You're right, "open air" is only a small piece of the puzzle. Here's a breakdown of the key differences and how to explain it in a way that your customer can understand

Key Differences: Distribution Lines vs. Branch Circuits

Think of it like a highway system versus a driveway: Voltage

This is the biggest factor.

Utility Lines (Distribution)

Use very high voltages (e.g., 12,000 volts, 34,500 volts, or even higher).

Your Circuit (Branch Circuit)

Use low voltages (120 volts, 240 volts).

Explanation for the Customer

"The power company uses extremely high voltages on those lines. It's like using a firehose to get water across a large yard. Your circuit is like using a garden hose. To get the same amount of water (electricity) across the same distance with a garden hose (120V), you need a much larger hose (bigger wire)."

Purpose/Load Profile



Utility Lines (Distribution)

Are designed to carry power to be stepped down by many transformers to service many end-users.

Your Circuit (Branch Circuit)

Supplies a limited amount of power to serve one or more outlets or equipment.

Voltage Drop



Utility Lines (Distribution)

Voltage drop is still a concern, but it's managed differently. They use voltage regulators at substations and along the line to maintain voltage within acceptable limits. They design the system to accept larger voltage drops.

Your Circuit (Branch Circuit)

Voltage drop is critical for safety and proper operation of appliances. Excessive voltage drop can cause lights to dim, motors to overheat, and electronic devices to malfunction. Code limits voltage drop to a small percentage (3% for branch circuits, 5% total).

Why High Voltage Allows Smaller Wires



Power Equation

Power (Watts) = Voltage (Volts) Current (Amps) To deliver the same amount of power (Watts), if you increase the voltage, you decrease the current. This is the key!

Smaller current means less voltage drop (Voltage Drop = Current Resistance). Therefore, you can use smaller wires.

Why the "Open Air" Factor is Minor (But Not Zero)



Cooling

Overhead wires dissipate heat much more efficiently than wires bundled inside conduit or walls. The open air allows for better cooling, which allows the wire to carry slightly more current without overheating.

Insulation

Overhead wires have a different type of insulation (or sometimes no insulation) compared to building wire. This can affect their ampacity.

Here's a more complete explanation you can use with your customer

"The difference comes down to voltage and the way the power company designs their system. They use extremely high voltages on those overhead lines, often thousands of volts. Think of it like this: Imagine you need to move a lot of water a long distance. You could use a narrow pipe and pump the water at very high pressure (high voltage). Or you could use a much wider pipe and pump the water at lower pressure (low voltage). Both methods move the same amount of water, but the high-pressure system uses a smaller pipe. Electricity works similarly. By using high voltage, the power company can send the same amount of power with less current (amps) flowing through the wires. Since voltage drop is directly related to current, less current means less voltage drop, allowing them to use smaller wires. Your circuit is a low-voltage circuit (120V or 240V). To get the same power over a half-mile at that low voltage, the current would be incredibly high, causing a massive voltage drop if we used a smaller wire. That voltage drop would make appliances run poorly or not at all and could even be dangerous. That's why we need such a large wire – to keep the voltage drop within safe and functional limits."

Important Considerations



Code Compliance

Emphasize that you must follow the National Electrical Code (NEC) and local codes, which have strict requirements for voltage drop and wire sizing to ensure safety and proper operation.

Cost vs. Performance

Explain that while using 500 kcmil aluminum is expensive, it's the only way to safely and reliably deliver 20 amps over that distance at the required voltage.

Alternative Solutions

Consider if there are other solutions, such as:

Moving the load closer to the power source

If possible, can they relocate the equipment to be closer to the main panel or another power source?

Adding a subpanel

Installing a subpanel closer to the load would significantly reduce the long run length.

Solar and Batteries:* Depending on the application, explore solar and battery storage as a potential alternative.By explaining the principles in a clear and understandable way, you can help your customer appreciate the importance of proper wiring and code compliance. You can also show them that you are knowledgeable and prioritize safety and performance. Flag for review