Electrical quick reference

31 topics, one line each, in the order Hone teaches them.

Hone is a place to practise a career, one idea a day. This sheet is the whole Electrical track at a glance: every idea it covers, in the order they are taught, one line each. It is a map rather than a lesson. Read opens the full explanation of an idea; Practise gives you a question on it. Both are free, and reading needs no account at all.

From a battery to a breaker panel · Current, voltage, resistance

what electricity isElectricity is charge on the move. Current (amps) is how much charge passes each second, voltage (volts) is the push that moves it, and resistance (ohms) is how hard the path makes that push work. Read: What is actually moving in the wire · Practise what electricity is

Ohm's lawVoltage pushes, resistance resists, and current is what results: V = I × R. Read: Ohm's law: the one relationship everything hangs on · Practise Ohm's law

power, in wattsPower is volts times amps: P = V × I, in watts. Read: Power: how much work the circuit is doing · Practise power, in watts

energy and the bill (kWh)Power is how fast energy is used; energy is power multiplied by time, and the meter on the house counts it in kilowatt-hours. Read: Watts are a rate; the bill is watts times hours · Practise energy and the bill (kWh)

AC and DCDirect current flows steadily in one direction, like a battery. Alternating current reverses over and over; in North America it completes 60 cycles every second, and the 120 V on the nameplate is its effective value, not its peak. Read: DC goes one way; AC turns around sixty times a second · Practise AC and DC

From a battery to a breaker panel · Series and parallel

series circuitsIn a series circuit the current has one path, so the resistances simply add and the same current flows through each part. Read: Series: one path, the resistances add · Practise series circuits

parallel circuitsIn parallel every branch sees the full voltage, the branch currents add, and the total resistance is always smaller than the smallest branch. Read: Parallel: more paths, less resistance · Practise parallel circuits

series and parallel togetherWhen a circuit has both, replace each parallel group with its single equivalent resistance, then add what is left in series. Read: Mixed circuits: shrink the parallel part first · Practise series and parallel together

the voltage dividerIn a series loop each part takes a share of the source voltage equal to its share of the total resistance: Vx = Vsource × Rx / Rtotal. Read: Where the volts go: in proportion to the ohms · Practise the voltage divider

when one branch opensIn parallel, an open branch simply stops carrying its current and the others carry on unchanged; in series, an open anywhere stops everything. Read: One branch opens: what the others feel · Practise when one branch opens

From a battery to a breaker panel · Reading and measuring

the multimeterA multimeter measures volts across two points, ohms through a part with the power off, continuity as a beep for a path with almost no resistance, and, on some meters, amps in series with the circuit. Read: The multimeter: four questions it can answer · Practise the multimeter

measuring current, and the clamp meterA meter in amps mode must be in series, so the whole current flows through it; a clamp meter reads the current from the magnetic field around one conductor without breaking the circuit. Read: Amps are measured in the path, or around it · Practise measuring current, and the clamp meter

reading a nameplateEvery appliance and motor carries a plate with its voltage, its watts or VA or horsepower, its amps, and its frequency; the amps line, or watts divided by volts, is the number the circuit is sized from. Read: The nameplate tells you what to size for · Practise reading a nameplate

120/240 V in a homeThe utility transformer has a center-tapped winding: each end is 120 V to the center, the ends are 240 V to each other, and the center is the neutral. Small loads go from one hot to neutral; big loads go hot to hot. Read: Two hots and a neutral: how 120 and 240 both come from one service · Practise 120/240 V in a home

From a battery to a breaker panel · Wire and protection

wire sizes (AWG)American Wire Gauge counts down as the wire gets thicker: 14 AWG is small, 6 AWG is big, and 1/0, 2/0 and up are bigger still. Every three gauge steps roughly doubles the copper; ten steps is ten times the area, measured in circular mils. Read: Wire sizes: the smaller the number, the bigger the wire · Practise wire sizes (AWG)

how much a wire can carryAmpacity is the current a conductor can carry continuously without its insulation getting too hot, and it comes from a table by wire size and by the temperature rating of the insulation and the terminals. Read: How much current a wire may carry · Practise how much a wire can carry

the 80 percent ruleA load that runs for three hours or more is continuous, and its circuit is sized at 125 percent of the load; said the other way, a breaker may carry only 80 percent of its rating continuously. Read: The 80 percent rule: a circuit that runs for hours is sized at 125 percent · Practise the 80 percent rule

sizing a breakerA breaker is sized to the ampacity of the wire it feeds, so the wire can never carry more than it is rated for; the load only decides whether the circuit is big enough, never how big the breaker may be. Read: The breaker protects the wire, not the appliance · Practise sizing a breaker

voltage drop over a runEvery foot of wire has resistance, so the far end of a long run sees less voltage than the panel; the drop is estimated with VD = 2 × K × I × L / CM, where K is 12.9 for copper, I is the amps, L the one-way length in feet and CM the wire's circular mils. Read: Voltage drop: the wire eats some of the volts on the way · Practise voltage drop over a run

conduit fillWires need room to be pulled and to shed heat, so with three or more conductors in a conduit their total cross-section may fill no more than 40 percent of the conduit's inside area; two conductors get 31 percent and one gets 53 percent. Read: Conduit fill: forty percent of the pipe is the most you may fill · Practise conduit fill

GFCI and AFCIA GFCI compares the current going out on the hot with the current coming back on the neutral and opens in a fraction of a second if about 5 mA is missing, because missing current is going through something else, maybe a person; an AFCI listens for the electrical signature of an arc, the sparking at a damaged cord or loose connection that starts fires. Read: GFCI and AFCI: two devices for two different ways to get hurt · Practise GFCI and AFCI

From a battery to a breaker panel · Safety

lockout, tagoutBefore working on equipment you turn it off, open the disconnect that feeds it, put your own lock and tag on that disconnect, release any energy still stored, and prove it dead with a meter; only then do hands go in, and only you remove your lock. Read: Lockout, tagout: the order that keeps the power off while you are inside · Practise lockout, tagout

test before touchBefore treating any conductor as dead, test your meter on a source you know is live, test the conductor, then test the known-live source again; the two live readings prove the meter was working during the dead reading in between. Read: Test before touch: live, dead, live · Practise test before touch

arc flashWhen a fault jumps across air, the arc is hotter than the surface of the sun for a fraction of a second and throws molten metal and a pressure wave; the hazard is measured in calories per square centimetre at your working distance, and 1.2 cal/cm² is enough for a second-degree burn. Read: Arc flash: the explosion that needs no contact · Practise arc flash

grounding and bondingGrounding ties the electrical system to the earth at one point, so voltages have a reference and lightning has somewhere to go; bonding ties every piece of metal that could become energised back to the source with a low-resistance path, so a fault becomes a big current that trips the breaker instead of a live box waiting for a hand. Read: Grounding connects to the earth; bonding connects the metal together · Practise grounding and bonding

the neutral is not a groundThe neutral is the return path for every 120 V load and carries current all the time; the equipment ground is bonded to the same bar at the service but carries current only during a fault. They are tied together at exactly one point and nowhere else, so a fault has a clear path and the ground wires never share the load's current. Read: The neutral carries current; the ground carries none, until the day it matters · Practise the neutral is not a ground

From a battery to a breaker panel · On the job

load calculation for a dwellingA dwelling's load is added up in a fixed order: general lighting at 3 VA per square foot, plus 1500 VA for each small-appliance circuit and the laundry, with a demand factor applied because not everything is on at once; then the range and dryer from their tables and the fixed appliances; the total in VA divided by 240 V is the service amps. Read: Sizing a house service: count the load the standard way · Practise load calculation for a dwelling

motor full-load currentThe nameplate gives full-load amps (FLA); the conductors are sized at 125 percent of it, and because a motor draws several times FLA for a moment when it starts, the breaker or fuse may be far larger than the wire's ampacity, with the motor's overload relay guarding the wire instead. Read: A motor is sized from its full-load current, and the breaker is allowed to be big · Practise motor full-load current

a 240 V circuit, end to endOne appliance, one circuit: read the nameplate, find the amps, apply 125 percent if the load is continuous, choose the next standard breaker at or above that, choose a wire whose ampacity covers the breaker, check the voltage drop for the run, and finish with the receptacle that matches the breaker. Read: A 240 V circuit end to end: from the nameplate to the receptacle · Practise a 240 V circuit, end to end

troubleshooting a dead outletAsk what changed, check the cheap things at the panel and the GFCI, test at the outlet with a meter, and only then de-energize, lock out and open it; the fault is almost always a tripped device or a loose connection at the last working outlet upstream. Read: A dead outlet: check in the order that costs least · Practise troubleshooting a dead outlet

reading a panel scheduleA panel schedule lists every breaker space by number, odd numbers down the left and even down the right, with each row alternating between the two hot legs; a two-pole 240 V breaker takes two spaces on the same side, one above the other, and so lands on both legs. Read: Reading a panel schedule: which breaker, which leg, which wire · Practise reading a panel schedule