“+48V” does not name one system. In road vehicles it means a nominal 48 V DC supply that sits alongside the familiar 12 V battery and feeds an electric machine and high-power accessories. In telecommunications, the same figure usually appears as a −48 V DC interface, with a negative-referenced naming convention and an operating range of 40.5–72 V DC. The number is shared; the architecture, polarity convention and governing standards are not. Identify the application before comparing anything.
Start with the application, not the number
Most confusion about 48 V comes from treating automotive and telecom usage as the same thing. They describe different equipment, different polarity conventions and different standards.
Automotive 48 V
In a road vehicle, 48 V refers to a nominal 48 V DC electrical system. ISO 21780:2020 covers electrical and electronic components designed for a road-vehicle electrical system operating at that nominal voltage. ISO’s record shows the edition was confirmed as current in 2026, so it remains the reference to cite for automotive components.
Telecom −48 V
In telecommunications, the figure is written as −48 V DC. This is an ICT power interface for network equipment, not a vehicle supply. ITU-T L.1210 (December 2025), which covers power-feeding for IMT-2020 and converged access networks, cites a common 40.5–72 V DC interface. The minus sign reflects the telecom naming convention and should not be read as a description of the automotive supply, which is nominal positive 48 V.
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Why vehicles are adding a 48 V system
The German Association of the Automotive Industry (VDA) says growing electrification has pushed 12 V systems toward their capacity limits. A higher-power 48 V system supports energy recovery, start-stop and hybrid-drive functions, and loads such as air conditioning, active chassis control, high-power heating and electric turbochargers. VDA puts it directly: “The 48-volt power system also forms the basis for the hybrid drive.”
The underlying physics is simple. For the same power, current falls as voltage rises (P = V × I). Moving a given amount of power at 48 V instead of 12 V means roughly a quarter of the current, which allows smaller conductors and lower resistive losses in the harness. That is the main engineering reason a 48 V bus is attractive for loads that would otherwise demand very large 12 V currents.
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A mild-hybrid example
onsemi’s 2021 technical explanation of mild hybrids describes one example architecture, not a standard for every vehicle sold as 48 V. It has:
- A 12 V bus for traditional equipment such as lighting, infotainment and body electronics.
- A 48 V bus for the electric machine and selected higher-power accessories.
- A bidirectional DC-DC converter linking the two buses.
In the configurations described, the 48 V battery does not propel the vehicle on its own. As onsemi’s Senior Engineer Jay Nagle put it in the same 2021 Q&A, “the 48−volt battery cannot propel the car on its own.” The 48 V system assists the drivetrain; the combustion engine or other propulsion remains essential.
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Supplier power figures, with their limits
onsemi’s 2021 note says a standard 12 V system “can put out a maximum of 2 to 4 kW” and a 48 V system “can deliver up to 10 − 20 kW.” These are supplier generalizations. The note does not tie them to a specific vehicle, load profile or test setup, so treat them as indicative of the design direction rather than as measured performance.
Telecom −48 V and the reach problem
The telecom interface is about delivering power to network equipment over cabling, often across distance. ITU-T L.1210 explains the central trade-off: lower DC voltage means higher currents for the same power, and those higher currents increase cable losses. In legacy cabling, that can limit how far power can be fed before losses become unacceptable.
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- Primary output voltage (vdc): 48
- Output current (a): 5
- Maximum output power (w): 240
- Family: NDR-240
- Family: standards-240 Input voltage (vac): 90 to 264
The same summary places 48 V in a wider set of options, including AC, DC up to 400 V, low-voltage DC and Power over Ethernet (PoE). The point is that the −48 V interface is one system choice for specific equipment, not a universal answer for all ICT loads. ITU-T L.1201 (March 2014, still listed in force) takes a broader view, describing architectures for DC systems up to 400 V in telecom centres, data centres and customer premises, including redundancy, power distribution and monitoring. It is useful background on DC power in telecom, but it is not a specification for −48 V.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Comparing the two uses
| Aspect | Automotive +48 V | Telecom −48 V |
|---|---|---|
| Application | Vehicle electrical distribution for mild-hybrid and hybrid functions | Power interface for ICT and network equipment |
| Voltage convention | Nominal positive 48 V DC vehicle supply | −48 V DC naming convention; 40.5–72 V DC interface range (ITU-T L.1210, December 2025) |
| Typical loads | Electric machine, air conditioning, active chassis control, high-power heating, electric turbochargers (VDA) | Network and ICT equipment; specific loads not stated in the cited ITU-T summary |
| Distribution concern | Same power at lower current than 12 V, reducing conductor size and resistive losses | Lower voltage means higher current and greater cable loss; reach can be limited in legacy cabling |
| Governing standard | ISO 21780:2020, covering electrical and electronic components (confirmed current in 2026) | Applicable ETSI interface specifications and ITU-T recommendations such as L.1210; the specific ETSI document is not stated in the cited summaries |
| Safety basis | Vehicle-specific design, protection and work requirements; not a single nominal-voltage rule | Installation and equipment requirements under the applicable standard and equipment documentation; not a single nominal-voltage rule |
Safety and standards: no blanket rule
Do not treat “48 V” as a safety determination on its own. A nominal voltage describes the system; it does not settle how components must be protected, insulated, labelled or serviced. onsemi’s 2021 note makes a broad safety observation, but the standards summaries available do not establish a universal safe-touch rule for every 48 V installation.
In practice, automotive work follows the vehicle’s own design and service documentation and the applicable component standard, while telecom installations follow the relevant ETSI and ITU-T requirements and the equipment manufacturer’s instructions. For detailed requirements, consult the full standard and the equipment documentation for the specific installation.
Further reading
- SAE International, 48-Volt Developments: a book-length reference on automotive 48 V systems, useful if you need more depth on vehicle architectures than a summary provides.
- ISO 21780:2020: the full automotive standard for components in a nominal 48 V DC road-vehicle system.
- ITU-T L.1210 and L.1201: the telecom power-feeding and DC power architecture recommendations discussed above.
Automotive 48 V DC-DC converters and power electronics are real system components, but choosing one depends on the vehicle’s voltage architecture, load profile and compatibility requirements, so a generic product choice is not offered here.
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