You're right to question whether the original arc flash study and labeling still applies after replacing the panelboards with enclosures and disconnect switches using the 10-foot tap rule. The answer is almost certainly no, and a new arc flash study is required. Here's a breakdown of why

Why the Original Arc Flash Study is Invalid:
Changes in Fault Current

Replacing a panelboard with a disconnect and enclosure almost always changes the available fault current. Panelboards inherently have impedance that limits fault current to some degree. A straight busbar configuration may have lower impedance, potentially increasing the fault current at the downstream equipment. This is the most critical factor in invalidating your old study. Increased fault current directly increases arc flash incident energy.

Change in Clearing Time

The upstream overcurrent protective device (OCPD, likely a breaker or fuses feeding the tap) is now clearing a fault downstream of a tap. The total impedance of the circuit has changed, and this will almost certainly affect the clearing time of the OCPD. Clearing time is directly related to arc flash incident energy. Longer clearing times mean higher incident energy.

Configuration Changes

The physical arrangement of conductors, bus bars, and enclosures affects arc flash incident energy. Even if fault current and clearing time didn't change (highly unlikely), the geometry change alone could alter the outcome.

Modified Upstream Protection

Often, tap rules require a change in the upstream overcurrent device. This will dramatically impact your clearing times and your incident energy calculations.

Changes in Working Distance

Even if the incident energy stayed the same, if the working distance has changed because of the new configuration, the arc flash boundary and PPE requirements could change.

The 10-Foot Tap Rule and Arc Flash

The 10-foot tap rule (NEC 240.21(B)(1)) allows a tap of a certain length without an overcurrent device at the tap point, provided certain conditions are met (ampacity of the tap conductors, protection against physical damage, etc.). However, the allowance of the tap doesn't negate the need to consider the potential arc flash hazards created by the tap. In fact, taps often increase arc flash hazards because they potentially increase available fault current and impact OCPD clearing times.

What You Need to Do


Re-Evaluate the Arc Flash Study

You must conduct a new arc flash study that includes these changes. This is not optional. OSHA and NFPA 70E require accurate arc flash assessments.2.

Update Arc Flash Labels

Based on the new study, generate and install new arc flash labels on each of the new enclosures/disconnects. These labels must accurately reflect the new incident energy, working distance, PPE requirements, and arc flash boundary.

Steps for the New Arc Flash Study



Collect Updated Data

This is crucial. Get accurate information on:

Upstream OCPD

Type, size, trip curves.

Conductor Length and Size

From the upstream OCPD to the enclosure/disconnect.

Bus Bar Material and Dimensions

Copper or aluminum, length, cross-sectional area.

Transformer Information

kVA, impedance, voltage.

Available Fault Current at the Source

This is often obtained from the utility company.

Enclosure Dimensions

Internal and external dimensions.

Working Distance

Document the actual working distance technicians will use.

Perform Fault Current Analysis

Calculate the fault current available at the enclosure/disconnect. This calculation must account for the impedance of the conductors, bus bars, and the source.

Determine Clearing Time

Determine the clearing time of the upstream OCPD for the calculated fault current. Use the manufacturer's time-current curves.

Calculate Incident Energy

Use appropriate arc flash calculation software (SKM PowerTools, EasyPower, ETAP, etc.) to calculate the incident energy at the working distance. These programs will consider the fault current, clearing time, and system voltage. It is highly recommended to use software for this.

Determine PPE Requirements

Based on the calculated incident energy, determine the required PPE category according to NFPA 70E.

Generate Arc Flash Labels

Create durable arc flash labels that include the incident energy, working distance, PPE category, arc flash boundary, and voltage.

Train Personnel

Ensure that all qualified electrical workers are trained on the new arc flash hazards and PPE requirements.

Important Considerations



Qualified Person

The arc flash study must be performed by a "qualified person" as defined in NFPA 70E. This usually means a licensed professional engineer with expertise in power systems analysis and arc flash hazard assessment.

Coordination Study

It's also important to review the overall system coordination study to ensure that the new protection scheme with the taps still provides adequate protection and coordination throughout the electrical system.

Documentation: Keep a detailed record of all data used, calculations performed, and assumptions made in the arc flash study.In summary, replacing the panelboards necessitates a new arc flash study. Do not rely on the old labels, as they no longer accurately reflect the potential hazards. Prioritize safety and ensure compliance with OSHA and NFPA 70E. Flag for review