Editing USB (section)

=== Low-power and high-power devices <span class="anchor" id="HOST"></span>===

This section describes the power distribution model of USB that existed before [[USB_hardware#USB_Power_Delivery|Power-Delivery]] (USB-PD). On devices that do not use PD, USB provides up to 4.5&nbsp;W through Type-A and Type-B connectors, and up to 15&nbsp;W through USB-C. All pre-PD USB power is provided at 5&nbsp;V.

For a host providing power to devices, USB has a concept of the ''unit load''. Any device may draw power of one unit, and devices may request more power in these discrete steps. It is not required that the host provide requested power, and a device may not draw more power than negotiated.

Devices that draw no more than one unit are said to be ''low-power'' devices. All devices must act as low-power devices when starting out as unconfigured. For USB devices up to USB&nbsp;2.0 a unit load is 100&nbsp;mA (or 500&nbsp;mW), while USB&nbsp;3.0 defines a unit load as 150&nbsp;mA (750&nbsp;mW). Full-featured USB-C can support low-power devices with a unit load of 250&nbsp;mA (or 1250&nbsp;mW).

Devices that draw more than one unit are ''high-power'' devices (such as typical 2.5-inch hard disk drives). USB up to 2.0 allows a host or hub to provide up to 2.5&nbsp;W to each device, in five discrete steps of 100&nbsp;mA, and SuperSpeed devices (USB 3.x) allows a host or a hub to provide up to 4.5&nbsp;W in six steps of 150&nbsp;mA.
USB-C allows for dual-lane operation of USB 3.x with larger unit load (250&nbsp;mA; up to 7.5&nbsp;W).<ref>{{Cite web |title=USB 3.2 Revision 1.1 - June 2022 |url=https://usb.org/document-library/usb-32-revision-11-june-2022 |access-date=2024-12-31 |website=usb.org |publisher= USB Implementers Forum |at=p470, sec. 11.4.5 Vbus Electrical Characteristics}}</ref> USB-C also allows for Type-C Current as a replacement for USB BC, signaling power availability in a simple way, without needing any data connection.<ref>{{Cite web |title=USB Type-C® Cable and Connector Specification Release 2.4 {{!}} USB-IF |url=https://usb.org/document-library/usb-type-cr-cable-and-connector-specification-release-24 |access-date=2024-12-31 |website=usb.org |publisher= USB Implementers Forum |at=p41, sec. 2.4 Vbus}}</ref>

{| class="wikitable sortable" style="margin:0 0 1em 1em;"
|+ USB power standards
|-
! Specification
! max current
! Voltage
! max power
|-
| Low-power device
| {{right|100&nbsp;mA}} || {{right|5&nbsp;V}}{{Efn |name="Vdrop" |The V{{sub |BUS}} supply from a low-powered hub port may drop to 4.40&nbsp;V.}} || {{right|0.50&nbsp;W}}
|-
| Low-power SuperSpeed / USB 3.x device
| {{right|150&nbsp;mA}} || {{right|5&nbsp;V}}{{Efn |name="Vdrop"}} || {{right|0.75&nbsp;W}}
|-
| High-power device
| {{right|500&nbsp;mA}}{{Efn|Up to five unit loads; with non-SuperSpeed devices, one unit load is 100&nbsp;mA.}} || {{right|5&nbsp;V}} || {{right|2.5&nbsp;W}}
|-
| High-power SuperSpeed / USB 3.x single-lane device
| {{right|900&nbsp;mA}}{{Efn|Up to six unit loads; with SuperSpeed devices, one unit load is 150&nbsp;mA.}} || {{right|5&nbsp;V}} || {{right|4.5&nbsp;W}}
|-
| High-power SuperSpeed / USB 3.x dual-lane device{{Efn|name="usbcOnly"|for USB-C only}}
| {{right|1.5&nbsp;A}}{{Efn|name="Vml"|Up to six unit loads; with multi-lane devices, one unit load is 250&nbsp;mA.}} || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
|-
| Battery Charging (BC)
| {{right|1.5&nbsp;A}}  || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
|-
| USB4{{Efn|Not Type-C current, only available after starting USB4 connection. Can be combined with Type-C current.}}
| {{right|1.5&nbsp;A}} || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
|-
| Type-C current 1.5&nbsp;A{{Efn|optional for every USB-C host port. Mandatory for USB-C ports with USB-BC or for even higher PD output.}}
| {{right|1.5&nbsp;A}} || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
|-
| Type-C current 3&nbsp;A{{Efn|optional for every USB-C host port. Mandatory for ports with even higher PD output.}}
| {{right|3&nbsp;A}} || {{right|5&nbsp;V}} || {{right|15&nbsp;W}}
|-
| [[USB Power Delivery|Power Delivery]] {{abbr|SPR|Standard Power Range}}{{Efn|name="usbcOnly"}}
| {{right|5&nbsp;A}}{{efn|name="req_5A_cable"|>3&nbsp;A (>60&nbsp;W) operation requires an electronically marked cable rated at 5&nbsp;A.}} || {{right|up to 20&nbsp;V}} || {{right|100&nbsp;W}}
|-
| Power Delivery {{abbr|EPR|Extended Power Range}}{{Efn|name="usbcOnly"}}
| {{right|5&nbsp;A}}{{efn|name="req_5A_cable"}} || {{right|up to 48&nbsp;V}}{{efn|name="req_EPR_cable"|>20&nbsp;V (>100&nbsp;W) operation requires an electronically marked Extended Power Range (EPR) cable.}} || {{right|240&nbsp;W}}
|-
| colspan=4 | {{notelist}}
|}

To recognize Battery Charging mode, a dedicated charging port places a resistance not exceeding 200&nbsp;Ω across the D+ and D− terminals. Shorted or near-shorted data lanes with less than 200&nbsp;Ω of resistance across the D+ and D− terminals signify a dedicated charging port (DCP) with indefinite charging rates.<ref name="USBBC1.2">{{cite web| url = http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip | title = Battery Charging Specification, Revision 1.2 | date = 7 December 2010 | access-date = 29 March 2016 | publisher = USB Implementers Forum | url-status=dead | archive-url = https://web.archive.org/web/20160328102350/http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip | archive-date = 28 March 2016 |page=45}}</ref><ref>{{cite web |title=OVERVIEW OF USB BATTERY CHARGING REVISION 1.2 AND THE IMPORTANT ROLE OF ADAPTER EMULATORS |url=https://pdfserv.maximintegrated.com/en/an/TUT5801.pdf |publisher=maxim integrated |page=3 |date=2014 |access-date=12 August 2021 |archive-date=4 July 2021 |archive-url=https://web.archive.org/web/20210704221311/https://pdfserv.maximintegrated.com/en/an/TUT5801.pdf |url-status=live}}</ref>

In addition to standard USB, there is a proprietary high-powered system known as [[PoweredUSB]], developed in the 1990s, and mainly used in point-of-sale terminals such as cash registers.