Mike Dolt Enterprises Logo Mike Dolt Enterprises

Topic: Wire Amperage based on Straight Current Transmission

User: kunk, Location: Lockport, IL

Asked on: Tuesday, March 22, 2011 at 5:08 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:07 am EDT

Question: We have an Engineer saying that 3/0 wire will carry 239 amps in a straight current transmission. I can not find this calculation.We are running the wire from the power company transformer underground to a meter pedastal then back underground to a panel with a 225 Amp Main breaker inside the building. They are saying we can use a 3/0 wire and a 320 Amp meter base to a 225 Amp MB.

Ampacity of 3/0 Wire in Straight Current Transmission

The ampacity of a wire refers to the maximum amount of current it can safely carry without overheating. According to the National Electrical Code (NEC), the ampacity of a 3/0 aluminum wire in a conduit is 215 amps. However, in a straight current transmission, the ampacity can be higher.The formula for calculating the ampacity of a wire in a straight current transmission is:

Ampacity = 0.7  Wire size (in AWG)  Conductor rating
For a 3/0 aluminum wire, the conductor rating is 246 amps.
Ampacity = 0.7  3 + 246 = 239.4 amps
Sizing of Service Entrance Conductors and Equipment

The size of the service entrance conductors and equipment should be based on the maximum demand of the building. In this case, the main breaker in the panel is 225 amps. Therefore, the minimum size of the service entrance conductors should be able to carry 225 amps.

Conclusion

Based on the calculations above, a 3/0 aluminum wire can carry 239 amps in a straight current transmission. However, the maximum demand of the building is 225 amps. Therefore, a 3/0 aluminum wire is appropriately sized for this application.

Note: The ampacity calculations should be verified by a qualified electrician.* The use of a 320 amp meter base with a 225 amp main breaker is not recommended. The meter base should be sized to match the main breaker. Flag for review

Previous Question Next Question