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What Size Wire for 125 Amp Breaker? Here is Your Answer!

Written by Edwin Jones / Fact checked by Andrew Wright

what size wire for 12 -amp breaker

What size wire for 125 amp breaker? The answer is rarely simple. You’ll need to take into consideration the required electrical load, distance, space available in the panel, temperature ratings, etc.

Specificity is the key in short. If you’re a homeowner and need 100A delivered to a 125 subpanel within 100 feet at 75 degrees C, for example, then you’ll need, according to universal recommendations, a #1 copper or a #1/0 aluminum wire. I’ll explain further and share some helpful tips below.

The Exact Electrical Load and Temperature Rating is Important


Going back to the example I share above, if we simply bump the load to 125A, you’ll already need a 2/0 aluminum wire instead, which is obviously entirely different from the 125 amp wire size copper I mentioned. It’s also advised to increase your circuit breaker capacity to 175A if that’s the case to ensure safety.

How do we calculate the load based on the circuit breaker’s ampere rating? The general rule of thumb is to multiply it by 0.8. For example, 175A x 0.8 is 140A, while a 150A breaker only supports a total of 120A. This is why it’s recommended to use a 175A and a bigger 125 amp wire size of 2/0.

What about the temperature rating? To me, there’s no better reference for it than this table. The wire you use determines the temperature rating you need to follow and the corresponding capacity recommendations.

As you’ll see, they’re still proportional to the usual breaker and wire size suggestions made in all countries worldwide, as represented in this other table approved by Canada’s National Electric Code.

I could not find a better video reference for what size to use based on the usual real-world appliances and their average electrical loads than the following. I recommend you watch it to gain more insights.

We Can’t Ignore Distance, Too

You’ll also need to calculate the exact distance you need when knowing the right 125 amp breaker wire size. If your project involves wiring from your main circuit breaker to your garage, then you’ll need to consider the distance in feet. It’s just as important as knowing the amps when calculating voltage drop index, after all.

Sometimes, I use this calculator to confirm my requirements. However, I don’t always stick to it, because I prefer calculating based on practical load. The practical load takes into consideration what each appliance needs to run instead of the possibility of the breaker tripping.

In most cases, I don’t need to calculate for voltage drop, especially if the distance doesn’t exceed the voltage (e.g. 360V can cover 360 feet). If it does or is as close as possible to it, that’s the only time you need to make voltage drop calculations.

According to the Electrical Engineering Portal, to calculate for voltage drop, you need to get the exact ampere-feet you’re dealing with. It’s usually calculated by multiplying load or circuit current (in amperes) with the exact length of your circuit. Afterward, divide it by 100 then multiply it by the VD values found in the tables.

What If You Don’t Have Enough Space Anymore in Your Panel


This is another common issue I need to address here because most homeowners find this out too late and end up with another problem. You find that the appropriate wire size can’t fit in the space available in your panel anymore. This is exactly why most electricians encourage homeowners to get as much space as possible for their main panel or subpanel.

Once I encounter cases like these, I generally advise people to invest in a subpanel next to the main panel. If it’s a 125 amp panel, then you can use a 100A breaker on the main panel to feed the breaker. The new sub panel must always have the same ampacity rating as the panel that is feeding it.

When in Doubt, Don’t Hesitate to Ask for Guidance From a Licensed Electrician

If you still feel lost (and that’s understandable, given the technicalities), there’s no shame in asking for help from your local electrician to know the ideal wire for 125 amp service. I’ll even stress that it’s the safest route to take if you aren’t too confident with your DIY skills when handling electric systems.

Incidentally, if you’re not already aware of it, make sure that your municipality actually allows unlicensed citizens to do electrical work. This goes for both residential and commercial buildings. After all, there’s always an inherent risk involved, not only to your family but also to the nearby properties and people as well should an accident lead to fire.


To sum it up, always know the specifics beforehand. Take note of the electrical load, temperature rating, and, sometimes, even the distance required for the entire wiring if it’s necessary. This is the only way for you to know what size wire for 125 amp breaker.

Should you need adjustments, don’t hesitate to consult a professional because anything less could only raise your risk. It will also save you money and time in the long run.

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