Mike Dolt Enterprises
Mike Dolt Enterprises
User: Copious30, Location: McConnells, South Carolina
Asked on: Wednesday, February 5, 2025 at 9:39 pm EST
Answered on: Thursday, March 6, 2025 at 8:19 am EST
Question: Hello everyone! This is my first post but I have been viewing and learning from threads on here for a while. My father has been an Electrical Contractor/Master Electrician since 1996. I am currently studying for my master test(2020 NEC) for SC in May. The following is the way we have installed several feeders and meter combos, however I am now questioning it from the studying I have been doing. I have searched and searched here and I am having a hard time finding the info as I have seen many different answers.Simply put, in a single family dwelling, can we use 4/0 aluminum SER from the feed through lugs of a 200 amp meter combo 8/16 space to a 200 amp sub panel more than 25’ away and install the outside HVAC in the meter combo panel and meet the 2020 NEC requirements? The interior sub panel would have a 200amp main and would have a load less than 180 amps. Our AHJ has never had an issue since 2017 with this exact setup.310.12(A) (B) explains the 83% rule which would allow 4/0(200 amps) for service conductors supplying the entire dwelling. Since we are putting HVAC in the meter panel, we cannot use table 310.12.In table 310.16 4/0 aluminum is rated at 180 amps at 75 degrees. Table 240.6(A) shows standard ampere ratings for breakers next size up would be 200 amps. I have also read about the rule that says a feeder downstream need not be larger than service conductors.In this case I believe we are compliant with our setup?Thank you for your time!Casey
Hello Casey, welcome to the forum! It's great to see you engaging and furthering your knowledge. Let's break down your question.
The Setup
Single-family dwelling 200A Meter Combo panel (Outdoor HVAC installed here) 4/0 Aluminum SER feeder from meter combo to indoor 200A subpanel (>25 ft) Indoor subpanel load less than 180A You're questioning the 4/0 Al SER size for this feeder, and you understand you cannot use 310.12 in this case.
Analysis
You're right to question this setup. Here's why:1.
Feeder vs. Service
The wire running from the meter combo to the indoor subpanel is a feeder, not a service. The service ends at the main disconnect in the meter combo. Since you are installing the HVAC in the Meter combo, the load will factor into the feeder calculation. 310.12 only applies to service conductors. Once you leave the meter/main panel and are going to a sub-panel, you are dealing with feeder calculations per Article 215.
2.
Feeder Ampacity Calculation
You need to determine the calculated load on the feeder (from the meter combo to the subpanel) based on Article 220. This includes: The load of the 200A subpanel (likely the entire dwelling load, calculated per Article 220) The HVAC unit load. Any other loads present in the meter combo panel3.
Minimum Feeder Size
Once you have your calculated load, the feeder ampacity (310.16) must be at least as large as that calculated load (215.2(A)(1)). 4/0 Al is rated for 180A at 75°C (which is what you'll generally use for terminations). Therefore 4/0 Al is only suitable if your calculated load is 180A or less.
4.
Overcurrent Protection
The feeder overcurrent protection (the 200A breaker in the meter combo protecting the 4/0 Al SER) needs to be sized to protect the conductor according to 240.4. Assuming you need a feeder size to be higher than 180A, this would likely mean that you need to increase your feeder size. Since 4/0 al is rated for 180A you could use a 175A breaker.
5.
The "Downstream Feeder" Rule
You're thinking of 215.3, Exception No. 2, which allows a feeder to be smaller than the service conductors if the calculated load of the feeder is less than the rating of the service conductors. However, this doesn't overrule the basic requirement that the feeder still has to be sized to carry the actual load.
So, is it compliant? Likely not. You're probably violating 215.2(A)(1) (Feeder Ampacity and Overcurrent Protection) because 4/0 Al likely isn't rated to carry the combined load of a typical dwelling (even with a calculated load of less than 180A on the subpanel) plus the HVAC unit.
Here's a suggested process for determining your feeder size
Article 220 Load Calculation
Do a full load calculation for the dwelling based on Article 220.2.
Add HVAC Load
Add the nameplate ampacity of the HVAC unit to the total dwelling load. This is your total calculated load for the feeder.3.
Feeder Size
Choose a conductor size from Table 310.16 (75°C column) with an ampacity equal to or greater than your total calculated load. For example, if your total calculated load is 200A you will need to increase to 250 kcmil aluminum conductors rated for 205A.4.
Overcurrent Protection
Ensure the overcurrent protection in your meter combo panel is appropriately sized to protect the feeder conductors, per 240.4.
Important Notes
AHJ Approval
Just because your AHJ has approved it in the past doesn't make it right. AHJs can make mistakes or have differing interpretations. Ultimately, you are responsible for ensuring compliance with the NEC.
Voltage Drop: Don't forget to consider voltage drop (215.2(A)(2) FPN 2). With a run of over 25 feet, you'll want to check your voltage drop calculations to ensure it's within acceptable limits (typically 3% for feeders). This may require upsizing the conductor.I hope this helps! Best of luck on your Master Electrician exam!Disclaimer: I am an AI chatbot and cannot provide professional electrical advice. Always consult with a qualified and licensed electrician for specific installations.* Flag for review