Inside a standard USB phone charger cable, the positive wire is red and carries +5V, while the negative (ground) wire is black. For USB-C cables, the positive is carried on the VBUS pins and the negative on the GND pins, but official manufacturers advise against cutting or modifying certified cables due to safety risks.
Which wire is positive and negative on a phone charger?
The universal color code for USB cables is red for positive (+5V) and black for negative (ground). In USB-C connectors, the positive power is supplied through VBUS pins and the negative through GND pins. If you are working with a charge-only cable, it may contain only these two wires, omitting the data wires entirely. Always unplug the charger before cutting or testing any wires, and note that Apple and Samsung advise against modifying certified cables because of potential battery and charging risks.
How to identify the positive wire with color codes
In most standard USB cables, the red wire is the positive +5V conductor. This is true for both data cables and charge-only cables. If you strip back the outer insulation and see a red wire, that is your positive. If the colors are faded or non-standard, you will need a multimeter to confirm. Some chargers have a small positive (+) sign embossed or marked on the side of the connector, indicating which wire is positive.
How to identify the negative wire with color codes
The negative wire is almost always black. In some cables, you may find a bare silver or copper wire alongside the colored wires; this bare wire also serves as ground (negative). If the cable has a black wire and a bare wire, both are connected to ground and can be treated as negative. When colors are missing or ambiguous, do not rely on guesswork, use a multimeter. Look for symbols such as "-", "GND," or a down arrow, which are commonly used to denote the negative wire on the USB connector or charger itself.
Using a multimeter to confirm positive and negative
- Set your multimeter to DC voltage (usually marked with a V and a straight line or dashes).
- Plug the charger into a power source but do not connect it to a phone.
- Touch the black probe to the wire you suspect is negative (black or bare).
- Touch the red probe to the wire you suspect is positive.
- If the multimeter displays a positive voltage reading (e.g., +5.0V), you have correctly identified the positive wire. If it displays a negative reading (e.g., -5.0V), swap your probes, the wire on the red probe is actually negative.
This method works even when colors are faded, missing, or non-standard, and it is the most reliable way to confirm polarity before making any connections.
USB-C and the reversible connector challenge
USB-C connectors have 24 pins arranged symmetrically, which allows the plug to be inserted either way. This design means that at the plug end, the VBUS (positive) and GND (negative) pins are duplicated on both sides, making it nearly impossible to identify which internal wire corresponds to which pin without a detailed pinout diagram. The only reliable method to find positive and negative in a USB-C cable is to strip the cable midway, where the individual wires are separate, rather than at the connector itself. Once exposed, look for the red (positive) and black (negative) wires as described above.
Safety and manufacturer warnings
Apple and Samsung both state that using uncertified or modified chargers can negatively affect device charging capabilities and cause battery issues. Always unplug the charger from the wall before cutting or testing wires. If you see any frayed or exposed wires, discard the cable immediately. Working with live wires carries a risk of short circuits, electric shock, or fire. Use insulated gloves while handling the wires to minimize the risk of electric shocks and burns. Avoid using metallic tools when dealing with the wires; opt for non-conductive tools such as plastic or rubber. Ensure you are working with a charger that matches the voltage requirements of your phone, as using an incompatible charger can lead to damage or pose a risk of explosion. If you are uncertain about identifying the positive and negative wires, it is best to seek assistance from a professional or someone with electrical expertise.
Frequently asked questions
Can I use any charger with my phone?
Most modern phones use a standard USB interface, but using an incompatible charger can damage your device or slow charging. Always use the charger that came with your phone or one certified by the manufacturer.
Can I use a fast charger with my phone?
Fast charging requires both the phone and the charger to support the same fast-charging standard. Using a fast charger on a phone that does not support it will not harm the device, but it will charge at normal speed.
Why does my phone charger get hot?
Some warmth is normal, but excessive heat indicates a problem such as a low-quality charger, a damaged cable, or charging in a high-temperature environment. If the charger becomes too hot to touch, stop using it and replace it.
Can I charge my phone overnight?
Yes, modern smartphones have overcharge protection that stops charging once the battery is full. However, keeping the phone plugged in for extended periods can cause minor battery degradation over time. Unplugging once fully charged is best for long-term battery health.
How do I tell if a cable is charge-only or data-capable?
A charge-only cable contains only the red (positive) and black (negative) wires. A data cable also includes green and white wires for data transfer. If you strip the cable and see only two wires, it is charge-only. For a complete overview of phone charger types and cables, see our Phone Charger Types and Cable Guide.
For the wider topic, see phone charger types and cables.
