Electrical fundamentals
The foundation under every job: verify dead circuits, know Ohm's Law cold, understand what your local code requires, and troubleshoot in a repeatable order.
The one rule that never bends
Treat every conductor as live until you have proven it dead with a tester you have just verified on a known-live circuit. Breaker off is not the same as dead.
Safety protocols come first
Nothing else on this page matters if the circuit is still live.
- Turn the breaker off, then lock or tape it off and label it so nobody flips it back on while you work.
- Verify dead every single time: test the wires with a non-contact tester, confirm with a meter across hot-to-neutral and hot-to-ground, then prove your tester still works on a circuit you know is live.
- Never trust the panel label. Test the actual conductors you are about to touch — mislabeled and double-fed circuits are common.
- Use the one-hand rule near anything that could be energized, and keep your other hand out of contact with grounded metal.
- Wear safety glasses, use insulated tools, and stand on a dry, non-conductive surface.
- Panel work: the main lugs stay energized even with the main breaker off. Treat the top of the panel as live at all times.
- Plug power tools into GFCI protection, keep cords out of water, and never work on wet surfaces or in a damp crawlspace without protection.
- Ladders near service conductors should be fiberglass, never aluminum.
Core principles worth memorizing
Four relationships explain most of what you will see in the field.
- Ohm's Law: Volts = Amps x Ohms. Rearranged: Amps = Volts / Ohms, Ohms = Volts / Amps.
- Power: Watts = Volts x Amps. A 1,500 W heater on 120 V draws 12.5 A — which is why it fills a 15 A circuit by itself.
- Series circuits share one path, so current is the same everywhere and voltage divides. Parallel circuits share voltage, and current divides — house wiring is parallel.
- Wire gauge is chosen for the breaker, not the load: 14 AWG copper for 15 A, 12 AWG for 20 A, 10 AWG for 30 A.
- Voltage drop: keep it under about 3% on a branch circuit. On long runs, go up one wire size.
- A tripping breaker is information, not a nuisance. It means current exceeded what the wire can safely carry, or a fault exists.
Ohm's Law calculator
V = I x R
Fill in any two values and clear the third to solve for it.
Resistance: 10.00 Ω
Power: 1440 W
Local codes, permits, and inspections
The NEC is the floor. Your city or county decides the rest.
- The NEC (NFPA 70) is a model code. It only applies where your state, county, or city adopts it — and many adopt an older edition with local amendments.
- Find your rules by searching your city or county name plus "electrical code amendments" or by calling the building department and asking which NEC edition is enforced.
- The AHJ (Authority Having Jurisdiction) — usually your local inspector — has the final say and can require more than the NEC does.
- Permits are typically required for new circuits, panel or service work, and rewiring. Simple device swaps often are not, but rules vary.
- Common inspection failures: missing or improper AFCI/GFCI protection, overfilled boxes, unsupported cable, missing cable clamps, no anti-short bushings, and unlabeled circuits.
- Some work — service upgrades, meter changes, anything touching the utility drop — legally requires a licensed electrician nearly everywhere.
- Insurance and resale both care about permits. Unpermitted work can be denied on a claim or flagged on inspection.
Essential electrician skills
Five capabilities that separate a safe electrician from a dangerous one. Each includes a role-aware tip and a hands-on tool.
Troubleshoot tripped circuits
Safely diagnose short circuits, ground faults, and overloaded panels using a multimeter.
- Start with a visual inspection: look for scorched devices, melted wire nuts, or burned breaker bus stabs before touching anything.
- Use a multimeter, not just a non-contact tester. A voltage test tells you if a conductor is live; an ohms test tells you if a short exists.
- Short circuit: hot touches neutral or ground with very low resistance. The breaker trips instantly. Isolate the circuit, then divide and conquer.
- Ground fault: hot contacts a grounded surface (box, metal fixture, water). A GFCI/AFCI detects the imbalance and trips.
- Overload: the running load is at or above the breaker rating. The breaker trips after some time or under heavy load.
- Never bypass a breaker or install a larger one without verifying the wire is sized for it. The breaker protects the wire, not the device.
- On AFCI breakers, a trip can be an arc fault, a ground fault, or a nuisance. Test the breaker and the wiring separately.
Tripped-circuit diagnostic
Pick the symptom that matches what you see. This is a starting guide, not a replacement for a meter.
Install and size main service panels
Properly calculate load requirements and set up breaker panels to code.
- Service size is determined by the calculated load, not by adding up every breaker rating. A 200 A panel with a 100 A calculated load is fine.
- The main service conductors, meter base, and main breaker must all be rated together as a system.
- Panel location matters: working clearance, illumination, dedicated space, and accessibility per NEC 110.26.
- Use the manufacturer’s listed breakers or classified breakers approved for that panel. Mixing brands can void listing and cause failures.
- Ground and neutral are bonded together only at the service equipment. Subpanels and detached buildings keep them isolated.
- Label every circuit at the panel. The NEC requires it, and it makes future troubleshooting and emergency response faster.
- Main lugs remain energized even with the main breaker off. Treat the service entrance area as live at all times.
Panel load calculator
estimated demand
Add the major loads. This is a simplified demand estimate; a real load calc uses NEC demand factors and non-coincident loads.
10.0 A
25.0 A
23.3 A
20.8 A
Total demand: 14,800 W
Approx. amps: 79.2 A
Suggested service: 100 A
Bend and run conduit
Accurately measure, bend, and secure EMT or PVC conduit for commercial and residential runs.
- Measure from the inside wall of the box to the back of the bend, then add the take-up for the bender size and conduit type.
- A 90° stub-up height is: total height minus take-up. Practice on scrap before cutting your final stick.
- Offset bends let the conduit run parallel to a wall after leaving a surface-mounted box. Measure the offset height and the distance.
- Support EMT within 3 feet of every box and every 10 feet thereafter; PVC typically every 3 feet and within 3 feet of a box.
- Use proper connectors and locknuts; set-screw connectors are common for EMT, and glued fittings for PVC.
- Conduit fill is limited by the total cross-sectional area of conductors. Never exceed 40% for three or more conductors.
- Deburr every cut end so the insulation is not damaged when pulling wire. Use a fish tape and pulling lubricant on long runs.
Conduit fill calculator
NEC Chapter 9
One to two conductors can fill up to 53% of the conduit cross-section. This calculator uses common EMT sizes and THHN/THWN-2 conductor areas.
Area: 0.040 in²
Fill percentage7.5%
Max fill for 3/4" EMT: 0.282 in². Current total: 0.040 in².
Execute proper grounding and bonding
Ensure every metal box, service drop, and subpanel is correctly bonded to prevent shock hazards.
- Grounding provides a path for fault current to return to the source. Bonding keeps all metal parts at the same potential.
- The grounding electrode system may include ground rods, a concrete-encased electrode (Ufer), a ground ring, or the metal water pipe.
- At the service, the neutral and equipment ground are bonded together. This is the only place that bond is allowed.
- In a subpanel, the neutral bar is isolated from the ground bar. Bonding them there creates parallel neutral paths and dangerous voltages.
- Every metal box, raceway, and enclosure must be bonded to the equipment grounding conductor.
- Receptacles and switches must be connected to the EGC. Self-grounding devices count only in listed configurations.
- A broken or missing EGC is a shock hazard. If you find old two-wire cable without a ground, do not use the box as the only ground path.
Grounding and bonding checklist
Use this before energizing a new service, subpanel, or major circuit. Bonding keeps conductive parts at the same potential; grounding provides the fault-current path.
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Wire a subpanel
Feed a garage, shop, or addition with a 4-wire feeder — and float the neutral.
- A subpanel is fed by a feeder, not a branch circuit: two hots, one neutral, and one equipment ground — four wires, never three.
- Size the feeder to the subpanel rating and verify the service can carry the extra load with a load calculation first. 60A covers a small shop; 100A suits an addition or detached building.
- The neutral bar in a subpanel must be isolated. Remove the bonding screw or strap, and terminate equipment grounds on a separate bar bonded to the enclosure.
- The neutral-to-ground bond happens exactly once, at the service. Bonding them again downstream puts neutral current on enclosures and raceways.
- A subpanel in a detached structure needs its own grounding electrode bonded to the ground bar, while the EGC still runs back with the feeder and the neutral stays isolated.
- The main lugs in the service stay energized with the main breaker off. Test everything you plan to touch, and torque every lug to the printed spec.
- Check voltage drop on long runs — over roughly 100 feet, upsizing a conductor size is usually cheaper than chasing dim lights and hot terminations later.
Feeder & subpanel sizer
240V single phase
Typical residential picks for a 4-wire feeder in conduit. Always confirm against NEC 310.16 and your local amendments.
Copper feeder
#6 Cu THHN
Aluminum feeder
#4 Al
Equipment ground
#10 Cu
Estimated voltage drop: 2.8 V (1.2%)
Within the recommended 3% at 80% of the feeder rating.
Remember: the subpanel neutral bar must be isolated and the bonding screw removed. Grounds go on their own bar bonded to the enclosure.