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What Size Wire for 400 Amp Service? – Sizing Guide

Written by Edwin Jones / Fact checked by Andrew Wright

What Size Wire for 400 Amp Service

A large home may demand a 400-amp service to help power additional appliances, like extra heating and air conditioning. But the question is, what size wire for 400 amp service?

A 1,000 kcmil aluminum or 600 kcmil copper wire is ideal for a 400-amp service. However, the wire to choose for the service panel may differ depending on certain factors, such as wire type and length.

It’s important to understand the features of different wires to know which model to use for your 400-amp service. This guide shares details about that concern, along with the factors to consider to help you choose the correct wire.


Wire Size for 400 Amp Service

Understanding the different wire types available for a 400a service is vital. Doing so helps reduce the risks of issues for the panel, the circuit, and the connected devices.

1. Wire Size Types

Wire types typically fall under four categories. Each classification has its unique pros and cons. The four kinds of cable sizes for 400 amps to watch out for are the following:

  • Thermoplastic High-Heat-Resistant Nylon-Coated (THHN)


A single conductor wire is commonly used in different buildings. As its name implies, its high heat and oil resistance can make it a suitable choice in a diagram with various appliances and types of machinery.

  • Thermoplastic High-Water-Resistance Nylon-Coated (THWN)


The THWN wire has similar traits as a THHN cable, except for the focused resistance. With THWN, you can take advantage of high water resistance, allowing the wire to resist temperatures of up to 167 degrees Fahrenheit (75 degrees Celsius) when wet.

  • Thermoplastic-Sheathed Cable (TPS)


Commonly known by its trade name Romex, a residential service will generally use this type of wire for a 400-amp electrical panel. It usually has non-metallic sheathing used for branch wiring.

  • Cross-Linked Polyethylene High-Heat and Water-Resistant Wire (XHHW)


A type of wire size for commercial and industrial use. An XHHW cable generally has a single conductor. However, it usually has thicker insulation than THHN models.

2. Requirements to Choose the Right Wire Size for 400-Amp Service

Apart from the wire type, reflect upon other requirements in choosing the correct wire for a 400-amp electrical panel. Determine the correct cable to use by asking yourself the following questions:


  • What is the wire’s flexibility?
  • What wire insulation to use?
  • Should you use copper or aluminum wire?
  • How many phases? (singel, parallel or three?)
  • Is it underground or not?

For instance, if you want a suitable 400 amp 3 phase wire size for your house, opt for 600 kcm copper wires or 1000 kcm alumnium wires. Meanwhile, if it’s -140°F underground, you will need a 1250 kcm aluminum wires instead.

Factors to Consider for Wire Size Selection

Aside from the elements already mentioned, think about other important factors that can affect your wire selection for a 400-amp panel. The things to consider include the following:

1. Voltage Drop

According to the National Electrical Code (NEC) guidelines mentioned in 210.19(A) (1), the recommended voltage drop shouldn’t exceed 3%. But the maximum combined voltage drop for both the feed and branch circuit should be 5% at most.

Note that the wire’s carried voltage will drop as it lengthens. So, here’s a wire size chart highlighting the wire length and which gauge wire to choose at a 3% voltage drop.


Wire Length Wire Size at 120 Volts Wire Size at 220 Volts
0 feet 600 kcmil Copper Wire 600 kcmil Copper Wire
300 feet 900 kcmil Copper Wire 600 kcmil Copper Wire
400 feet 1,250 kcmil Copper Wire 700 kcmil Copper Wire
500 feet 1,500 kcmil Copper Wire 800 kcmil Copper Wire

2. Current Type

Calculate the wire size to use for a 400-amp electrical panel using the current type. Note that a single-phase current generally uses different cables than a three-phase service box.

Solve for the wire to use for a single-phase current by using the following formula:

\[ n = -39 \times \log_{\frac{9}{2}}(R) + 36 \]

wherein “R” is the wire’s diameter

If you’re searching for a 400-amp 3-phase wire size, multiply the result of the above formula by √3/2.

Frequently Asked Questions

What Size Wire Do I Need for a 400-Amp Parallel Service?

A typical 400-amp service using parallel service conductors generally requires 500 kcmil or 750 kcmil aluminum cables. Note that these wire sizes are usually for commercial use and aren’t often ideal for residences.

What Is The Ground Wire Size For 400-Amp Service?

The correct ground wire size to use for a 400-amp service typically depends on the gauge cable used for the panel. Assuming that the service panel uses a 700 kcmil copper wire, a ground wire that can handle 5,000 amps should suffice.


In conclusion, the process of determining what size wire for 400 amp service depends on different factors. Still, 1,000 kcmil aluminum or 400 kcmil copper wires are fairly common choices.

Nonetheless, you must also think about other factors, including the wire’s type and length. Also, don’t forget to ask questions about your goals for the setup to determine the correct wire size to use for the 400-amp electrical box.

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