Crash course: Ohm's Law for electricians with photos (part 3)

Part 3: Putting Ohm's Law to work on the truck

Parts 1 and 2 covered the math and the wheel. This one is about jobsite use. When the meter reads weird, when a breaker keeps tripping, when a homeowner swears the outlet "shocked" the lamp, Ohm's Law tells you what is actually happening before you start pulling devices.

Keep V = IR, P = VI, and the derived forms in your head. Everything below is just those three equations applied to copper, loads, and faults you see every week.

Voltage drop calls that stop callbacks

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3 percent, with total drop on feeder plus branch not exceeding 5 percent. That is a recommendation, not a rule, but it is the number inspectors and customers will quote back at you.

The field formula for single phase: VD = (2 x K x I x L) / CM. K is 12.9 for copper, 21.2 for aluminum. L is one way length in feet. CM is circular mils from Chapter 9 Table 8. Solve for CM when you need to upsize.

• 120V, 20A continuous load, 150 ft one way, #12 Cu (6530 CM): VD = (2 x 12.9 x 16 x 150) / 6530 = 9.5V, or 7.9 percent. Bump to #10.
• Same load on #10 Cu (10380 CM): VD = 5.96V, 4.97 percent. Borderline. Go #8 if the load is motor driven.
• Long runs to detached garages, well pumps, and pole barns are where #14 and #12 quietly cook for years.

If a customer reports lights dimming when the AC kicks on, measure voltage at the panel and at the load while the compressor starts. More than 5 percent dip across the branch and you have a conductor problem, not a utility problem.

Sizing for heat, not just amps

P = I squared R is the equation that explains why loose lugs burn. Resistance at a bad termination might be 0.05 ohm instead of 0.0005 ohm. At 30A, that is 45 watts dumped into a wire nut or a breaker stab. That is a soldering iron sitting inside your panel.

NEC 110.14(D) requires torque values per listing, and 2017 onward made it enforceable with a calibrated tool. The math is why. Double the current, quadruple the heat. A 50A circuit through a marginal connection produces four times the heat of a 25A circuit through the same connection.

• Check terminations on any circuit that has tripped under load.
• Discoloration on insulation near a lug means it has been over 90 C. Cut back and re-terminate.
• Aluminum branch circuits: AL/CU rated devices, antioxidant, and torque. Every time.

Reading a clamp meter through Ohm's Law

A clamp meter gives you current. The panel schedule gives you voltage. The nameplate gives you power. Any two will get you the third, and any disagreement is a clue.

Example: 240V baseboard heater nameplate says 1500W. You clamp it and read 4.2A. Expected current is 1500 / 240 = 6.25A. The element is partially open, or supply voltage is low. Measure voltage at the heater. If voltage is good, the element is failing.

Example: 120V receptacle circuit pulling 18A with nothing visible plugged in. Something on that circuit is drawing 2160W. Refrigerator compressor stuck on, bathroom heater left running, or a hidden load in a finished basement. Walk the circuit map.

Fault current and why it matters at the service

NEC 110.9 requires equipment intended to interrupt fault current to have an interrupting rating sufficient for the available fault current. NEC 110.24 requires the available fault current to be marked at the service. Ohm's Law underwrites both.

Available fault current at the panel is roughly utility transformer infinite bus current divided by transformer impedance, then reduced by service conductor impedance. The point in the field: a 200A panel rated 10kAIC sitting 15 feet from a 75 kVA pad mount transformer can see fault current well above 10kA. That is a panel rated to explode under fault.

If you are tying into an existing service near a large transformer, get the available fault current from the POCO before you spec the equipment. Series rated combinations only count if every component in the chain is listed for it.

Quick mental math for the field

You will not pull out a calculator on a ladder. Memorize a few anchor points and interpolate.

1. Watts to amps at 120V: divide watts by 120. A 1500W hair dryer is 12.5A. A 1800W microwave is 15A and should be on its own circuit per NEC 210.11(C)(1) logic for kitchens.
2. Watts to amps at 240V: divide watts by 240. A 4500W water heater is 18.75A, sized at 25A breaker per NEC 422.13.
3. Three phase watts to amps: P / (1.732 x VL x PF). At 480V three phase with PF 0.9, divide kW by 0.748 to get amps. A 50 kW load is roughly 67A.
4. Resistance check on a known load: R = V squared / P. A 1500W 240V heater should read about 38 ohms cold. Way off and the element is bad.

What to carry in your head

Three equations, three derived forms, six constants for copper and aluminum K values, and the 3 and 5 percent voltage drop targets. That set covers ninety percent of diagnostic work between the meter base and the last device on the circuit.

Part 4 will go into motor circuits, locked rotor amps, and why NEC 430 sizes overcurrent protection differently from branch circuit conductors. Same Ohm's Law underneath, different rules on top.