Yes, there are several potential code issues when using a 50A breaker to feed a 70A fused disconnect upstream of a motor. Here's a breakdown of the concerns, primarily based on the National Electrical Code (NEC)

1. Overcurrent Protection for the Disconnect Switch:
NEC 240.21(B)(3): Feeder Taps

While feeder taps allow reduced conductor size, they are very specific and unlikely to be met in your scenario. These rules are designed to provide proper short-circuit protection. If you can meet all the requirements for a feeder tap, you might be okay, but it's complex. The key things to consider are that a feeder tap will likely not work here because the length to the disconnect is to long or the conductors are not sized appropriately.

General Principle

Generally, the overcurrent protective device (the 50A breaker) protecting the conductors feeding the 70A disconnect must have a rating no higher than the ampacity of the conductors and the rating of the disconnect switch. You are likely violating this general rule.

2. Conductor Ampacity



NEC 310.16 and related tables

The conductors feeding the 70A disconnect must have an ampacity equal to or greater than the 50A breaker AND large enough to carry the motor load. It's highly probable that the existing conductors are sized for 70A. Using a 50A breaker means the conductors may be significantly oversized, which isn't directly illegal, but it points to a design mismatch and potential inefficiency.

3. Motor Overload Protection vs. Short-Circuit/Ground-Fault Protection



NEC Article 430 (Motors, Motor Circuits, and Controllers)

This is the key article to consult. Motor circuits have two distinct levels of protection:

Overload Protection

This protects the motor from running at too high a current for too long (e.g., a jammed load). This is typically provided by overload relays built into the motor starter or a separate overload device. These are sized based on the motor's nameplate current.

Short-Circuit and Ground-Fault Protection

This protects the circuit from catastrophic failures like shorts to ground or phase-to-phase shorts. This is what the breaker and the fuses in the disconnect are intended for.

NEC 430.52(C) and (D): Motor Branch-Circuit Short-Circuit and Ground-Fault Protection

This section is critical. It specifies the maximum allowable rating or setting for the short-circuit and ground-fault protective device (your breaker) based on the motor's Full Load Amps (FLA) and the type of motor. The breaker rating can be higher than the motor FLA, but only up to certain limits. For inverse time circuit breakers, the maximum is typically 250% of the motor's FLA. For fuses (the 70A disconnect), the maximum is often higher (e.g., 300% for non-time-delay fuses, 175% for time-delay fuses).

Why This is a Problem (likely)


Undersized Protection

If the motor FLA is anywhere near a value that requires a breaker rating of more than 50A to comply with NEC 430.52, you are violating the code. You're potentially depriving the motor circuit of adequate short-circuit protection.
2.

Incorrect Coordination

The 50A breaker may trip before the 70A fuses in the disconnect in the event of a fault. This is not ideal coordination and can lead to unnecessary shutdowns and increased stress on the system.

What You Need to Do


Determine the Motor's Full Load Amps (FLA)

This is on the motor's nameplate.2.

Calculate the Maximum Allowable Breaker Rating

Using NEC 430.52(C), based on the motor's FLA and type of motor, determine the maximum breaker rating permitted.
3.

Verify Conductor Ampacity

Ensure the conductors feeding the disconnect are rated to carry at least the motor's FLA and meet the requirements of 310.16.
4.

Coordination Study

Ideally, perform a coordination study to ensure the 50A breaker and the 70A fuses will operate in the correct sequence to minimize disruptions and maximize safety.

Possible Solutions (after proper calculations and code review)



Increase the Breaker Size (Most Likely)

If the calculations from NEC 430.52 allow, replace the 50A breaker with a breaker of the appropriate size to protect the motor and comply with the code. This might require also upgrading the conductors feeding the disconnect to match the new breaker's ampacity.

Reduce Fuse Size

Lower the fuse size in the 70A disconnect, provided this doesn't create nuisance tripping.

Change Disconnect to a Breaker

Replace the fused disconnect with a properly sized circuit breaker. This provides both overload and short-circuit protection in a single device and can simplify coordination.

Important Considerations



Existing Installation

Just because something is existing doesn't mean it's code-compliant or safe. Previous work might have been done incorrectly or to outdated codes.

Local Codes

Local jurisdictions may have amendments or stricter interpretations of the NEC.*

Qualified Electrician

This work MUST be performed by a qualified and licensed electrician. Motor circuits can be complex, and incorrect wiring or protection can lead to equipment damage, fire hazards, and personal injury.

In summary, using a 50A breaker to feed a 70A fused disconnect and then a motor is highly suspect and likely a code violation. You need to perform the calculations outlined above and consult with a qualified electrician to ensure the circuit is safe, code-compliant, and properly protected. Flag for review