Alright, many homeowners have a problem with wire sizes. Here, we will focus on the 25-amp wire size. Some parts of this might be a bit complex, but when you boil it down, we are simply going to address these two questions:
- “What wire gauge do I need for a 25 amp breaker?” This one is actually very simple; we just need to account for the 80% NEC breaker rule.
- “What size wire do I need for 25 amps? What if my appliance is 50 feet, 100 feet, 150 feet, or 200 feet away from the outlet?” Figuring out which gauge wire you need for 25 amps is simple; if the wire is short. If we have a long wire, we have to account for voltage drop. Here we can quickly go from 12 AWG copper wires at 0 feet to thicker 4 AWG, 2 AWG, and 1 AWG wires at 100 feet wire length.
Now, let’s take a structured approach. First, we will start with a 25 amp breaker wire size that’s easy to figure out (we will show you how you can determine wire size for any amp breaker. In the second part, we will look at wire size for a 25 amp circuit that’s some distance away. Because this involves a little bit of voltage drop math and NEC Chapters, we will simplify it by offering you these two very useful resources:
- 25 Amp Wire Size Calculator. This calculator automatically determines the wire size (AWG or kcmil copper wire) needed for 25 amps at a given voltage (V), wire length (L), and allowable voltage drop percentage (3%, 5%, 10%, and so on). You can freely use it; just make sure you read the note below the calculator about the minimum wire size for 25 amp circuits.
- 25 Amp Wire Size Charts (2 of them). We have calculated the adequate wire sizes for 25 amp circuits at 0 feet to 500 feet wire length for each 3% and 10% allowable voltage drop and summarized all the results in two useful charts you can consult.
Alright, let’s start with the easy 25A breaker part:
25 Amp Breaker Wire Size
25A breaker is one of the standard-sized breakers. It’s a lot less popular than 15A, 20A, and 30A breakers, but if you use the 25 amp breaker, you need to know which is the correct sized wire you should wire it with.
The obvious choice would be a 12 AWG copper wire since it has 25A ampacity. That means that it can handle a 25 amp current, and since we have a 25 amp breaker, a wire that can handle 25 amps seems to be the right choice. But it’s not.
Here we have to account for NEC’s 80% rule for breakers (also referred to as the 125% breaker rule). Namely, we consult the National Electrical Code NEC “Chapter 2 Wiring and Protection“, and specifically “210.20 Overcurrent Protection” part that states, “Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.”
In simple English, we cannot load more than 80% of the specified amps on the breaker. In the case of the 25 amp breaker:
- We can’t put 25 amps on it; that would be 100% of the max. capacity.
- We can put up to 20 amps on it; that is 80% of the max. capacity.
So, when we are looking for a 25 amp breaker wire size, we are actually looking for a wire that can handle 20 amps. Which wire can handle 20 amps? Simple. We check this NEC 210.16 table for copper and aluminum wires and check the ampacities at median 75°C (167°F) temperature. Here is the screenshot of that table for copper and aluminum wires with underlined 20A wires we need for the 25 amp breaker:
From these tables, we see very clearly that:
- The correctly sized copper wire for a 25 amp breaker is 14 AWG copper wire.
- The correct sized aluminum wire for a 25 amp breaker is 12 AWG aluminum wire.
Both of these wires have a 20A ampacity; which is exactly what we need since when NEC 80% rule is applied to a 25 amp breaker, we need wires that can handle up to 20 amps.
In practice, some people do automatically use 12 AWG wires for 25 amp breakers, but that’s not necessary at all. If we use copper wires, we can go with 12 AWG copper wire.
This solves the 25 amp breaker problem. Let’s now look at a 25 amp wire size problem:
How To Calculate Wire Size For 25 Amp Circuit? (Example)
For 25 amp service, we need a wire that can handle 25 amps. Just looking at the ampacity tables above, we can quickly see that we can use:
- 12 AWG copper wire for 25 amp service (since it has 25A ampacity).
- 10 AWG aluminum wire for 25 amp service (since it has 30A ampacity).
That’s all good and well if we use short wires. However, when we use longer wires (50 feet or more), we have to account for voltage drop.
Voltage Drop: This is a phenomenon where in long wires the initial voltage (Example: 120V) decreases (to 115V, for example). This is because the conductor itself – the wire – presents a certain resistance to the flow of current. The longer the wire, the more resistance we have.
Now, if the voltage drops, we have to use thicker wires with lower resistance to get the 25 amp current at the end of the wire. What we actually do to offset this voltage drop is use thicker wires with less resistivity. In the case of 25 amp service, we move from the 12 AWG copper wire, to 10 AWG, 8 AWG, 6 AWG, 4 AWG, 3 AWG, and so on, wires (even 250-2000 kcmil copper wires in 12V circuits).
Thus we can’t use the 12 AWG copper wire if we want to send a 25 amp draw 100 feet away (120V circuit).
The NEC Code also prescribes by how much voltage can drop. Here are the 3 main maximum allowable voltage drop buckets:
- 3% allowable voltage drop for branch circuits. Example: For a 25 amp current on a 120V circuit 100 feet away from the source, we have to use 6 AWG copper wire to be within the 3% allowable voltage drop.
- 5% allowable voltage drop for feeders. Example: For a 25 amp current on a 120V circuit 100 feet away from the source, we have to use 8 AWG copper wire to be within the 5% allowable voltage drop.
- 10% allowable voltage drop for non-critical circuits, like general appliances and lighting. Example: For a 25 amp current on a 120V circuit 100 feet away from the source, we have to use 12 AWG copper wire to be within the 10% allowable voltage drop.
Here is a bit of math insight into how to account for voltage drop. We can calculate the 25 amp voltage drop using this equation:
Vd (25 Amps) = 2 × L × K × 25 Amps / CM
L is wire length, and K is specific resistivity (12.9 ohms for copper wires, and 21.2 ohms for aluminum wires). CM or Circular Mils is the cross-section of the wire we need. When we calculate the CM, we can convert CM to AWG using this conversion chart. We express CM from this equation like this:
CM = 2 × L × K × 25 Amps / Vd
Let’s solve one example to illustrate how this calculation is done before we look at the calculator that does this automatically. We will use the 25 amp service 100 feet away (using copper wires) on a 120V circuit, and with a 5% allowable voltage drop. This means that voltage drop (Vd) is 5% out of 120V; that’s 6V.
Let’s put all of this in the equation and calculate circular mils of copper wire we need for this 25 amp service:
CM = 2 × 100 × 12.9 × 25 Amps / 6V = 10,750 Circular Mils
We see that we need a wire that has a cross-section area of at least 10,750 circular mils. Now let’s look at which wire this is. Here is the screenshot of the AWG to CM chart:
Here we see that:
- 10 AWG copper wire is too thin; it has 10,383 circular mils, and we need at least 10,750 circular mils.
- 9 AWG copper wire has more than 10,750 circular mils but this is not an electric wire (electric wires are 14, 12, 10, 8, 6, 4, 3, 2, 1 AWG and so on).
- 8 AWG copper wire has 16,509 circular mils; this is more than the minimum required 10,750 circular mils and is an electric wire. This is the correctly sized wire for this circuit.
Now, all of these considerations are a bit dull, and the math is a bit complex. That’s why all of these calculations and considerations are accounted for in the following 25 Amp Wire Size Calculator. Here it is, followed by the all-important 25 Amp Wire Size Charts:
25 Amp Wire Size Calculator (Accounted For Voltage Drop)
Note: This calculator accounts for voltage drop precisely. Regardless of that, for a 25-amp circuit, you always need a minimum of 12 AWG copper wire or 10 AWG aluminum wire.
Let’s look at how this calculator works:
Supposedly, you want to run a 200 feet 120V wire that delivers 25 amps. The allowable voltage drop is 3%. To calculate this, slide the 1st wire length slider to ‘200’, the 2nd voltage slider to ‘120’, and the 3rd allowable voltage drop percentage to ‘3’. Here is the result:
For this 25 amp circuit, we would need a 4 AWG copper wire to stay within the 3% allowable voltage drop.
This is just one example. We have calculated 25 amp wire gauge sizes for 12V, 120V, and 220V circuits at a distance of 0, 50, 100, 150, 200, 300, 400, and 500 feet away from the source for both 3% and 10% allowable voltage drop. All the results of required wire gauges are summarized in these all-encompassing charts (1st one is for 3% voltage drop, 2nd one is for 10% voltage drop):
25 Amp Wire Gauge Source 0-500 Feet Away (3% Allowable Voltage Drop)
|Wire Length (3% Vd):||25 Amp Wire Size At 12V:||25 Amp Wire Size At 120V:||25 Amp Wire Size At 220V:|
|0 Feet||12 AWG Copper Wire||12 AWG Copper Wire||12 AWG Copper Wire|
|50 Feet||1/0 AWG Copper Wire||10 AWG Copper Wire||12 AWG Copper Wire|
|100 Feet||4/0 AWG Copper Wire||6 AWG Copper Wire||10 AWG Copper Wire|
|150 Feet||300 kcmil Copper Wire||4 AWG Copper Wire||8 AWG Copper Wire|
|200 Feet||250 kcmil Copper Wire||4 AWG Copper Wire||6 AWG Copper Wire|
|300 Feet||600 kcmil Copper Wire||2 AWG Copper Wire||4 AWG Copper Wire|
|400 Feet||750 kcmil Copper Wire||1 AWG Copper Wire||4 AWG Copper Wire|
|500 Feet||900 kcmil Copper Wire||1/0 AWG Copper Wire||3 AWG Copper Wire|
Here we clearly see which wire gauge we need to deliver 25 amp current at a distance at 12V, 120V, and 220V voltages. For example, if you were to run a 400 feet wire, you would need:
- 750 kcmil copper wire for 25A 12V circuit. The 12V voltage drop is the most severe, and thus we have to use very thick wires.
- 1 AWG copper wire for 25A 120V circuit.
- 4 AWG copper wire for 25A 220V circuit.
Now, let’s have a look at how these wire sizes change when we allow for 10% voltage drop for a 25 amp service:
25 Amp Wire Gauge Source 0-500 Feet Away (10% Allowable Voltage Drop)
|Wire Length (10% Vd):||25 Amp Wire Size At 12V:||25 Amp Wire Size At 120V:||25 Amp Wire Size At 220V:|
|0 Feet||12 AWG Copper Wire||12 AWG Copper Wire||12 AWG Copper Wire|
|50 Feet||4 AWG Copper Wire||12 AWG Copper Wire||12 AWG Copper Wire|
|100 Feet||2 AWG Copper Wire||12 AWG Copper Wire||12 AWG Copper Wire|
|150 Feet||1 AWG Copper Wire||10 AWG Copper Wire||12 AWG Copper Wire|
|200 Feet||2/0 AWG Copper Wire||8 AWG Copper Wire||12 AWG Copper Wire|
|300 Feet||3/0 AWG Copper Wire||8 AWG Copper Wire||10 AWG Copper Wire|
|400 Feet||250 kcmil Copper Wire||6 AWG Copper Wire||8 AWG Copper Wire|
|500 Feet||300 kcmil Copper Wire||4 AWG Copper Wire||8 AWG Copper Wire|
For a 10% allowable voltage drop, we don’t need as thick wires as for a 3% voltage drop. If we use the same 25 amp service 400 feet away at a 10% allowable voltage drop, we would use:
- 250 kcmil copper wire for 25A 12V service.
- 6 AWG copper wire for 25A 120V service.
- 8 AWG copper wire for 25A 220V service.
Using the calculator and these two charts, allowing with the explanation of how to calculate the 25 amp wire size at a distance, you now have quite a lot of knowledge of how to size a 25 amp wire. If you have and questions or would need our help, you can use the comment section below, give us some numbers, and we will help you out.