Okay, let's break down this situation and address the best approach according to the NEC (National Electrical Code) and good electrical practices.

Understanding the Situation



Welder Requirements

50A outlet, 208V/3-phase Full Load Amps (FLA): 38A Inrush Current: 78A

Panel

100A/3-pole, 208V/3-phase breaker.

Problem

The 100A conductor size required by the 100A breaker is too large to terminate at the 50A receptacle.

Proposed Solution

Install a disconnect switch between the breaker and the receptacle to reduce wire size.

Analysis and Recommendations


Legality of the 100A Breaker

Is it code compliant to begin with to use a 100A breaker for a 38A welder? NEC 630.11(A) states the overcurrent protection for welders shall not exceed 200% of the rated primary current of the welder. In your case, 38A 2 = 76A. Therefore, a 100A breaker DOES NOT meet code. Instead, consider a 70A breaker, which will meet code.2.

Disconnect Switch and Fusing - MUST DO



You MUST use a fused disconnect switch. You are correct that the available fault current at the receptacle is likely to be higher than the withstand rating of the 50A receptacle itself. A fused disconnect provides both a local means of disconnecting power for servicing the welder and critical fault current protection. Fuse Sizing

Fuse size depends on the breaker rating. With a 100A breaker, you can still use the 70A fuse. There is nothing wrong with this, as long as the welder is protected. With a 70A breaker, you should follow the 70A fuse. There is nothing wrong with this, as long as the welder is protected. DO NOT exceed the breaker rating. The goal is to protect the receptacle and the downstream wiring, not necessarily the welder itself (although that is a welcome side effect).3.

Wire Sizing and Termination

Since you are using a fused disconnect switch, you can size the conductors after the disconnect switch based on the fuse size (or welder's full load amperage, if lower). For a 70A fuse, you would need conductors rated for at least 70A. Consult NEC Table 310.16 (and use the appropriate correction/adjustment factors based on the installation conditions) to determine the required conductor size. For example, 4 AWG copper THHN is rated for 95A, so it would be acceptable. The conductors from the fused disconnect to the 50A receptacle must be sized so that you can properly and safely terminate them at the 50A receptacle. Use the correct sized lugs and connectors for all terminations.4.

Receptacle Rating

Ensure the 50A receptacle is rated for the voltage (208V, 3-phase) and amperage requirements. It also needs to have a short circuit current rating that meets or exceeds the available fault current if no fused disconnect were to be used.

Putting it All Together - Revised Plan


Change the breaker

If your welder has a full load amperage of 38A, replace your 100A breaker to a 70A breaker to conform with NEC 630.11(A)2.

Install a Fused Disconnect Switch

Mount a fused disconnect switch near the receptacle. The enclosure should be rated appropriately for the environment (indoor, outdoor, etc.)3.

Wire Size (from panel to disconnect)

The wires going from the panel to the disconnect must be sized to match the rating of the 70A breaker (e.g., probably 6 AWG copper THHN).4.

Fuse the Disconnect

Use 70A fuses in the disconnect switch.5.

Wire Size (from disconnect to receptacle)

Size the wires from the disconnect to the 50A receptacle based on the 70A fuse rating (e.g., probably 4 AWG copper THHN), ensuring you can properly terminate them at the receptacle.6.

Proper Terminations

Use appropriately sized lugs and connectors for all connections. Torque connections to the manufacturer's specifications.7.

Labeling

Clearly label the disconnect switch.

Important Considerations



Local Codes

Always check with your local electrical inspector for any specific requirements or interpretations of the NEC in your jurisdiction.

Qualified Electrician

This type of electrical work should only be performed by a qualified and licensed electrician.

Safety

Always de-energize the circuit before working on any electrical components. Use appropriate personal protective equipment (PPE).*

Arc Flash

Consider the potential arc flash hazard and take appropriate precautions.

In summary, installing a fused disconnect switch is the correct approach to protect the receptacle from fault current. The fuse size should be sized accordingly based on the breaker rating (or the full load amperage of the welder, if lower). Wire sizes should be determined based on the applicable NEC tables and adjusted for any derating factors. Always consult with a qualified electrician and adhere to local codes. Flag for review