DC Voltage: What is it? (Circuit Symbol & Wire Color Codes)

What is DC Voltage?

DC voltage means a voltage that does not reverse polarity. It produces, or would produce, a DC current. AC voltage reverses polarity and produces alternating current.

DC here also covers quantities that do not regularly reverse polarity, or whose frequency is zero or near zero. AC covers quantities that reverse polarity at a frequency above zero.

Voltage is the electric potential difference between two points, equal to work per unit charge (V = W/Q). Charge separation creates that difference. Current is not required for a voltage to exist.

When charge is moved against the field, its potential energy changes. That potential difference is the voltage between the two points.

The two common supply forms are AC and DC. Voltage taken from a DC source is DC voltage.

Ideal DC voltage has a fixed polarity and a steady magnitude. It is written V_DC. The frequency of an ideal DC source is zero. Real DC supplies can have ripple, but they do not reverse polarity the way AC does.

DC Voltage Symbol

Unicode character U+2393 “⎓” is a DC voltage symbol. Circuit drawings also use a battery symbol (long and short parallel lines) or a circle marked + and −.

Schematics show several DC sources. A battery is the source drawn most often. The figure below is the battery symbol.

Another common DC source symbol is shown below.

An ideal DC voltage source has zero internal resistance. A real source always has some internal resistance.

An ideal source holds a fixed terminal voltage at any current. A real source drops I times its internal resistance inside the source, so terminal voltage falls as load current rises.

The V-I curves of an ideal source and a real DC source are shown below.

DC Voltage Wire Color Code

Wiring systems use colour (and labels) so each conductor can be identified during install and later work.

Colour-code tables often say “DC power” rather than “DC voltage.” Those words are not the same quantity. Here “DC power” only means the circuit is DC rather than AC.

IEC DC Power Circuit Wire Color Codes

IEC 60445 and related installation rules set conductor identification for much of the world. The US National Electrical Code (NEC) is a separate document and does not copy IEC colours.

A widely published IEC-style DC colour set is in the table below (brown L+, grey L−, blue midwire). IEC 60445 also lists preferred DC colours red (+) and white (−) at equipment. Always follow the standard named on the drawing.

Function	IEC Standard
	Label	Color
Protective earth	PE	Green-Yellow
Two-wire unearthed DC system
Positive	L+	Brown
Negative	L-	Grey
Two-wire earthed DC system
Positive (Negative earthed) circuit	L+	Brown
Negative (Negative earthed) circuit	M	Blue
Positive (Positive earthed) circuit	M	Blue
Negative (Positive earthed) circuit	L-	Grey
Three-wire earthed DC system
Positive	L+	Brown
Mid-wire	M	Blue
Negative	L-	Grey

US DC Power Circuit Wire Color Codes

The NEC requires DC conductors to be identified. It does not publish an IEC-style colour table for ungrounded DC. US practice often uses red for positive and black for negative, as the next table shows in parentheses.

Common US DC colours for grounded systems, as collected in the table below, use red, black and white. Confirm against the NEC edition and the job spec; the table is practice, not a substitute for the code book.

Function	US National Electrical recommended code
	Label	Color
Protective ground	PG	Bare, Green, or Green-Yellow
Two-wire ungrounded DC system
Positive	L+	No recommendation (Red)
Negative	L-	No recommendation (Black)
Two-wire grounded DC system
Positive (Negative grounded) circuit	L+	Red
Negative (Negative grounded) circuit	N	White
Positive (Positive grounded) circuit	N	White
Negative (Positive grounded) circuit	L-	Black
Three-wire earthed DC system
Positive	L+	Red
Mid-wire	N	White
Negative	L-	Black

Difference Between DC Voltage and AC Voltage

The stored comparison table is below. Some rows (power factor on DC, a single efficiency ranking) mix AC terms with DC and should not be read as universal laws.

	DC Voltage	AC Voltage
Definition	The DC voltage induces constant value direct current.	The AC voltage is a force that derives alternating current.
Frequency	Zero	50Hz/60Hz
Symbol
Polarity	Constant	Changes with respect to time
Power factor	Zero	Lie between 0 to 1
Source	DC generator, Battery, or cell	AC generator
Passive parameter	Resistance	Impedance
Efficiency	Low	High
Direction	Constant	Vary

How to Reduce DC Voltage

Sometimes a DC source is higher than the load rating, so the voltage must be reduced.

Example: a 9 V battery feeding a circuit that needs 3 V. The extra 6 V must be dropped somewhere or the source must be regulated down.

Crude drops use series diodes and resistors. A regulator is the usual choice when the load current varies.

How to Reduce DC Voltage With Diodes

A diode conducts when it is forward biased. A forward drop then appears across its terminals.

Small silicon diodes drop about 0.6 to 0.7 V, germanium about 0.25 to 0.3 V, and a small Schottky diode often about 0.2 to 0.4 V, depending on current.

Series diodes add their forward drops. The circuit below shows that chain.

The example needs about 6 V of drop between a 9 V battery and a 3 V load. At 0.7 V per silicon diode that is about 8 to 10 diodes, not 4 or 5. The figure is stored as-is and may show a shorter string.

The first anode goes to battery positive. Each next anode goes to the previous cathode. Add more diodes in that chain if a larger drop is required. Diode drop still varies with current, so this is a rough method.

How to Reduce DC Voltage With Resistors

Two resistors in series make a voltage divider. The split is accurate only if the load on the tap is light compared with the divider current, or if you include the load in the calculation.

Two equal resistors, with no load on the join, give half the input voltage at the join.

For other ratios, use the unloaded divider formula below:

   

The formula above gives the voltage across R₂ when both resistances are known. It does not by itself give the ohm value of R₂.

Choose one resistor plus the input voltage and the wanted output. Put those values in the next equation to find R₂.

   

Example: 5 V in, 3 V out, and one 10 Ω resistor already chosen. Find R₂.

   

   

How to Step-up DC Voltage

To step up DC is to raise its voltage. A common DC-DC converter for that job is a boost Converter.

A boost converter is a switched-mode supply. The usual power stage uses a diode and a transistor plus an inductor and a capacitor.

Filters cut voltage ripple. Typical parts are capacitors, or capacitors with inductors, on the input, the output or both.

In the lossless case, input power equals output power, so output current falls as output voltage rises. A real converter also dissipates some power as heat.

In AC systems a transformer steps voltage up or down. Alternating current makes a changing flux, which induces a secondary voltage by electromagnetic induction.

Steady direct current does not produce that changing flux, so a plain transformer does not step DC. Isolated DC-DC converters chop the DC first, then use a transformer on the AC that chopping creates.

A boost converter steps DC voltage up. A buck converter steps DC voltage down.

How is DC Voltage Represented on Multimeters?

A multimeter measures voltage, current and resistance. Meters differ in size and in which extra functions they include.

To measure voltage, select a DC volts range at or above the expected value and put the probes in the voltage jacks.

The same meter can read AC or DC. For DC, select the DC volts function (often marked V⎓ or VDC), not AC volts.

Manual-range digital multimeters often use 2 V, 20 V and 200 V steps. For a voltage under 20 V, the 20 V range is the usual choice. Autoranging meters pick the range for you. Connect red to the more positive point or the display shows a minus sign.