SATA Power Cable: Three Rails, Fifteen Pins, One Trap
Quick answer / TL;DR: Short answer: a SATA power connector supplies 3.3 V, 5 V and 12 V on fifteen pins, with each rail rated at about 1.5 A per pin across three pins, giving roughly 4.5 A per rail. The 3.3 V rail is often unused, and on drives that implement Power Disable it can stop the drive from spinning. If a drive does not spin up on a SATA power connector but works on a Molex adapter, the 3.3 V rail is the cause and the fix is to isolate that pin or use a four-pin adapter.
Copper all the way through, or a copper skin over aluminium: the cut decides.The SATA power connector has fifteen pins carrying three voltages: 3.3 V, 5 V and 12 V, each with its own ground return. It replaced the older four-pin Molex connector because it can supply each rail independently and because it is keyed and hot-pluggable in a way the Molex connector is not.
It also introduced a compatibility problem that still catches people: the 3.3 V rail can trigger the Power Disable feature on some drives, which makes them appear dead when they are perfectly functional.
The Pin Assignment
Fifteen pins in three groups of five. Each group carries one voltage followed by a ground, repeated three times, so that current is shared across three pins per rail rather than concentrated on one.
| Pins | Rail | Typical use |
|---|
| 1, 2, 3 | 3.3 V | often unused, legacy support | 4, 5, 6 | ground | return for 3.3 V | 7, 8, 9 | 5 V | electronics, 2.5 inch drives | 10, 11, 12 | ground | return for 5 V | 13, 14, 15 | 12 V | motor, 3.5 inch drives |
Splitting each rail across three pins is what allows the connector to carry meaningful current in a small housing. A single pin is rated around 1.5 A, so three pins give about 4.5 A per rail, which is adequate for a 3.5 inch drive that draws most of its power from 12 V at spin-up.
What Each Rail Actually Powers
The division of labour explains why a 2.5 inch drive and a 3.5 inch drive behave differently when a rail is marginal.
| Device | 3.3 V | 5 V | 12 V | Note |
|---|
| 2.5 inch SSD | rarely | yes, main supply | no | low current, tolerant | 2.5 inch HDD | rarely | yes, motor and logic | no | spin-up current on 5 V | 3.5 inch HDD | rarely | yes, logic | yes, motor | 12 V spin-up is the peak | Slim optical drive | no | yes | yes | via slimline connector |
The 3.3 V Problem
Some drives implement a feature called Power Disable, which uses the 3.3 V rail as a signal rather than as a supply. When 3.3 V is present, the drive stays powered down. This was intended for remote management in data centres and it is a genuine hazard in a desktop.
- Symptom: the drive does not spin up, is not detected, and appears dead
- Test: connect it through a four-pin Molex to SATA adapter, which has no 3.3 V
- Fix: use an adapter permanently, or isolate pins 1 to 3 on the SATA connector
- Affected devices: some large hard disks, particularly those designed for data centre use
- Not affected: most consumer SSDs and 2.5 inch drives
Real-World Applications
Desktop with a modular power supply
Use the supplied SATA power cables rather than splitters where possible. A modular supply has enough connectors for a normal build, and each additional splitter adds contacts and a current limit.
Drive that will not spin up
Before returning the drive, test it on a Molex to SATA adapter. If it spins, the 3.3 V rail was triggering Power Disable and the drive is fine. This is one of the few cases where a two-dollar adapter is the correct permanent fix.
NAS with many drives
Spin-up current is the peak load, and several drives starting together can exceed a rail's capability. Staggered spin-up, where the controller starts drives in sequence, is worth enabling. It is a controller feature, not a cable one.
Older power supply without SATA connectors
A Molex to SATA adapter is the standard solution. Choose one with a moulded body rather than a snap-together shell, because the moulded version has no internal contacts that can work loose.
Related reading: Pinout detail and wiring — blogs/news/sata-power-cable-pinout
Related reading: Splitters and current limits — blogs/news/sata-power-splitter
Standards and Verification
The SATA power connector, its pin assignment and its electrical ratings are defined in the specification published by the Serial ATA International Organization. The connector is keyed and designed for blind mating inside a chassis.
Voltage tolerance follows the ATX power supply specification for the rails themselves, with 12 V, 5 V and 3.3 V each having a permitted tolerance band. A rail outside tolerance causes instability that looks like a drive fault.
Verification is practical: measure the rail voltages at the connector under load with a multimeter, and confirm the drive spins up and remains detected through a sustained transfer. A rail that sags under load indicates a supply or cabling limitation rather than a drive problem.
Never force a SATA power connector. It is keyed with an L-shaped profile and can be inserted only one way, and forcing it bends the pins in a way that is not repairable.
Frequently Asked Questions
What voltages does SATA power provide?
3.3 V, 5 V and 12 V, each with its own ground return, on a 15-pin connector. Each rail uses three pins to share the current.
Why does my drive not spin up with a SATA power connector?
The 3.3 V rail may be triggering the Power Disable feature. Test with a four-pin Molex to SATA adapter, which has no 3.3 V, and if the drive spins the cause is confirmed.
Can I plug SATA power in backwards?
No. The connector is keyed. Forcing it will bend pins, so check the orientation rather than applying more force.
How much current can a SATA power connector supply?
Roughly 4.5 A per rail, based on about 1.5 A per pin across three pins. Peak spin-up current on the 12 V rail is the usual design constraint.
Do I need a Molex adapter for an old power supply?
Only if the supply has no SATA connectors. Choose a moulded adapter rather than one with a snap-together shell, which can open and expose live contacts.
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