Okay, let's break down NEC 240.21(B)(5) and see if your proposed setup for the septic treatment plant feeder and taps is compliant.

Understanding NEC 240.21(B)(5) - Feeder Taps Not Over 25 Feet Long

This section of the NEC allows a tap from a feeder without immediate overcurrent protection at the tap point under specific conditions. Here's the breakdown: (1) Length

The tap conductor cannot be longer than 25 feet.

(2) Ampacity

The ampacity of the tap conductor must not be less than 1/3 of the overcurrent device protecting the feeder conductors from which it is tapped.

(3) Protection from Physical Damage

The tap conductors are suitably protected from physical damage in an approved manner.

(4) Termination

The tap conductors terminate in a single circuit breaker or a single set of fuses that will limit the load to the ampacity of the tap conductors. This single circuit breaker or set of fuses shall be permitted to supply any number of additional overcurrent devices on its load side.

Analyzing Your Proposed Setup1. 70 Amp Feeder

You have a 70 amp feeder originating inside the building and running underground to an outdoor wireway.2.

Wireway

The underground feeder terminates in a wireway.3.

Taps

You plan to tap this feeder with: Two 20 amp taps (using #12 AWG conductors) One 30 amp tap (using #10 AWG conductors)4.

Terminations

Each tap terminates in its own circuit breaker inside the wireway.5.

Loads

The taps feed three controllers in the septic treatment plant: Two 20 amp, 2-pole (208V) controllers One 30 amp, 3-pole (208V) controller

Compliance Check

Let's analyze if this setup meets the requirements of NEC 240.21(B)(5): Length

We need to assume that the length of each of the taps is 25' or less.

Ampacity

This is where things get a little tricky and where your setup might be non-compliant, depending on how you interpret the code.

20 Amp Taps (#12 AWG)

The ampacity of #12 AWG is typically 20 amps (check NEC Table 310.16 for specific conditions). One third of the feeder overcurrent device (70 amps) is approximately 23.3 amps. Since 20 amps is less than 23.3 amps, on its face it would seem that your #12 tap is not large enough to take 1/3 of the load from the 70 amp feeder. Note that Table 310.16 would allow you to use the 20A rating of the #12 tap (20A 125% = 25A), since you are using a circuit breaker with a maximum size of 20A. However, 240.21(B)(5) requires the tap's ampacity to be 1/3 of the rating of the source conductor's protection, which is not being met.

30 Amp Tap (#10 AWG)

The ampacity of #10 AWG is typically 30 amps (again, check NEC Table 310.16). One third of the feeder overcurrent device (70 amps) is approximately 23.3 amps. Since 30 amps is greater than 23.3 amps, on its face it would seem that your #10 tap is large enough to take 1/3 of the load from the 70 amp feeder. Again, while Table 310.16 would allow you to use the 30A rating of the #10 tap (30A 125% = 37.5A), since you are using a circuit breaker with a maximum size of 30A, 240.21(B)(5) requires the tap's ampacity to be 1/3 of the rating of the source conductor's protection, which is being met in this scenario.

Protection from Physical Damage

Assuming the wireway and underground conduit provide adequate protection.

Termination

The taps terminate in individual circuit breakers. This is where your setup likely fails. NEC 240.21(B)(5) requires the tap conductors to terminate in a single circuit breaker or set of fuses. You are using multiple circuit breakers at the end of the taps.

Addressing the Termination Requirement

The key here is the "single circuit breaker or a single set of fuses" requirement. This is designed to ensure that the tap conductors are protected at their ampacity by one overcurrent device, not multiple.

Why Your Setup Doesn't Work (Probably)

Because the code states that the ampacity of the tap conductor must not be less than 1/3 of the overcurrent device protecting the feeder conductors from which it is tapped, your use of #12 conductors for the 20 amp taps seems to fail that rule.

Possible Solutions1. Reduce the Feeder Breaker

If you could reduce the main feeder breaker to, say, 60 amps (and your load calculations allow it), the #12 AWG conductors might meet the 1/3 ampacity requirement, but your #10 AWG tap would be too large. You also may have issues with voltage drop.2.

Individual Overcurrent Protection at the Tap

Install individual overcurrent protection (fuses or circuit breakers) at the point where you tap the feeder in the wireway. This eliminates the need to comply with 240.21(B)(5) entirely, as you're no longer relying on a tap rule. Use overcurrent protection sized to protect the tap conductors (20 amps for #12, 30 amps for #10).
3.

Increase Tap Conductor Size

Instead of #12 conductors, use #8 AWG conductors.4.

Redesign

Consider running separate feeders (if possible) from the main panel in the building to each controller in the septic treatment plant. This eliminates the tap issue altogether.

Important Considerations Load Calculations

Make sure you perform accurate load calculations to ensure that the 70 amp feeder and all tap conductors are adequately sized for the connected loads, including any motor starting currents.

Voltage Drop

Calculate voltage drop to ensure it's within acceptable limits, especially with underground runs. This might require increasing conductor sizes.

Local Amendments

Check for any local amendments to the NEC that might further restrict or modify these rules.

AHJ Approval

Always consult with your local Authority Having Jurisdiction (AHJ) before starting any electrical work to ensure compliance with all applicable codes and regulations. They have the final say on whether your installation meets the code.

Disclaimer: I am an AI chatbot and cannot provide official electrical code interpretations. This information is for general guidance only and should not be considered a substitute for professional advice from a licensed electrician or electrical engineer. You MUST consult with qualified professionals for any electrical work. Flag for review