Alright, figuring out wire size for a 15 amp service and 15 amp breaker seems easy in theory. In practice, we have to account for the distance from the source (15 amp wire size for 15 amp service) and the 80% NEC rule for 15 amp breaker sizing. We will show you exactly how to determine the following:
- Wire size for 15 amp breaker. We will need to account for the 80% National Electrical Code (NEC) to adequately size a 15A breaker wire for the continuous load.
- Wire gauge for 15 amp circuit. This includes the 15 amp copper and aluminum AWG wire, as well as 15 amp wire for services 50 feet, 100 feet, 150 feet, and 200 feet away from the source (up to 500 feet). Here you will get an easy-to-use 15 Amp Service Wire Size Calculator, complete with a copper wire size chart for 15 amp service for 12V, 120V, and 220V circuits at 3% and 10% allowable voltage drop.
Here are the two cases we will cover according to the electrical code:
- “I have a 15 amp breaker; what gauge wire should I be using?” This is quite easy to figure out; we only need to account for the 80% NEC rule.
- “I want 15 amp service at a distance (50, 100, 150, 200 feet); what gauge wire and what size breaker should I use?” This is a little more difficult to calculate because we need to account for the voltage drop across the 50, 100, 150, and 200 feet distance.
Let’s first tackle the 20 amp breaker wire size (easier), and then discuss the 20 amp service some distance away:
Wire Size For 15 Amp Breaker
Almost all amp breakers can be loaded only to 80% of their max. capacity. The point of the amp breaker – an overcurrent protective device or OCPD – is to protect your circuit from current surges.
This is for continuous load uses like powering water heaters, space heaters, branch circuits, and so on. These cover most of the electric services we use. In the NEC Article 100, the continuous load is defined as “A load where the maximum current is expected to continue for 3 hours or more.”
That means that we can’t put 15 amps on a 15 amp breaker. Rather, we can only put 80% of that current draw on the breaker:
15A Breaker Allowed Amps = 15A × 0.8 = 12 Amps
We see that we can put at most 12 amps of a 15 amp breaker. That means we have to find a wire gauge that can handle at least 12 amps. To figure that out, we consult this NEC 310.16 table which specifies the ampacity of copper and aluminum wires. Here is a screenshot of the copper and aluminum wire ampacities in the vicinity of 12A from this table:
In accordance with this NEC 310.16 table, the smallest ampacity wires are the 14 gauge copper and 12 gauge aluminum wires (with 20A ampacity). We only need a wire that can handle 12 amps but, to comply with the NEC code, we use:
- 14 AWG copper wire for 15 amp breaker.
- 12 AWG aluminum wire for 15 amp breaker.
You can use either one when wiring your 15-amp circuit.
Now, this was the easy part. Let’s take the more complex question of what size wire you need for a 15-amp service at a distance:
15 Amp Wire Size For Services And Sub-Panels Some Distance Away
If you want a wire that can deliver 15 amp service at 0 or very short distances, you can use the 14 AWG copper or 12 AWG aluminum wire. However, with most of these services and sub-panels, there is a considerable distance from the electric source to the sub-panel or end-consumption electric unit.
Whenever we see long-distance wiring, we have to account for voltage drop. Voltage drop can be significant in longer wires because the wire itself is not only an electric conduit but also an electricity consumer as well. The result is that we need to use a bigger initial amperage to get the 15 amps at the end of the wire.
Voltage drop for a 15 amp wire can be calculated like this:
Voltage Drop = 2 × Wire Length × K (Copper or Aluminum) × 15 Amps / CM
K is the conductor-specific resistance per length. This is 12.9 ohms per foot of wire for copper wires and 21.2 ohms for aluminum wires. CM or circular mils is a measure of a cross-section of the wire, and can be linked to AWG or kcmil wire sizes (we’ll do that).
To get data on how many circular mils a specific AWG or kcmil wire has, we need to check the NEC Chapter 9 tables – specifically this ‘Table 8 Conductor Properties‘ or this AWG to CM conversion chart (here is the screenshot):
We see that 14 AWG wire has a cross-section of 4,107 circular mils (CM). Now we can calculate the 14 AWG copper wire voltage drop at a 100 ft distance for 15 amp service using the above equation like this:
Voltage Drop (14 AWG Copper, 15A, 100ft) = 2 × 100 Feet × 12.9 Ohms × 15 Amps / 4,107 CM = 9.42V Decrease
We can see that the voltage in a 100 ft 14 AWG copper wire drops by 9.42V. Here are a few examples of how this 9.42V voltage drop in a 100 feet 14 AWG copper wire affects the amp draw:
- At 12V DC current, 9.42V represents a 78.5% decrease in voltage. The resulting voltage is now only 2.58V.
- At 120V 1-Phase AC current, 9.42V represents a 7.85% decrease in voltage. The resulting voltage is now 117.42V
- At 220V DC current, 9.42V represents a 4.28% decrease in voltage. The resulting voltage is now 217.42V.
On top of this, we have to account for allowable voltage drop. Normally, we have these allowable voltage drop percentages:
- 3% voltage drop for critical circuits like branch circuits.
- 5% voltage drop for feeders.
- 10% voltage drop for non-critical circuits like general lighting and general appliances.
At what distance from the source should we switch from 14 AWG to 12 AWG copper wire, or even to a thicker 10 AWG wire, if we were to satisfy the 3%, 5%, or 10% allowable voltage drop? There is a lot of calculation for that.
To make this complex calculation much simpler, we have designed a 15 Amp Wire Size Calculator. You insert the wire length to the source, specify the voltage and allowable voltage drop percentage, and the calculator will account for voltage drop and automatically determine the wire size you need for a 15 amp service some distance away.
To help you out, we also did the math for 15 amp service from 0 to 500 feet away for 12V, 120V, and 220V voltages (copper wires), and presented the results in the chart below:
15 Amp Wire Size Calculator
Here is how this calculator works:
Let’s say you have a 120V circuit and want to send 15 amps to a sub-panel 200 feet away. This is a branched circuit, so we allow for a 3% voltage drop. What size wire do we need to facilitate this 15 amp service? Just slide the 1st wire length slider to ‘200’, 2nd voltage slider to ‘120’, and the 3rd allowable voltage drop percentage to ‘3’, and you get the result:
For this 15A service, we need a 6 AWG copper wire.
We have prepared 3% and 10% allowable voltage drop charts that include the copper wires sizes needed to facilitate 15 amp service at 12V, 120V, and 220V. Here are these 2 charts
15 Amp Service At A Distance Chart (3% Allowable Voltage Drop)
|Wire Length (3% Vd):||15 Amp Wire Size At 12V:||15 Amp Wire Size At 120V:||15 Amp Wire Size At 220V:|
|0 Feet||16 AWG Copper Wire||16 AWG Copper Wire||16 AWG Copper Wire|
|50 Feet||12 AWG Copper Wire||12 AWG Copper Wire||14 AWG Copper Wire|
|100 Feet||2/0 AWG Copper Wire||8 AWG Copper Wire||12 AWG Copper Wire|
|150 Feet||3/0 AWG Copper Wire||8 AWG Copper Wire||10 AWG Copper Wire|
|200 Feet||250 kcmil Copper Wire||6 AWG Copper Wire||8 AWG Copper Wire|
|300 Feet||350 kcmil Copper Wire||4 AWG Copper Wire||6 AWG Copper Wire|
|400 Feet||500 kcmil Copper Wire||3 AWG Copper Wire||6 AWG Copper Wire|
|500 Feet||600 kcmil Copper Wire||2 AWG Copper Wire||4 AWG Copper Wire|
As you can see, with very low voltage (12V), you have to quickly go for very thick kcmil copper wires at a few 100 feet distance from the source.
Here is an example of what size copper wires you need for a 15 amp circuit 200 feet away at a 3% allowable voltage drop at different voltages:
- At 12V, you would use 250 kcmil copper wire.
- At 120V, you would use 6 AWG copper wire.
- At 220V, you would use 8 AWG copper wire.
Now, let’s have a look at the 10% allowable voltage drop; here you won’t have to use such thick wires for 15 amp service some distance away:
15 Amp Service At A Distance Chart (10% Allowable Voltage Drop)
|Wire Length (10% Vd):||15 Amp Wire Size At 12V:||15 Amp Wire Size At 120V:||15 Amp Wire Size At 220V:|
|0 Feet||16 AWG Copper Wire||16 AWG Copper Wire||16 AWG Copper Wire|
|50 Feet||8 AWG Copper Wire||16 AWG Copper Wire||16 AWG Copper Wire|
|100 Feet||4 AWG Copper Wire||14 AWG Copper Wire||16 AWG Copper Wire|
|150 Feet||3 AWG Copper Wire||12 AWG Copper Wire||14 AWG Copper Wire|
|200 Feet||2 AWG Copper Wire||12 AWG Copper Wire||14 AWG Copper Wire|
|300 Feet||1/0 AWG Copper Wire||10 AWG Copper Wire||12 AWG Copper Wire|
|400 Feet||2/0 AWG Copper Wire||8 AWG Copper Wire||10 AWG Copper Wire|
|500 Feet||3/0 AWG Copper Wire||8 AWG Copper Wire||10 AWG Copper Wire|
As we can see, at a 10% allowable voltage drop, you can run a 500 feet 220V wire and still use quite a thin 10 AWG copper wire.
At a distance of 200 feet away from the source and 10% allowable voltage drop, we use these wires:
- At 12V, you would use 2 AWG copper wire.
- At 120V, you would use 12 AWG copper wire.
- At 220V, you would use 14 AWG copper wire.
If you compare this with a 3% allowable voltage drop, you can see that wire sizes don’t increase as quickly at a 10% allowable voltage drop.
Overall, we understand that sizing a wire for a 15 amp service by accounting for voltage drop can be quite difficult. Hopefully, these cases illustrate well how to do the math. On top of that, you can use the calculator and these 2 charts to get an idea of what size wire you need for a 15 amp draw.
If you have any questions or would like for us to do some math for you, you can use the comment section below, and we’ll help you out.
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