You need the correct wire size for the job. To calculate wire size, you will need to use ampacity and NEC 80% rule. We will show you how to easily calculate wire size by hand, using a wire gauge calculator, and at the end, you will also find a complete chart of what size wire you need for how any amp service.
Namely, the goal of all wire size calculations is to avoid:
- Choosing too small a wire. Example: 4 AWG wire for 100 amps is too small. This will fry potentially fry the circuit.
- Choosing too big a wire. Example: 2/0 AWH wire for 100 amps is too big. These bigger wires are just unnecessary overkill.
To adequately calculate the size of wire you need, you only have to apply two rules:
- Ampacity rule. Every AWG gauge wire has its own ampacity; a metric that tells you how many amps that wire can handle. Example: 4 AWG copper wire has 85A ampacity at a median 75°C temperature. That means a 4 AWG wire can handle 85 amps of current.
- NEC 80% rule. National Electrical Code (NEC) states that ‘any wire can be loaded to only 80% of its ampacity rating (for continuous loads)’. Example: 4 AWG wire can handle 85A. However, you should only load it to 80% of that; that’s 68 amps maximum. This is a safety precaution, preventing your wires from potentially catching fire.
With these two rules, you can calculate wire size for any amp service. Here is how we are going to help you to calculate the wire size yourself:
- Calculate wire size by hand using the wire size calculation formula. We will learn how you can calculate wire size step-by-step, using the ampacity and NEC 80% rule for DC and AC current.
- Use a wire gauge calculator. This DC wire size calculator will help you calculate the wire gauge automatically. Example: For 100 amp service, you will a wire with at least 125A ampacity. That means you will need a 1 AWG wire for 100 amps since it has 130A ampacity.
- Wire size chart. In this chart, you can find wire sizes for 1 amp, 10 amp, 20 amp, 30 amp, 40 amp, 50 amp, 60 amp, 70 amp, 80 amp, 90 amp, 100 amp, 150 amp, and 200 amp services, to name just a few.
Let’s illustrate how to calculate wire size, and then proceed to the automatic wire size calculator and the wire gauge charts:
How To Calculate Wire Size? (Using Wire Size Calculation Formula)
Here is the step-by-step process how to calculate wire size:
- Determine how many amp service you need. Let’s say we need a 100 amp service and would require a wire for 100 amps.
- Multiply these amps by a 1.25 factor to get the minimum required wire ampacity. This is to account for NEC 80% rule. Example: If you need a 100 amp wire, multiply 100 amps by 1.25; you get 125A. This is the minimum ampacity your 100 amp wire needs.
- Consult the wire ampacity chart. This is a chart that specifies how many amps can any AWG wire handle; here is the full AWG wire gauge chart for all 44 AWG wires. Check the ampacity; the ampacity of the wire should be the same or higher than the ampacity of the wire we calculated in Step 2. Example: For 100 amp wire, we need a wire with at least 125A ampacity. From the ampacity chart, we see that 2 AWG wire has 115A ampacity and 1 AWG wire has 130A ampacity. Since 130A is higher than 125A, the 1 AWG wire is the right choice for 100 amp service.
For quicker calculation, you can use this simple wire size calculation formula:
Wire Size = Amps Needed × 1.25 = Min. Ampacity Needed
When you get the minimum ampacity needed, you can check the wire ampacity chart to get the correctly sized wire.
To make all this process even easier, you can use the wire gauge calculator that dynamically calculates the minimal ampacity wire you need:
Wire Gauge Calculator (Get Ampacity, Choose Wire Size)
Simply input how many amps you want the wire to handle and you will get the min. ampacity that wire needs. After that, you can consult the wire ampacity wire and choose the correct wire size (we will show you how below):
Let’s explain how this calculator works using a 100 amp wire size as an example:
You just slide the slides to ‘100’ and get that a 100 amp wire should have at least 125A ampacity. To choose the wire size, just check the ampacity chart here and look for a wire with at least 125A ampacity. That’s it. If you have selected the 1 AWG gauge wire, you have the correct wire size for 100 amp service.
To help you out, we have summarized wire sizes for the most common amp needs in this chart:
Wire Size Chart
|Amps Needed (A):||Min. Ampacity Wire (A):||Gauge Wire Size:|
|1 Amp||1.25 Amps||28 AWG|
|5 Amps||6.25 Amps||22 AWG|
|10 Amps||10.25 Amps||20 AWG|
|15 Amps||18.75 Amps||14 AWG|
|20 Amps||25.00 Amps||12 AWG|
|25 Amps||31.25 Amps||10 AWG|
|30 Amps||37.50 Amps||8 AWG|
|40 Amps||50.00 Amps||8 AWG|
|50 Amps||62.50 Amps||6 AWG|
|60 Amps||75.00 Amps||4 AWG|
|70 Amps||87.50 Amps||3 AWG|
|80 Amps||100.00 Amps||3 AWG|
|90 Amps||112.50 Amps||2 AWG|
|100 Amps||125.00 Amps||1 AWG|
|110 Amps||137.50 Amps||0 (1/0) AWG|
|120 Amps||150.00 Amps||0 (1/0) AWG|
|130 Amps||162.50 Amps||00 (2/0) AWG|
|140 Amps||175.00 Amps||00 (2/0) AWG|
|150 Amps||187.50 Amps||000 (3/0) AWG|
|160 Amps||200.00 Amps||000 (3/0) AWG|
|170 Amps||212.50 Amps||0000 (4/0) AWG|
|180 Amps||225.00 Amps||0000 (4/0) AWG|
|190 Amps||237.50 Amps||250 kcmil (250 MCM)|
|200 Amps||250.00 Amps||250 kcmil (250 MCM)|
|250 Amps||312.50 Amps||400 kcmil (400 MCM)|
|300 Amps||375.00 Amps||500 kcmil (500 MCM)|
|350 Amps||437.50 Amps||700 kcmil (700 MCM)|
|400 Amps||500.00 Amps||900 kcmil (900 MCM)|
|500 Amps||625.00 Amps||1500 kcmil (1500 MCM)|
You can just check the table and see what size wire you need.
Example: What size wire do you need for 50 amp service? You need a wire with at least 62.5A ampacity. The correct choice is the 6 AWG wire because it has a 65A ampacity (more than 62.5A).
With this information, you can now correctly choose a wire size for any service.
Note: If you have sub panels 100 feet, 150 feet, 200 feet away, and so on, you should choose bigger wires (to account for voltage drop). You can consult these articles to get a better grip on how to calculate wire size at a distance from the source of electricity:
- 20 amp wire size.
- 30 amp wire size.
- 40 amp wire size.
- 50 amp wire size + sub panel at a 100 feet distance.
- 60 amp wire size + sub panel at a 100 feet distance.
- 100 amp wire size at a distance of 100 feet and 150 feet.
- 200 amp wire size.
If you want to calculate a specific wire size and haven’t found the answer here, you can use the comments below, describe the situation and we’ll try to help you out.