make sure the transformer’s kVA rating is large enough to handle the load.
Frequently Asked Questions
- Is this calculation the same for all voltages? Yes, the 1.732 multiplier applies regardless of whether your shop has 240V or 460V single-phase. The amp values will differ based on voltage, but the calculation is always the same.
- What size phase converter do I need? That depends on multiple factors, including the load’s horsepower or amperage, load type (resistive, inductive, or motor load), current draw at startup, and more. We offer a sizing calculator as a starting point, and our team of application engineers is readily available for sizing recommendations.
- What do I do if I don’t have enough single-phase power? Depending on the context, you may need to contact the utility company to upgrade your service. They can help tailor a solution based on the three-phase equipment you plan to run.
The Next Steps for Finding the Right Phase Converter
Once you’ve confirmed your single-phase breaker panel can handle the three-phase load, the next step is selecting the right phase converter for your setup.
NAPCco’s selection of rotary and digital phase converters is built for any application, from light motor loads to medium workshops and large industrial facilities; No matter the electrical setup, we offer a phase converter that will fit your needs and use case.
