Crash course: Voltage, amperage, and resistance basics for apprentices (part 4)

The big three, and why they matter on every call

Voltage pushes, amperage flows, resistance pushes back. Every troubleshooting call, every load calc, every conductor sizing decision comes back to those three values and how they relate. Get fluent with them and the rest of the trade gets easier.

This is part 4 of the crash course. By now you should be comfortable reading a meter and identifying conductors. Here we tie the electrical theory to what you actually do in the field: sizing, troubleshooting, and not getting hurt.

Voltage: the pressure behind the work

Voltage (V) is electrical potential difference, measured between two points. It does not flow. It exists. A 120V circuit has 120 volts of potential between the ungrounded conductor and the grounded conductor whether or not anything is plugged in.

Common system voltages you will see daily: 120/240V single-phase residential, 120/208V wye in light commercial, 277/480V wye in commercial and industrial. NEC 220.5(A) tells you to use nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, and 600 for load calculations unless other voltages are specified.

Voltage drop is where theory meets the jobsite. NEC 210.19(A) Informational Note No. 4 recommends branch circuits not exceed 3% drop, with combined feeder and branch not exceeding 5%. It is a recommendation, not a requirement, but inspectors and engineers will hold you to it on long runs.

Field tip: On any home run over 100 feet, do the voltage drop math before you pull. Upsizing a 12 AWG to 10 AWG on the front end is cheaper than re-pulling after the EC fails commissioning.

Amperage: what actually does the damage

Amperage (I) is current, the rate of electron flow, measured in amps. This is what heats conductors, trips breakers, and stops hearts. Voltage gets the headlines but amperage does the work, and the harm.

Conductor ampacity lives in NEC Table 310.16 for the conductors you pull most. Termination temperature ratings come from 110.14(C), and that is the rule that catches apprentices: a 90 degree C insulated conductor terminated on a 75 degree C lug uses the 75 degree C column. The insulation does not raise the rating of the termination.

Continuous loads (three hours or more) get sized at 125% per 210.19(A)(1) and 215.2(A)(1). A 16 amp continuous load needs a 20 amp circuit minimum. Miss this on a sign circuit or a parking lot lighting feeder and you will be back on a service call when the breaker keeps tripping at dusk.

Resistance: the part you cannot see

Resistance (R) is opposition to current flow, measured in ohms. Conductors have it, terminations have it, loads are made of it. Bad resistance is what burns up panels: a loose lug, a corroded splice, a backstabbed receptacle that has been cycled 10,000 times.

Ohm's Law ties the three together: V = I x R. Rearranged, you get I = V / R and R = V / I. Memorize it. Use it. When a 120V circuit reads 60V at the load, you have resistance somewhere it does not belong.

• Tight terminations: torque to manufacturer spec, required by 110.14(D).
• Clean splices: stranded to stranded, no nicked strands, listed connectors only.
• Bonding: low resistance fault path back to the source, per 250.4(A)(5).
• Insulation: high resistance between conductors and to ground, megger when in doubt.

Putting it together on a real call

Customer says a kitchen receptacle is dead. You meter it: 0V hot to neutral, 0V hot to ground. Most apprentices stop there. Do not. Check upstream. Pull the next receptacle on the circuit. If it is hot, the dead one has an open. If it is also dead, walk it back to the panel.

Now you find a backstabbed receptacle two boxes back with a charred neutral. That is resistance you could not see, heating up over years, finally opening the circuit. Replace with side-wired terminations, torqued. Document it. The GFCI requirement under 210.8(A) probably applies if this is within six feet of the sink, so verify the protection scheme while you are in there.

Three values, one Ohm's Law, and a meter. That is most of residential and commercial troubleshooting.

Field tip: A non-contact tester tells you something is energized. It does not tell you the voltage, the polarity, or whether the neutral is open. Pull the meter out. Every time.

What to drill until it is automatic

Theory is useless if you have to look it up at the top of a lift. The values and rules below should be reflexive before you sit for your journeyman exam.

1. Ohm's Law and the power formula (P = V x I) without thinking.
2. Common system voltages and which loads run on each.
3. Ampacity for 14, 12, 10, 8, 6 AWG copper at 75 degree C from Table 310.16.
4. Continuous load sizing at 125%, per 210.19(A)(1).
5. Voltage drop targets: 3% branch, 5% combined.
6. GFCI and AFCI requirements under 210.8 and 210.12.

Part 5 of the crash course will move from these fundamentals into series and parallel circuits, and how they change the math when loads share a path. Until then, meter everything, torque everything, and trust the numbers over the feeling in your gut.