What size wire for 400 amp service?

What size wire for a 400 amp service? Full Guide

400 amp wire

The best wire size for a 400 amp service is a 1,000 kcmil wire. This wire size will ensure the best possible service for your building.

Which applications use 400 amp service?

A 400 amp service is needed in industrial, commercial and institutional applications. These applications may include schools, hospitals, factories and other large buildings.

What are the benefits of a 400 amp service?

The main benefits of a 400 amp service are that it can provide more power to your building, helping you to run more appliances or devices at once. It can also help to improve the reliability of your electrical service. In addition, a 400 amp service is more future-proof than a smaller service, meaning it can handle more electrical demand as your business grows.

The speed at which technology is moving each year will continue to put a strain on your business' electrical service. Having a 400 amp service can help you to keep pace with your needs, without having to install another power source early on.

What decides wire size for a 400 amp service?

When installing a new electrical service, the type of cable that is used will be determined by two factors:

  • The voltage drop allowed by code (typically 3%)
  • The ampacity required by the load.

The National Electric Code establishes that the maximum allowable voltage drop from one end of the circuit to the other is 3%. Voltage drops greater than this can lead to unreliable equipment operation and poor performance due to excessive heating in conductors and terminal equipment. To ensure that you stay within this limit, it's important that you use quality aluminum or copper wire for your application.

If you're looking for an upgrade to your current electrical service or are in need of a new one for a new building, a 400 amp service is the best option. This service will provide you with enough power to keep pace with your needs.

What exactly is voltage drop?

Voltage drop is described as the loss of voltage in a circuit due to the resistance of the wire. This drop in voltage can be caused by a number of factors, including the length and size of the wire, as well as the current running through it. The greater the current, the greater the voltage drop.

When sizing your wire for a 400 amp service, it's important to keep voltage drop in mind. The National Electrical Code (NEC) dictates that the maximum allowable voltage drop in any circuit should not exceed 3%. This includes both the hot and neutral conductors.

What happens if you choose the wrong wire size?

Choosing the wrong wire size can result in inadequate power supply to the load, which can lead to equipment failure. Voltage drop issues will also affect how appliances operate.

Choosing the right wire size for your 400 amp service is important, but it's equally important to consider other factors. After all, larger wire has a higher resistance per foot than smaller wire so there are many things that must be considered when choosing the right wire.

For example, you need to consider your voltage drop percentage limit by using NEC Volts-Ohm's Law. You'll find that since you're already at 3% voltage drop, it may not be worth increasing cost and energy efficiency by increasing wire size further unless absolutely necessary.

What is the NEC Volts-Ohm's Law for 400 amp service?

NEC Volts-Ohm's Law states that your maximum allowable voltage drop across any wire is equal to 3% plus 4 tenths of one percent times the square root of the total current in amperes. This means it is not cost effective to run larger wire if only a small amount of power will be used per foot, but as you double or triple the power load per foot, then more energy efficient options are available.

Figuring out which size wire is right for your application can get complicated. That's why it's best to use an NEC compliant electrical calculator or contact a licensed professional electrician before final decision.


This blog post went over many different things to consider when selecting the right wire size for your electrical service. Keep in mind that these are just general guidelines, and that local codes may have specific requirements that must be followed. Always consult with a licensed electrician to get an accurate recommendation for your individual project.

When it comes to 400 amp service, you will want to use the best wire possible for your application. This means using a wire that is up to the task, and has been NEC compliantly certified. There are many factors to consider when making this decision, such as the amps being delivered, the length of the run, and the gauge of wire needed. In most cases, it's best to use copper wiring for your electrical service. However, aluminum wiring can also be used in some applications.

60 Amps

Read this post to know all you need to know about using 60 amps.


20 Amps

All you need to know about using 20 amps. Some of the best 20 amp products are also recommended.


15 Amps

Knowing how to use 15 amps is very important. This post goes over everything you need to know.


40 Amp Breaker Wire Size

At some point in time you might want to install a 40 amp breaker. This post will tell you all you need to know about the correct 40 wire size.


As an Amazon Associate I earn from qualifying purchases.

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.