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What Size Wire for 400 Amp Service: (Essential Info Guide)

When wiring your home or a building, choosing the right size wire for your electrical service is crucial.

The right size copper wire for a 400 amp service is 600 kcmil. But if you’re working with aluminum or copper-clad aluminum wires, you’ll want a 1,000 kcmil size wire instead. Nonetheless, factors like distance and voltage drop can come into play, so it’s essential to account for those variables.

Let’s get down to the nitty-gritty and dig into the proper wire sizes you need for a 400 amp service.

Determining Wire Size for 400 Amp Service

Copper Wires

exposed ends of a copper wire

I’ve always found copper wires a great choice due to their high conductivity and durability. For a 400 Amp service, 600 kcmil copper wire will be your ideal choice.

But don’t forget about the ground wire: you’ll need 3 AWG copper.

Every installation is unique, so these numbers might change based on voltage drop, temperature, and distance.

Aluminum Wires

electrical cable wires in colors blue, brown, yellow-green, and black

Now, if you prefer to work with aluminum wires, they can be more cost-effective and lightweight than copper.

For a 400 Amp service, you’ll want to use a 1,000 kcmil aluminum wire. And for the ground wire, go with a 1 AWG aluminum wire.

Here’s a quick summary of the ideal wire sizes:

  • Copper: 600 kcmil for the main wire, 3 AWG for the ground wire
  • Aluminum: 1,000 kcmil for the main wire, 1 AWG for the ground wire

That said, checking with a professional electrician is still recommended to ensure you select the correct wire sizes for your project.

Whether you choose copper or aluminum, using the right wire size will ensure the safety and efficiency of your electrical system.

Wire Size Charts and National Electrical Code

Wire size charts and the National Electrical Code (NEC) are important when planning electrical projects.

The charts are based on the National Electrical Code (NEC), and they help you ensure that your wires are suited to handle the right amount of amps safely. Thus, it’s crucial to follow the NEC guidelines.

Now, let me share a handy little chart for wire sizes and their maximum amp ratings per the NEC:

Wire Size (AWG/kcmil)60°C (140°F)75°C (167°F)90°C (194°F)
4/0 Aluminum180A205A230A
600 kcmil Copper445A475A505A
1,000 kcmil Aluminum485A545A615A

Factors Influencing Wire Size Selection


Distance plays a huge role in choosing the right wire size for a 400 amp service.

The longer the distance, the larger the wire size you’ll need. Why? It helps to reduce voltage drop and electrical loss along the length of the wire.


Temperature is another vital factor in wire size selection. As the temperature around the wiring changes, so does its ability to carry electrical current.

Hotter temperatures require larger wire sizes, while smaller ones can use smaller ones. Remember that, especially when working in environments with extreme temperature fluctuations.

Voltage Drop

Alright, let’s chat about voltage drop. You might think it’s a bummer, but it’s a key consideration when selecting wire size.

If your voltage drops are too high, your electrical devices might experience performance issues or damage.

So, choosing the right wire size helps alleviate voltage drop problems and keeps those electrical devices running smoothly.


You can’t talk about wire size without mentioning amperage. The wire size you select must handle the amps that your 400 amp service puts out.

For example, a 400 amp service requires 600 kcmil copper wire or 1,000 kcmil aluminum wire. Catering to the amperage ensures safe and reliable electrical connections.


Last but not least, let’s discuss pressure. Wire connections experience “pressure,” including physical stress, corrosion, and dirt.

Pressures like these can negatively affect the wire’s performance and service life.

So, consider pressure when selecting wire size, especially if your project involves underground or overhead power lines.

Keep those wires secure, clean, and protected, and you’re good to go!

Remember, considering factors such as distance, temperature, voltage drop, amperage, and pressure will ultimately help you select the perfect wire size for your 400 amp service. Happy wiring!




  • “Electrical Wiring Residential” by Ray C. Mullin and Phil Simmons


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About Alex Robertson

AvatarCertifications: B.M.E.
Education: University Of Denver - Mechanical Engineering
Lives In: Denver Colorado

Hi, I’m Alex! I’m a co-founder, content strategist, and writer and a close friend of our co-owner, Sam Orlovsky. I received my Bachelor of Mechanical Engineering (B.M.E.) degree from Denver, where we studied together. My passion for technical and creative writing has led me to help Sam with this project.

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