Crash course: Voltage, amperage, and resistance basics step by step (part 5)

Voltage, amperage, resistance: the working definitions

Voltage is electrical pressure, measured in volts (V). Amperage is the rate of flow, measured in amps (A). Resistance opposes flow, measured in ohms (Ω). Every troubleshooting call you take comes back to these three.

Ohm's Law ties them together: V = I × R. Rearrange it as needed. If you know two values, you can solve for the third. This is not theory, it is the math behind every circuit you touch.

Power (watts) rides alongside: P = V × I. A 1500W heater on 120V pulls 12.5A. That number tells you breaker size, conductor size, and whether the existing circuit can handle it.

Voltage in the field

Residential and light commercial in the US runs 120/240V single phase. Commercial three phase is typically 208Y/120V or 480Y/277V. Know which system you are on before you put leads on anything. NEC 210.6 covers branch circuit voltage limitations.

Voltage drop matters on long runs. NEC 210.19(A) Informational Note No. 4 recommends keeping branch circuit drop under 3%, and total drop (feeder plus branch) under 5%. Skip this and you get nuisance trips, motor overheat, and dim lights at the far end of a barn or shop.

Quick rule for copper: a 12 AWG circuit at 20A starts losing too much voltage past about 75 feet on 120V. Bump to 10 AWG when the run gets long, do not just trust the breaker.

• 120V: receptacles, lighting, small appliances
• 240V: dryers, ranges, water heaters, well pumps
• 277V: commercial lighting fed from 480Y/277V
• 480V: motors, HVAC, industrial loads

Amperage and conductor sizing

Every conductor has an ampacity, the maximum current it can carry without overheating. NEC Table 310.16 is the reference for most installations at 60°C, 75°C, and 90°C terminations. Most breakers and lugs are rated 75°C, so size off that column unless you have documented 90°C terminations.

Continuous loads (three hours or more) get sized at 125% per NEC 210.19(A)(1) and 215.2(A)(1). A 16A continuous load needs a 20A circuit minimum. Forget this and your breaker will trip on a hot afternoon and the customer will call you back on your dime.

Watch derating. NEC 310.15(C)(1) hits you when more than three current carrying conductors share a raceway. Ambient temperature correction from 310.15(B) stacks on top of that. A 20A circuit in a 110°F attic with seven other hots in the pipe is not really a 20A circuit anymore.

If you are pulling a long homerun through a hot attic with multiple other circuits, do the derate math before you cut conductor. Upsizing once is cheaper than pulling it twice.

Resistance, grounding, and fault paths

Resistance shows up in three places that matter daily: the load itself, the conductors, and the equipment grounding path. A clean ground fault path needs low impedance so the breaker can clear quickly. NEC 250.4(A)(5) requires the fault current path to be permanent, low impedance, and capable of carrying the fault current.

When you measure a heater element, motor winding, or solenoid coil, you are reading load resistance. Open winding reads OL. Shorted winding reads near zero. Compare phase to phase on a three phase motor, the legs should match within a few percent.

Bond and ground are not the same thing. Bonding ties metal parts together so they sit at the same potential. Grounding ties the system to earth. NEC 250 separates these clearly, and mixing them up is how you end up with energized equipment frames and a customer holding a hot dryer handle.

Putting Ohm's Law to work on a service call

You get called on a 240V baseboard heater that is not putting out heat. Nameplate says 2000W. Quick math: I = P/V = 2000/240 = 8.33A. Element resistance should be R = V/I = 240/8.33 = roughly 28.8Ω cold.

Pull power, lock out, meter the element. If you read OL, the element is open. If you read near zero, it is shorted. If you read 29Ω, the element is fine and your problem is upstream: thermostat, contactor, breaker, or a loose connection cooking under a wirenut.

1. Verify nameplate voltage and wattage
2. Calculate expected current and resistance
3. De-energize and lock out per NEC 110.25 and OSHA
4. Measure resistance, compare to expected
5. If the load checks out, work back toward the panel

Always meter your meter. Check it on a known live circuit before and after you take a reading. A meter that lies once will lie again.

Common mistakes that cost callbacks

Sizing a circuit by breaker alone, ignoring continuous load and derate. A 20A breaker on overloaded 12 AWG in a hot attic will trip, and replacing the breaker without fixing the conductor sizing just kicks the problem down the road.

Confusing voltage to ground with voltage between conductors. On a 208Y/120V system, line to line is 208V, line to neutral is 120V. On 480Y/277V, line to line is 480V, line to neutral is 277V. Know which you are reading before you size equipment.

Skipping the resistance check on a dead load. Voltage testing tells you the circuit is energized. Resistance testing tells you whether the load itself is the failure. Both belong in your sequence, every call.