The ohm (symbol: Ω) is the SI derived unit of electrical resistance, defined as the resistance between two points of a conductor when a constant potential difference of one volt produces a current of one ampere. In any real circuit or installation, resistance changes the voltage available at the load (via voltage drop) and converts electrical energy into thermal energy (heat), dictating exactly how much current will flow for a given applied voltage.

The Modern Definition of the Ohm in SI Units

For decades, electrical units were tied to physical artifacts or specific electrochemical cells. That changed fundamentally with the 2019 redefinition of the SI base units by NIST. Today, the ohm is no longer defined by a physical coil of wire in a vault. Instead, it is derived from fundamental constants of the universe—specifically, the Planck constant (h) and the elementary charge (e).

In modern metrology, the ohm is realized using the quantum Hall effect. The von Klitzing constant (RK), which equals h/e2, provides an exact, universally reproducible resistance value of approximately 25,812.807 Ω. This means a national standards lab can calibrate a resistance standard without needing to compare it to a physical reference artifact, ensuring your bench multimeter's readings trace back to the fundamental laws of physics.

The Water Pipe Analogy (Used Once)
Think of resistance like the friction inside a water pipe. A higher ohm value means more pressure (voltage) is required to push the same volume of water (current) through the pipe. If the pipe is narrow or rough (high resistance), you lose pressure (voltage drop) along the length of the pipe, and the friction generates a tiny amount of heat.

What People Commonly Confuse It With

Hobbyists and junior technicians frequently confuse strict DC resistance (measured in ohms) with impedance (Z) and reactance (X). While impedance and reactance are also measured in ohms, they apply to alternating current (AC) circuits. Reactance involves energy storage in magnetic or electric fields (inductors and capacitors) and introduces phase shifts between voltage and current. When you measure a component with a standard DC multimeter, you are only measuring the real, resistive part of the circuit, not its total AC impedance.

Worked Numeric Example: Sizing a Feeder Based on Ohmic Loss

Understanding the ohm isn't just academic; it prevents fires and equipment failure on the jobsite. Let's look at a real-world voltage drop calculation using actual AWG wire resistance to see how ohmic losses change a circuit's behavior.

The Scenario: You are wiring a 120V branch circuit to power a 12A space heater. The panel is 100 feet away from the outlet. You decide to use standard 14 AWG THHN copper wire.

  • Load Current (I): 12 Amps
  • Wire Length: 100 feet out, 100 feet back = 200 feet total circuit length.
  • 14 AWG Copper Resistance: According to NEC Chapter 9, Table 8, 14 AWG solid copper has a resistance of 2.525 Ω per 1,000 feet at 20°C.

Step 1: Calculate Total Wire Resistance (R)
R = (200 ft / 1,000 ft) × 2.525 Ω = 0.505 Ω

Step 2: Calculate Voltage Drop (V_drop)
Using Ohm's Law (V = I × R):
V_drop = 12A × 0.505 Ω = 6.06 Volts

Step 3: Evaluate the Result
The heater will only see 113.94V (120V - 6.06V). The percentage of voltage drop is (6.06 / 120) × 100 = 5.05%. NEC-style guidance recommends keeping branch circuit voltage drop under 3%. Furthermore, that 0.505 Ω of resistance is dissipating power as heat inside your walls: P = I²R = 144 × 0.505 = 72.7 Watts of wasted heat.

The Fix: Step up to 12 AWG copper wire, which has a resistance of 1.588 Ω/kft. The new resistance is 0.317 Ω, dropping the voltage loss to 3.8V (3.1%), bringing the installation into acceptable limits and running significantly cooler.

Where You Meet the Ohm in Practice

Whether you are debugging an ESP32 breakout board or terminating a 200A subpanel feeder, the ohm is the primary metric you use to verify circuit integrity.

Bench Electronics: Tolerance and Temperature

When selecting resistors for a precision op-amp feedback loop, the nominal ohm value isn't enough; you must account for tolerance and temperature coefficient (TCR). A standard 5% carbon film resistor might read anywhere from 9,500 Ω to 10,500 Ω on a 10kΩ part. For precision ADC voltage dividers, you need 1% or 0.1% metal film resistors. Furthermore, resistance changes with temperature. Copper wire increases in resistance by about 0.4% per degree Celsius. A motor winding that measures 2.0 Ω cold might measure 2.4 Ω at operating temperature, which will alter your startup current calculations.

Jobsite Electrical: Megohms and Milliohms

In heavy electrical work, standard ohms are rarely the focus. Instead, you deal in extremes:

  • Megohms (MΩ): Used to measure insulation resistance. A healthy 600V THHN cable should read well over 100 MΩ when tested with a Megger (megohmmeter) at 500V or 1000V. If it reads below 1 MΩ, the insulation is compromised and the wire must be replaced.
  • Milliohms (mΩ): Used to measure contact resistance. A properly torqued breaker lug or a clean ground bond should read in the micro-ohm or low milliohm range. If a 200A main breaker terminal reads 50 mΩ (0.05 Ω), it will generate 2,000 Watts of heat at full load (P = 200² × 0.05), guaranteeing a melted lug and a potential fire.

Frequently Asked Questions About Ohm SI Units

Is the ohm a base SI unit or a derived SI unit?

The ohm is a derived SI unit. It is not one of the seven base units (like the meter, kilogram, or ampere). Instead, it is expressed in terms of base units as kilogram meter squared per second cubed per ampere squared (kg⋅m²⋅s⁻³⋅A⁻²). In practical terms, it is simply defined as one volt per one ampere (V/A).

Why did the SI definition of the ohm change in 2019?

Before May 2019, electrical units were tied to physical artifacts and specific empirical experiments (like the Weston cell for voltage). The BIPM SI Brochure outlines how the 2019 redefinition tied the ohm to the von Klitzing constant via the quantum Hall effect. This change ensures that the ohm is universally reproducible with extreme precision in any advanced metrology lab, independent of physical artifacts that can degrade or be damaged over time.

What is the difference between an ohm and a milliohm in practical testing?

An ohm (Ω) is used for standard circuit components and wire runs, while a milliohm (mΩ, or 1/1000th of an ohm) is used to measure the quality of electrical connections. You use standard ohms to verify a heating element or a resistor, but you use milliohms to verify that a busbar joint, a breaker terminal, or a grounding electrode conductor has a near-zero resistance connection. High milliohm readings at connections indicate loose torque, oxidation, or arcing hazards.

Can I use a standard multimeter to measure SI-traceable ohms?

No. A standard bench or handheld digital multimeter (DMM) typically has a DC resistance accuracy of 0.1% to 1%, which is fine for DIY and general troubleshooting but useless for metrology. To measure SI-traceable resistance, calibration labs use specialized Kelvin (4-wire) bridges and quantum Hall resistance standards that eliminate test lead resistance and thermal EMF errors, achieving accuracies down to parts per billion. For practical bench work, understanding basic resistance principles with a standard 4.5-digit DMM is more than sufficient.