Yes, it is very likely a code violation due to ampacity derating. Here's a breakdown of why and what to consider

The Problem: Ampacity Derating
30 Conductors in a Conduit

The primary issue is the sheer number of current-carrying conductors (CCCs) in a single conduit. The NEC (National Electrical Code) severely limits the ampacity of conductors when more than three are bundled together in a raceway or cable. This is because heat builds up, and THHN insulation can be damaged if it gets too hot.

NEC 310.15(C)(1)

This is the section of the NEC that deals with ampacity adjustment factors for more than three current-carrying conductors in a raceway or cable. It mandates derating.

The Derating Process

You'll need to:1.

Determine the Base Ampacity

Look up the ampacity of #3 THHN copper in the 75°C column of NEC Table 310.16. (Typically, it's 85 amps, assuming termination temperature limitations).
2.

Apply the Adjustment Factor

Find the adjustment factor in NEC Table 310.15(C)(1). With 30 conductors, the adjustment factor will be very low, likely around 40% - 45%.
3.

Calculate the Adjusted Ampacity

Multiply the base ampacity by the adjustment factor. Example: 85 amps 0.40 = 34 amps.

Example Scenario (Illustrative)

Let's assume a 40% derating factor for this example. #3 THHN ampacity (75°C column): 85 amps Derating Factor (30 conductors): 40% (0.40) Adjusted Ampacity: 85 amps 0.40 = 34 ampsThis means each #3 THHN conductor is only effectively carrying 34 amps.

The Issue with 100A Load Centers



10 Load Centers 100A = 1000A Potential Load

If all 10 load centers draw their full rated 100 amps simultaneously, that's a demand of 1000 amps.

30 Conductors at 34 amps (Adjusted)

30 conductors 34 amps = 1020 amps total capacity. While this looks like it might be enough, you need to consider several additional factors:

Voltage Drop

With that many conductors and that much load, you'll likely have significant voltage drop over the 8-foot run. Voltage drop not only reduces the performance of the loads but also increases the current draw, exacerbating the derating issue.

Demand Factors

Are the apartments really going to draw 100 amps each simultaneously? You need to apply demand factors (as allowed by the NEC, Article 220) to realistically estimate the actual load. This requires knowledge of the types of loads in each apartment (lighting, appliances, HVAC, etc.).

Neutral Conductors

Are all 30 conductors current-carrying? If some are neutrals carrying primarily balanced loads, you might be able to exclude them from the derating calculation (see NEC 310.15(C)(1)(a)).

However, with apartment buildings, it's very likely that the neutrals are carrying a significant amount of current due to harmonic loads, and therefore must be considered. Grounding/Bonding

Consider the need for an equipment grounding conductor (EGC). If you have more than one circuit per load center, you need a grounding conductor.

Overcurrent Protection

Even if you can theoretically meet the load, the overcurrent protection devices (breakers or fuses) must properly protect the conductors at their derated ampacity.

Possible Solutions (and Considerations)


Multiple Conduits

This is the most common solution. Divide the conductors into multiple smaller conduits so you have fewer than four CCCs per conduit. This greatly reduces or eliminates the derating requirement. You'll need to ensure proper sizing of each conduit.2.

Larger Conductors

Instead of #3 THHN, you could use larger conductors (e.g., #1/0 or #2/0) that have a higher base ampacity. Even after derating, they might provide sufficient capacity. However, even with larger conductors, you may still run into space constraints within the conduit and the load gutter.3.

Parallel Conductors

In some cases, you can run conductors in parallel. This requires careful attention to NEC requirements (NEC 310.10(H)). All parallel runs must be the same length, material, size, insulation type, and terminated in the same manner.
4.

Feeder Calculation and Demand Factors

Thoroughly perform a feeder calculation based on the NEC using appropriate demand factors for apartment buildings. This might show that the actual load is significantly less than 1000 amps. This is crucial. However, it does not guarantee a pass.5.

Change the Load Gutter

A larger load gutter might allow you to physically separate the conductors more, mitigating some of the heat buildup (but this likely won't eliminate the derating requirement).

Important Considerations for Your Plans



Show Your Calculations

Clearly show all your ampacity calculations, including the base ampacity, the adjustment factors, and the final derated ampacity.

NEC References

Cite the specific sections of the NEC you are using for your calculations.

Demand Factors

Justify the demand factors you are using, referencing the appropriate table in the NEC (Article 220).

Overcurrent Protection

Specify the correct size and type of overcurrent protection devices (breakers or fuses).

Professional Responsibility

As the person creating the electrical plans, you have a professional responsibility to ensure the design is safe and meets code. If you're unsure, consult with a licensed electrical engineer or a more experienced electrician.*

AHJ Approval

The Authority Having Jurisdiction (AHJ, usually the local building inspector) has the final say. Be prepared to defend your design and answer their questions.

In conclusion, the initial configuration with 30 #3 THHN conductors in a single 4" conduit is highly unlikely to meet code due to ampacity derating. You MUST perform detailed calculations and explore alternative solutions to ensure a safe and compliant installation. It is strongly recommended to involve a licensed electrical engineer for a project of this scope and complexity. Flag for review