# How do I measure the voltage coming out of a USB port?

If I attach a USB cable to a USB port, and then strip the wire at the end, then how do I connect a multimeter to it to measure the voltage?

• Remember that drawing any more than 50 mA before asking for more (if you do at all) is a violation of the spec and potentially damaging to the bus (and all other devices on it). With the USB Charging Specification, devices are also able to draw up to 1.8 A from a USB port (if allowed). Commented Oct 21, 2010 at 19:18
• it is 100mA without asking, not 50. Commented Oct 22, 2010 at 3:01
• Just to make sure awakeFromNib is clear, the 100mA limit has nothing to do with measuring the voltage with a multimeter since the multimeter will not load the circuit much. The only case that this does come into play is if he connects a large load, such as a 10 ohm resistor, across the vbus and ground pins. Doesn't sound like this is what he is asking about though. Commented Oct 22, 2010 at 5:49
• How do you get past the 100mA limit? Commented Oct 23, 2010 at 5:24
• As part of the enumeration, you negotiate how much power is needed. Commented Aug 28, 2015 at 15:31

The standard pin numbers and colours in a USB cable are shown below.

Pin Number   Cable Colour   Function
1          Red         VBUS (5 volts)
2          White       D-
3          Green       D+
4          Black       Ground


If there is no device connected to the USB port then the maximum current that the port can support may be quite low. Once a device has enumerated itself on the port then it can negotiate for up to 500mA from the host controller.

To measure the voltage then you either touch the meter probes onto the stripped ends of the wires or, if your cable end is a large connector, then touch the probes (without shorting them together) onto pins 1 and 4 of the connector. The power pins on the USB connector are easy to identify as they are longer than the data pins. This is so that they mate first and ensures that the power is connected before the data.

• Ok, so how do I hook up my multimeter to the wires to verify the voltage that I am getting. Commented Oct 21, 2010 at 9:16