Many people get confused when it comes to circuit breakers and the right wires to use. Let’s dispel the confusion surrounding the question, “What size wire for 70 amp breaker?”
According to official recommendations based on breaker rating, the minimum wire size you should use is 4 gauge copper or 2 gauge aluminum. You may go lower but it’s risky. This also doesn’t factor in the wire’s total running length, which may or may not necessitate upsizing the wire size.
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Always Factor in Voltage Drop and the Wire’s Running Length
The answer stated above is derived from the recommendations of the American National Electric Code, which also include ambient temperature. While it’s safest to stick to the guidelines, you’ll arrive at the best answer if you consider all the essentials, including voltage drop and the running length.
Based on this voltage drop calculator, the minimum wire size requirements can greatly change even with a slight uptick or downtick in voltage drop. Do take note that voltage drop only becomes a major factor for upsizing if you have long running lengths of, at least, 100 or 110 feet and above.
Here Are Sample Scenarios
Let’s say the total cable run is 200 feet, with, of course, 70 amps as the main panel or subpanel’s ampacity. It’s a given that most American households use 240V appliances, so let’s set that.
With a voltage drop of 2.74%, the calculator actually recommends a 3 gauge copper wire instead. If we decrease it to exactly 2%, you may even need to use 1 AWG already. On the other hand, with a 5% voltage drop (which is still passable!) in that same setup, the calculator now recommends exactly the one on the NEC: 4 gauge copper.
Again, these are minimum requirements, so you shouldn’t go below them as much as possible. As you can also see, not all cases are an exact fit to what the standard 70 amp wire size chart would depict once voltage drop and running length come into the picture.
But we can also argue that, unless you’re building a subpanel on your detached garage that will connect with the main panel or a similar project with the same scale, you don’t have to consider upsizing and departing from the numbers on a standard wire gauge amp chart.
When Should You Consider Upsizing?
The voltage drop will be a crucial consideration for upsizing; however, it’s not always mandatory. Here are the conditions most electricians set when deciding when to upsize wire:
- The most important is calculating based on the loads you’re anticipating (i.e. your needs). If you’re building a workshop that you’re planning to expand over time, choosing a thicker wire may be necessary.
If this is the case, you may have to settle for a higher amp breaker since its rating should always match the wire’s own.
- As I mentioned before, if the total length of the cable exceeds 100 feet, it’s time to think about upsizing, but even then, it may still be better to follow the recommendations of a standard breaker wire size chart with regard to amp rating of wire, especially if you know what appliances you’ll be running at the same time – with voltage drop and cable length considered.
Still, many will argue that it’s better to upsize since increasing the conductor’s diameter aids in reducing voltage drop with arguably marginal consequences.
I hope that these points have enlightened you about the dynamic nature of this topic and that the specific query, “What gauge wire for 70 amp breaker?” doesn’t take long to evolve into a question that can be more meaningfully answered by a qualified electrical engineer or electrician.
Frequently Asked Question
How to convert copper wire sizes to aluminum?
To get the appropriate size aluminum wire based on copper recommendations, simply follow the NEC’s practice of using AWG sizes that are twice as large as the latter. However, this is not an absolute rule, especially if ambient temperatures are high and if voltage drop is considered.
It’s better to adopt the practice of converting from one to the other and vice versa by selecting the equivalent or larger ampacity while maintaining the same temperature rating.
So to sum up my answer to “What size wire for 70 amp breaker?”, you need to use, at the minimum, a 4 AWG for copper or 2 AWG for aluminum. Calculators allow you to be more in-depth and confirm recommendations, so don’t hesitate to use them.
Never use a smaller wire that can’t handle your breaker’s amp rating. Consult an electrician if you’re having doubts.
I am Andrew Wright. I established this blog after eight years of experience designing, installing, and maintaining electrical power systems. I love my job, and I have always wanted to offer others the necessary help so they can take care of their houses.