Okay, let's break down this situation and figure out the best, safest, and most cost-effective solution for you. You're right to be concerned about voltage drop over that distance.

Analyzing Your Needs and Options Current Load

You've got a garage door opener at 1.5A. Four LED shop lights will likely draw very little current (LEDs are efficient). Even estimating each light at 0.5A (which is likely high), that's only 2A total. Your maximum, simultaneous load is probably around 3.5A. Realistically, it will be lower because the lights will likely already be on when the garage door opener starts.

Future Proofing

This is the key consideration. You say you only plan to park cars. But, plans change. Even if you don't plan to use power tools, future owners might. A sub-panel provides flexibility.

Addressing the Conflicting Advice Electrician Relative (10/2 UF in Conduit)

This might work, but it's cutting it close and doesn't offer much flexibility. 10/2 UF copper wire can carry 30 amps. However, you're dealing with a long run and voltage drop. While voltage drop is a performance issue (things might not work optimally), excessive voltage drop can also lead to heat buildup and be a safety concern.

House Electrician (100A Sub-Panel)

This is overkill right now, but it's the most future-proof. It's also the most expensive. If you're sure you'll never need more than lighting and the opener, it's a waste of money.

Understanding Voltage Drop

Voltage drop is the decrease in voltage that occurs along a conductor due to its resistance. Longer distances and higher currents result in greater voltage drop. The National Electrical Code (NEC) recommends limiting voltage drop to 3% for branch circuits and 5% overall.

Calculations and Recommendations

Let's do some quick and dirty voltage drop calculations. I'm going to assume a 120V source voltage and a load of 3.5A (the maximum you think you'll ever draw). Using 10 AWG Copper

The resistance of 10 AWG copper is about 1.018 ohms per 1000 feet. For a 250-foot run (500 feet of wire total – there and back), the resistance is approximately 0.509 ohms. Voltage drop = Current x Resistance = 3.5A x 0.509 ohms = 1.78V Voltage drop percentage = (Voltage drop / Source Voltage) x 100% = (1.78V / 120V) x 100% = 1.48%

Based on this limited calculation, 10 AWG copper might be okay for just the garage door opener and LED lights. However, this is based on ideal conditions and a low load. A better option: 8/2 UF Copper Using 8/2 UF copper you can carry up to 40A and the resistance of 8 AWG copper is about 0.641 ohms per 1000 feet. For a 250-foot run (500 feet of wire total – there and back), the resistance is approximately 0.321 ohms. Voltage drop = Current x Resistance = 3.5A x 0.321 ohms = 1.12V Voltage drop percentage = (Voltage drop / Source Voltage) x 100% = (1.12V / 120V) x 100% = 0.93%Recommendations

Here's my recommendation, balancing cost, safety, and future flexibility:1. Run a Sub-Panel, but Smaller Than 100A

Instead of the 100A sub-panel, consider a 60A or even a 30A sub-panel. This gives you plenty of room for the opener and lights, plus a few outlets for future use (e.g., charging a car battery, a small shop vac, a radio, etc.).

60A Sub-Panel

You could run 6/3 UF copper wire (with a ground) to a 60A sub-panel. 6 AWG copper is rated for 55A. This is a good balance of cost and future capacity. The resistance of 6 AWG copper is about 0.403 ohms per 1000 feet. For a 250-foot run (500 feet of wire total – there and back), the resistance is approximately 0.202 ohms. Voltage drop = Current x Resistance = 3.5A x 0.202 ohms = 0.71V Voltage drop percentage = (Voltage drop / Source Voltage) x 100% = (0.71V / 120V) x 100% = 0.59%

30A Sub-Panel

You could run 8/3 UF copper wire (with a ground) to a 30A sub-panel. This is the "smallest" I would run.2.

Proper Wiring

Absolutely use the correct size conduit for the wire you choose. Bury it at the required depth for your local codes (typically 18-24 inches). Use appropriate fittings and seal the ends of the conduit to prevent water intrusion.3.

Grounding

Ensure a proper grounding system. You'll need a grounding electrode system (ground rod) at the new garage, connected to the sub-panel.4.

GFCI Protection

All receptacles in the garage must be GFCI protected. You can either use GFCI breakers in the sub-panel or GFCI receptacles.5.

Pulling Wires

Lubricate the wires with wire-pulling lubricant to make the job easier.6.

Permits and Inspection

Get the necessary permits and have your work inspected by a qualified electrical inspector. This is crucial for safety and code compliance.7.

Neutral and Ground Separation

At the sub-panel in the new garage, the neutral and ground wires must be kept separate. They are only bonded together at the main panel in the detached garage. This is very important!

Why a Sub-Panel is Better (Even at the Minimum Size)



Safety

Fuses/breakers in the sub-panel will protect the wiring in the new garage.

Convenience

If there's a short circuit in the new garage, it will trip a breaker in the sub-panel, not in the main garage, making it easier to troubleshoot.

Future Use

You might decide later to add a small welder, a battery charger, or a workbench with a few power tools. A sub-panel gives you the capacity to do so.

Resale Value

A sub-panel makes the property more attractive to future buyers.

Important Considerations



Local Codes

Always follow your local electrical codes. They supersede any general advice.

Professional Advice

While I can give you general guidance, it's always best to consult with a qualified, licensed electrician who can assess your specific situation and provide tailored recommendations.

Voltage Drop Calculator

Use an online voltage drop calculator (there are many available) to get a more precise calculation, inputting the exact wire type, length, and anticipated load. Southwire and other wire manufacturers have good ones.

Aluminum Wire: I have only been calculating for copper. Aluminum is another option but has different resistance factors. Aluminum is a less desirable option compared to copper.In summary, while running 10/2 UF might technically* work, I strongly advise against it. A 60A or even a 30A sub-panel with the appropriate wire size is a much safer, more versatile, and ultimately better solution.Good luck with your project! Flag for review