Mike Dolt Enterprises
Mike Dolt Enterprises
User: tim89s, Location: Brush Prairie WA
Asked on: Wednesday, March 23, 2011 at 7:47 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:11 am EDT
Question: I am installing an 800 amp panel. Calculated load is 750 amps. I plan on running two 3" conduits with 500 kcmil in each of them. Only 200 amps of the 800 amp panel will be using a neutral. The rest of the loads are stricly 3 phase. I was told by my project manager that you can take 125% of the 200 amps and size your neutral according to that. 1.25 x 200 = 250 amps. Since I am running parallel would I install 125 amp rated wire in each conduit? He didn't tell me what code section he got that from, I am interested to know. Any help would be greatThanks Tim
Code Reference
National Electrical Code (NEC) Article 220.61(B)
Explanation
For unbalanced loads, such as in your case where only a portion of the load uses a neutral, you can use a smaller neutral conductor provided it meets the following conditions: The calculated neutral current is less than 200 amps. The overcurrent protection device for the neutral is not greater than 200% of the neutral ampacity.In your case, the calculated neutral current is 200 amps. Since this is less than 200 amps, you can use a neutral conductor with an ampacity of 250 amps (125% of 200 amps).
Conduit Sizing for Parallel Conductors
Since you are running parallel conductors, you would need to adjust the conduit size accordingly. For 2 conductors in parallel, the conduit would need to be sized based on the ampacity of each individual conductor.In your case, you are using 500 kcmil conductors, which have an ampacity of 325 amps. Therefore, each conduit would need to be sized for 325 amps, which would require a
3" conduit.
Conclusion
Based on the NEC and the information provided, you would need to install two 3" conduits, each with 500 kcmil conductors for the 3-phase loads, and
125 amp rated wire in each conduit for the neutral. Flag for review