Okay, let's break down the issues regarding NM cable, exterior service panels, and potential code violations. Your boss has a point, and while the picture you showed might be legal in some interpretations, it raises significant concerns.

Here's a breakdown of the key considerations and relevant code sections (based on the NEC - National Electrical Code, which is widely adopted in the US; other codes may vary)



NM Cable in Damp or Wet Locations (Exterior)

This is the core issue.

NEC 334.12(B)(4)

This is a critical section. It prohibits NM cable from being installed in damp or wet locations. An exterior service panel, by definition, is exposed to the elements, making it a damp or wet location.

Exception

The exception allows NM cable to be embedded in plaster, concrete, or brick. That does not apply in this case.

Why is this a problem? NM cable is not designed to withstand moisture. Exposure to moisture can cause corrosion of the conductors, degradation of the insulation, and ultimately, short circuits, ground faults, and fire hazards. Service Entrance vs. Subpanel

It's important to clarify if you're talking about a service entrance panel (where the utility power comes into the house) or a subpanel (fed from the main service panel). The requirements are often different. I will assume that this is a service entrance panel since you stated service panel.

Knockouts and Weatherproofing



NEC 314.15

Boxes, conduit bodies, or fittings installed in wet locations must be approved for use in wet locations. This means they should be designed to prevent water from entering. Standard knockouts aren't inherently weatherproof.

NEC 314.17(B) (Removing Knockouts)

Unused openings in boxes, conduit bodies, or fittings must be closed or plugged.

Issue

Your boss's concern about "a million KOs" is valid. Each penetration is a potential entry point for water and insects. Even if you use KO seals, it's difficult to guarantee long-term waterproofing, especially with multiple penetrations. Over time, seals can degrade due to UV exposure, temperature changes, and physical damage.

Chase and Protection from Physical Damage

If you're using a chase (conduit), you generally need to transition to a wiring method suitable for that environment.

NEC 300.4(F) (Protection From Physical Damage)

Where subject to physical damage, conductors must be protected. NM cable is relatively fragile and not designed for exposed locations where it could be easily damaged.

Why the Picture Might Seem Okay (But Isn't Recommended)

The picture could be argued as "okay" if the following conditions were met (but it's still a very bad idea and likely to be rejected by an inspector):1.

Extremely Short Run

The NM cable run from the interior wall to the back of the panel is very short, and immediately enters the panel.2.

Protected from Physical Damage

The NM cable is routed in a way where it is extremely unlikely that any form of physical damage could occur.3.

Driploop

If the NM is coming out of the wall above the panel a proper driploop must be made.

Alternatives and Code-Compliant Solutions



Transition to a Suitable Wiring Method

The correct approach is to transition to a wiring method approved for wet locations before entering the exterior panel. Common options include:

Conduit (Rigid Metal Conduit - RMC, Intermediate Metal Conduit - IMC, Rigid PVC Conduit)

Run conduit from inside the house to the exterior panel. Transition from NM cable inside the house to individual THHN/THWN conductors inside the conduit. THHN/THWN is rated for wet locations. Use appropriate conduit fittings (weatherproof connectors) at both ends.

UF Cable (Underground Feeder Cable)

UF cable is designed for direct burial and wet locations. You could transition to UF cable inside the house and run it directly to the panel, using appropriate connectors.

Weatherproof Hubs/Connectors

Use listed and labeled weatherproof hubs or connectors to attach the conduit or cable to the panel. These provide a watertight seal.*

Sealing

Use appropriate sealant around the conduit/cable entry points to further prevent water intrusion.

In summary, your boss is right to be concerned. Using NM cable directly into the back of an exterior service panel is generally a code violation (NEC 334.12(B)(4)) and a poor practice that can lead to safety hazards. Transition to a suitable wiring method (conduit with THHN/THWN or UF cable) for a safe and code-compliant installation. Always consult your local electrical codes and your local electrical inspector for the final determination of the proper way to complete your project. Flag for review