Mike Dolt Enterprises
Mike Dolt Enterprises
User: JDye, Location: Missouri
Asked on: Tuesday, February 18, 2025 at 3:23 pm EST
Answered on: Thursday, March 6, 2025 at 8:47 am EST
Question: Hello everyone and thank you. I have been a long time forum reader and I'm now currently in the process of Mike Holt's training on the 2023 cycle. I'm sure this type of question has come up before but I had trouble finding it.That being said, what am I missing?400 amp 208y/120 three phase main panel service wire size for commercial property. The client moving in is a micro brewery with the only load information being "our family electrician said we maxed out our 200 amp and we need a 400 amp." That's all we have to go with for load calculations. The landlords want us to install the service and they provided a 400 amp main panel. The wire will be aluminum and ran in 2 runs parallel. Each run will be in PVC underground from the utilities disconnect to the panel.Assuming that most of the loads won't be 3 phase, I'm calculating the neutral as a current carrying conductor. Wire will be modern wire and either of the RHW-2 or THWN-2 multi-rated jacketed wire family. Lugs are 75C.If I use the 75C column for the aluminum wire, then derate to 80% { 310.15 (c)(1) 4 current carrying conductors} I have to use a 350kcmil wire to get up to exactly 400 amps. The lugs on the meter can and disconnect provided by the city only go up to (2) 250kcmil. I know it's a 320 amp continuous and 400 amp peak meter. Anything above that and the city puts you on demand. If I use the 90C column to derate my wire, I can get by with a 300kcmil but that still doesn't explain why my factory lugs are only good for up to (2) 250kcmil. And by going to the 90C column for my wire wouldn't I violate 110.14 (c)?If the meter can is only good for 320 amps, is that what I size to? If I do then (2) 250kcmil at 75C gets me to 328 amps which is over 320 amp but a long ways away from 400. At 90C (2) 250kcmil only gets me to 368 amps. I feel like having 400 amp fuses (city's fused disconnect), and a 400 amp main, I should be a lot closer to that 400 amp mark and if the meter fails, that's the city's problem to replace. I know you can round up breaker size to the next size breaker { 240.4 (b) } but I have no way of knowing the trip settings on the used main breaker the client provided.I had felt confident in my in my 350kcmil calculations until I noticed the factory installed lugs weren't big enough. Then I started to second guess myself and now I feel like I'm missing something drastic and possibly over-sizing my wire but I want to do it right. If I have missed something please quote the reference number so that I may read it myself.Again thank you.
Okay, let's break this down step-by-step. You're right to be concerned about all the factors you've identified. Here's a structured approach to resolving the issues
1. Understanding the System Limitations:
Meter Can Rating (320A Continuous, 400A Peak)
This is a critical piece of information. This limits the continuous current that the utility will allow through the meter. The fact that you know this is a critical piece of information to keep in the front of your brain.
Lug Ratings (Up to 2 x 250 kcmil)
This is another major constraint. The termination points must be rated for the conductor size, or you will be violating the code.
400A Fused Disconnect & Main Panel
These components have the potential to handle 400A, but the overall system is constrained by the meter can and lug ratings.
2. Addressing Your Concerns and Calculations
Overcurrent Protection
It's crucial to determine the actual overcurrent protection installed. You mentioned 400A fuses.
Neutral as a Current-Carrying Conductor
In this scenario, with a microbrewery and primarily single-phase loads, treating the neutral as a current-carrying conductor is likely necessary per 310.15(E)(1)(a). This requires derating the ampacity.
75°C vs. 90°C Terminations
This is a key area where confusion often arises. You're correct in pointing out 110.14(C). Here's the breakdown:
110.14(C)(1)(b)(3)
The default termination temperature rating for equipment rated over 100A is
75°C unless marked otherwise. What it means
You
MUST use the 75°C ampacity column in Table 310.16 when determining the allowable ampacity of the conductor at the termination point. This is non-negotiable unless the equipment is specifically listed and marked for 90°C terminations (rare for standard panels and disconnects at this size). Derating for Multiple Conductors
You correctly identified 310.15(C)(1)(a). With 4 current-carrying conductors in a raceway (3 phases + neutral), you derate to 80%.
3. The Core Problem and Solution
The fundamental problem is this: you are trying to install a 400A service on a system with significant limitations (320A continuous meter, lug size). You cannot simply ignore these limitations. You must design the service to operate safely and within the utility's parameters.
Here's the recommended approach
Communicate with the Utility
Crucial! Discuss the limitations of the meter can. Ask if a larger meter can can be installed and whether the utility can increase the allowable amperage.2.
Design Based on the Weakest Link
Assume that the utility cannot allow any upgrades on the city side. You must design the entire system to operate within the 320A continuous rating of the meter can and the 250 kcmil lug limits on the panel and disconnect.3.
Calculate for 320A Continuous (With the Current Limitations)
Parallel Conductors
320A / 2 = 160A per conductor.
Derating
To account for the neutral, you need to use 160A/0.8 = 200A.
Conductor Size (75°C)
Refer to Table 310.16 (Aluminium) and select the conductor that's rated for at least 200A at the 75°C column. A 2/0 Aluminum (175A) is not big enough, 3/0 aluminum (200A) would fit and provide 320 amps with 4 current carrying conductors after being ran in parallel.
Therefore you should use (2) sets of 3/0 aluminum per phase (including the neutral).4. Grounding/Bonding
Ensure proper sizing of the grounding electrode conductor (GEC) and equipment grounding conductors (EGC) per Article 250, based on the service entrance conductor size (the 3/0 Aluminum) to avoid having them as the weakest link.5.
Main Breaker and Fuses
While you have a 400A main breaker and fuses on the disconnect, consider replacing the main breaker to a 350A breaker. It is okay to have a higher-rated disconnect, but it is better to have the disconnect match the main breaker.
Important Considerations
Future Load Growth
Even though the client says they need 400A, their actual initial load is likely far less. If you size for 320A now, the system might be perfectly adequate for years. If there are future plans for expansion, provide conduit sizing for the future upgrade.
Code References to Focus On
110.3(B)
Listed and Labeled Equipment. You must install equipment according to its listing and labeling.
110.14(C)
Termination Temperature. As discussed above.
215.2
Minimum Rating and Size of Feeder Conductors. Applies to the feeder from the service to the brewery's main panel.
220
Load Calculations - even if the electrician gave load estimates, a formal load calculation is always recommended.
230
Services
250
Grounding and Bonding
310
Conductors for General Wiring
In Summary:
You've done a great job identifying potential problems. You're not missing anything obvious. The key is to understand the limitations of the existing equipment and design the service to operate safely and code-compliantly within those limitations. The utility must be involved in any decision to exceed the meter can's rating. Your duty is to inform them and design the best and most appropriate solution for the situation that you have. Flag for review