Quick answer
Electrical resistance is the opposition a material puts up against the flow of electric current, measured in ohms (Ω), where one ohm is one volt per ampere.
Copper and aluminum have low resistance, so they make good wires. Plastic, glass and dry wood have high resistance, so they insulate. Wherever current meets resistance, some energy turns into heat.
Quick facts
The key facts about Electrical Resistance, with sources:
Key takeaways
- Resistance is how hard a material fights current. Low resistance carries current; high resistance blocks it.
- Ohm’s law links it all: amps = volts ÷ ohms. Lower resistance at the same voltage means more current.
- Resistance turns some energy into heat. That is how heaters work, and why bad joints start fires.
- Wet skin has far lower resistance than dry skin, so shocks in damp places are worse.
- Thin or long wires have more resistance, which causes voltage drop and heat.
Resistance in plain terms
Picture water in a hose. A wide, smooth hose lets water flow freely. A thin or kinked hose holds it back. Resistance is the electrical version of that kink.
NIST lists the ohm as the SI unit of electric resistance and defines one ohm as one volt per ampere 1. So a part with 10 ohms lets 12 amps flow at 120 volts.
OSHA sorts materials by resistance. Metals generally offer very little resistance and are called conductors. Glass, plastic, porcelain, clay, pottery and dry wood slow or stop the flow and are called insulators 2. Every home wire uses both: a metal core inside a plastic jacket.
How resistance shapes current and heat
Ohm’s law comes straight from the NIST definition: volts equal amps times ohms 1. In a home, voltage stays near 120 or 240 volts.
So the resistance of what you plug in decides how many amps flow. A heater element has low resistance, so it pulls a lot of current. A night light has high resistance, so it pulls very little.
When current passes through resistance, energy turns into heat. Toasters, space heaters and water heaters are built to do exactly that. Virginia Tech Extension lists water heaters at 4,500 to 5,500 watts, almost all of it turned into heat 3.
Wires are built to do the opposite. Their resistance is kept low so they stay cool. When a joint loosens or corrodes, its resistance rises at one small spot. That spot can get hot enough to char insulation, even if the breaker never trips.
Amps = Volts ÷ Ohms · Heat (watts) = Amps × Amps × Ohms
Ohms- Resistance of the path
Volts- 120 V or 240 V at a home outlet
Amps × Amps × Ohms- Power turned into heat in that resistance; follows from W = V × A and V = A × Ω
Source: [1]
Materials sorted by resistance
| Group | Examples | Use in a home |
|---|---|---|
| Conductors (low resistance) | Copper, aluminum and other metals 2 | Wires, busbars, terminals |
| Unexpected conductors | The earth, wet wood, wet skin 2 | Why grounding works, and why water is a hazard |
| Insulators (high resistance) | Glass, plastic, porcelain, clay, dry wood 2 | Wire jackets, outlet bodies, fuse bodies |
| Resistive loads | Heater coils, toaster wires, old filament bulbs | Turning electricity into heat or light |
| Air | Normally an insulator; conducts in an arc or lightning 2 | Gaps in switches and breakers |
Example: why a loose connection gets hot
Illustrative numbers for a 15 A load on a 120 V circuit. Heat at a point = amps² × ohms, from the NIST unit definitions 1. The resistances are examples, not measured values.
| Connection condition | Resistance at joint (example) | Heat at that one spot |
|---|---|---|
| Tight, clean joint | 0.005 Ω | 15 × 15 × 0.005 ≈ 1.1 W |
| Slightly loose | 0.05 Ω | 15 × 15 × 0.05 ≈ 11 W |
| Loose and corroded | 0.2 Ω | 15 × 15 × 0.2 = 45 W |
- Same 15 ACurrent through every jointBreaker sees no fault
- 40×More resistance at the bad joint0.005 to 0.2 Ω
- 40×More heat at that spotAbout 1 W to 45 W
Source: [1]
Forty-five watts in a spot the size of a screw head is like a small soldering iron inside a wall box. The breaker sees only 15 amps, so it does not trip. This is one reason backstab connections and aluminum wiring get extra attention.
Where resistance shows up for a homeowner
You see it in wire sizes. Thinner wire has more resistance per foot, so it carries fewer amps. That is why 14 AWG wire sits on 15-amp circuits and thicker 12 AWG on 20-amp circuits. Our wire gauge page explains the sizes.
You see it in voltage drop. Long runs to a shed or garage lose some voltage along the way because the wire has resistance. Electricians may upsize the wire to keep the drop small.
You feel it as warmth. A warm plug on a heater cord is resistance at work. A hot plug or outlet means too much resistance or too much current.
When resistance helps and when it hurts
Useful resistance
Harmful resistance
- Loose or corroded joints make hot spots that can start fires.
- Long, thin wires waste energy as heat and drop voltage.
- Wet skin loses much of its protection 2.
What resistance numbers cannot show
A simple ohm reading on a dead circuit cannot always find a bad joint. Some faults only heat up under load. Electricians use other checks, such as infrared scans or voltage-drop tests under load.
AC loads with motors and electronics add other effects beyond simple resistance. Together these are called impedance. For a homeowner, the plain idea of resistance is enough to understand heat and safety.
How resistance affects cost
We do not quote prices. Resistance shows up in a job’s cost through these drivers. See what drives electrical project costs.
- Wire size. Bigger wire has lower resistance and costs more per foot.
- Run length. Long runs may need upsizing to limit voltage drop.
- Repair of hot spots. Fixing burned connections can mean new devices and cutting into walls.
- Material choice. Copper has lower resistance than aluminum of the same size.
- Energy loss. Undersized long runs waste a little energy as heat every hour they carry load.
How an electrician tracks down resistance problems
- Asks what you noticed: warmth, smell, flicker or dimming.
- Checks outlets and switches on the affected circuit, with power off where needed.
- Tests under load, or uses a thermal camera to find warm spots.
- Tightens, replaces or rebuilds bad connections and devices.
- Pulls a permit if the repair needs one, then retests.
Keeping resistance where it belongs
- Do not use outlets where plugs fit loosely. Ask an electrician to replace them.
- Keep cords out of water and damp areas.
- Replace cords with cracked jackets; do not tape them.
- Book an electrical inspection for older homes, where connections may have loosened over decades.
Resistance, shocks and fire
Your body has resistance too. OSHA notes that dry skin has fairly high resistance, but moist or wet skin acts as a conductor 2. Lower resistance means more current at the same voltage, and OSHA lists 50 to 150 milliamps as possibly fatal 2.
High resistance in the wrong place starts fires. The CPSC estimates an annual average of 17,100 home fires from electrical distribution equipment in 2019 to 2021, and installed wiring was the largest category in each of those years, at 7,400 to 8,100 fires 4.
Warm or discolored outlet plates, buzzing and a burning smell are warning signs.
Water lowers your resistance
Keep hair dryers, radios and cords away from sinks and tubs. Never touch a switch or appliance with wet hands. If an outlet is scorched or warm, stop using it and call an electrician.
Rules shaped by resistance
Many code rules exist to keep resistance low where current flows. Wire sizes, approved connectors and tightening rules all serve that aim. The National Electrical Code, NFPA 70, sets them; its 2026 edition is free to read online 5.
Grounding rules rely on a low-resistance path. OSHA describes the equipment grounding conductor as a low-resistance wire that sends fault current to ground, so breakers can trip 2. Our grounding and bonding guide goes further.
Resistance compared with related terms
| Term | What it is | Unit |
|---|---|---|
| Resistance | Opposition to current | Ohm (Ω) |
| Voltage | The push against that opposition | Volt |
| Current | The flow that results | Amp |
| Voltage drop | Voltage lost across a wire’s resistance | Volts or percent |
| Continuity | Whether a path exists at all | Pass / fail |
Myths about resistance
- Myth Higher resistance is always safer.
- Reality In insulation, yes. In a connection, high resistance makes heat and fire risk.
- Myth A breaker will catch a bad joint.
- Reality A breaker reacts to too much current. A hot joint can form at normal current.
- Myth Water itself conducts well.
- Reality OSHA notes pure water is a poor conductor; impurities make it conduct 2. Tap water and sweat have those impurities.
What changes by state
Resistance is physics, so it does not change by state. What changes is the housing stock. Older homes have older connections and sometimes aluminum branch wiring, which needs special connectors. Our research on housing age and electrical systems shows where older homes cluster. The code edition your inspector uses also varies.
When resistance matters to you
Resistance matters when something feels warm that should not, when lights dim on long runs, or when you buy an older home. Treat warmth at outlets, switches and plugs as a sign to call an electrician. Plan long feeds to sheds and garages with the voltage drop calculator.
Trust your hand on the faceplate
Outlet and switch plates should feel room temperature. A plate that feels warm while a heater or kettle runs is worth an electrician’s visit.
Questions about Electrical Resistance
What is electrical resistance?
Electrical resistance is how much a material opposes the flow of current. It is measured in ohms. NIST defines one ohm as one volt per amp. Metals have low resistance and carry current well; plastic, glass and dry wood have high resistance and act as insulators.
What is Ohm’s law?
Ohm’s law says volts equal amps times ohms. It follows from how the ohm is defined. In a home, voltage is fixed near 120 or 240 volts, so a load with lower resistance draws more current. A heater element draws more amps than a lamp for that reason.
Why does resistance create heat?
When current pushes through resistance, some electrical energy turns into heat. The heat equals the current squared times the resistance. Heaters use this on purpose. In wiring, it is a side effect kept small by using thick enough wire and tight connections.
Does wire size affect resistance?
Yes. Thinner and longer wires have more resistance. That is why each wire gauge has a current limit and why long runs can need thicker wire to limit voltage drop. Copper also has lower resistance than aluminum of the same size.
Why is wet skin more dangerous?
OSHA explains that dry skin has fairly high resistance, but moist or wet skin acts as a conductor. Lower resistance lets more current flow through the body at the same voltage. That is why bathrooms, kitchens and outdoor outlets need GFCI protection.
Can high resistance cause a fire?
Yes. A loose or corroded connection has high resistance at one point. Normal current through that point can make enough heat to char insulation. A breaker may not trip, because the current stays normal. Warm outlets or a burning smell need an electrician.
What has the lowest electrical resistance?
Metals such as copper, silver and aluminum have very low resistance, which is why they are used for wires. OSHA groups metals as conductors. Copper is the common choice in homes because it balances low resistance, cost and ease of use.
How is resistance measured?
With an ohmmeter, usually a setting on a multimeter, on a circuit with the power off. Electricians use it to check heating elements, continuity and insulation. Homeowners should not test inside boxes or panels; leave that to a licensed electrician.
What is the difference between resistance and impedance?
Resistance is the plain opposition to current, the same for AC and DC. Impedance is the total opposition in an AC circuit, adding effects from coils and capacitors in motors and electronics. For heaters and wiring, resistance and impedance are nearly the same. For motors, impedance is the better measure.
Sources
- NIST, SI Units: Electric Current (ohm and volt definitions), retrieved .
- OSHA 3075, Controlling Electrical Hazards, retrieved .
- Virginia Cooperative Extension, How to Estimate Appliance Energy Use (pub. 2901-9014), retrieved .
- U.S. CPSC, 2019–2021 Residential Fire Loss Estimates (May 2024), retrieved .
- NFPA 70, National Electrical Code, 2026 edition (free access), retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Unit definitions from NIST; conductor, insulator and skin facts from OSHA 3075; fire figures from CPSC. The loose-connection table uses labelled example resistances to show the math, not measurements.
Suggest a correction. We fix errors and say what changed.