Overclocking, Current Testing

In my original review I did some direct power measurements while overclocked. What does the data look like in that scenario, with power targets raised by 30% but with only a 3.5% overclock in GPU frequency?

The Witcher 3 (1080p) power draw, RX 480 Overclocked, Click to Enlarge

First, power measurement is noisier in this case, likely for a couple of reasons. First, the clock speed is jumping around as I showed you before due to thermal constraints and the instantaneous power draw follows that path. Also, as you draw more power through a path like the motherboard, high current draw will cause voltage droop, and the system fights with itself to keep power consistent through the card. As you go over spec on any power supply design, variance will increase in how that power is presented.

Though the noise makes it harder to see, when overclocked our Radeon RX 480 is drawing more than 190 watts and is hitting nearly 200 watts in some cases!

The Witcher 3 (1080p) power draw, RX 480 Overclocked, Click to Enlarge

When we zoom in we find that the motherboard is actually providing more than 95 watts of power over the +12V line and maintains the 5 watts from the +3.3V line, proving that we are indeed getting more than 100 watts through a PCIe connection that is only rated at 75 watts. The 6-pin PCIe power cable is almost crossing that 100 watt barrier too.

Because this is with overclocked settings, AMD is not really responsible for the specification breach in this specific instance, even though the company has talked about the overclocking headroom available on the RX 480 directly.

In the Tom’s Hardware testing, rather than measure power consumption at a resolution that gamers will actually use with the RX 480, they standardized on Metro: Last Light running at 4K. Just to see if we could duplicate those power consumption results, I fired the RX 480 back up and attempted to run the same testing.

Metro: Last Light (4K) power draw, RX 480, Click to Enlarge

The results are compelling: at stock settings the RX 480 is 160+ watts on average and spikes to 170 watts several times.

Metro: Last Light (4K) power draw, RX 480, Click to Enlarge

Taking a closer look reveals that the motherboard PCI Express connection is supplying 80-84 watts of power over the +12V rail continuously, while the +3.3V rail hovers just below 5 watts. This is definitely a concern for the RX 480 design, but to what degree?

The PCI Express specification language is more specific on currents than wattage limits, calling for a maximum of 5.5A over the +12V line and 3A over the +3.3V. Our direct power measurement tools and hardware allow us to not just measure power but directly record the amperage and voltage.

Metro: Last Light (4K) current and voltage, RX 480, Click to Enlarge

This graph shows that result, running Metro: Last Light at 4K with the Radeon RX 480 at stock settings. The green line is the amperage being used by the +12V on the motherboard PCI Express connection and the blue represents the same over the 6-pin power connection. The motherboard is pulling more than 6.5A through the slot continuously during gaming and spikes over 7A a few times as well. That is a 27% delta in peak current draw from the PCI Express specification. The blue line for the 6-pin connection is just slightly lower.

At the top, you have a red and white line representing the voltage signal of the +12V rails from the motherboard PCIe slot and the 6-pin connection. Notice the drop on the white line of the motherboard +12V rail – during game play it is actually running at 11.5V while the PCIe 6-pin cable is a much safer 11.9V. When we exit the game at the 18:25:27 timestamp, power draw drops, current drops and the voltage returns to the same 11.9V we would expect to find. Looking back at the red line, the differences in power handling capability of the two sources become clear, as the 6-pin voltage barely flinches at the same current swings that caused 0.4V droop from the motherboard-supplied +12V source.

That voltage droop is caused by the current draw over the PCI Express connection, pins, and traces through the motherboard. Doing some quick math (0.5V drop at nearly 7A) tells us that the pins and traces are directly dissipating 3 watts of power in this state! What might be even worse for this voltage droop is that it affects all other PCI Express slots on our Rampage V Extreme motherboard when the primary slot was loaded to this degree. Any other add-in card that you run in the system with an RX 480 drawing this much power will be forced to run at the lower voltage. PCI Express does build in a tolerance level of +/- 8% for this value, so the rest of the system should remain stable, but one question would be what happens to that voltage when someone attempts quad-crossfire with overclocked RX 480s?

Does any of this matter?

It seems clear at this point that the new AMD Radeon RX 480 does in fact draw more power through both the motherboard PCI Express connection and the 6-pin power connection than specifications state it should even when running at stock settings in certain gaming scenarios. The overdraw on the 6-pin cable is likely a non-issue; with power coming directly from the power supply and not passing through your motherboard and the fact that most cabling is built to handle higher power draw than we are seeing here, it’s very low on my list of concerns. The motherboard power draw is definitely something to keep an eye on though, especially given the voltage droop seen when motherboard traces are loaded to that degree.

The highest power draw I measured with the RX 480 at stock settings showed 80-85 watts of power draw at over 7A on the +12V line and 4.5-5.0 watts of power draw on the 3.3V line. These were consistent power draw numbers, not intermittent spikes, and users have a right to know how it works. When overclocked, we witnessed motherboard PCIe slot +12V power draw at 95+ watts!

  +12V +12V Current (Max) +12V Power
PCIE Specification 12 volts +/- 8% 5.5A 66W
RX 480 Stock 11.55V 6.96A 80.5W
RX 480 OC 11.45V 8.29A 95W
% Outside Spec 0% 50.7% 43.9%

The numbers above represent a “bad case” scenario but are by no means a “worst case” scenario for the RX 480.

I asked around our friends in the motherboard business for some feedback on this issue – is it something that users should be concerned about or are modern day motherboards built to handle this type of variance? One vendor told me directly that while spikes as high as 95 watts of power draw through the PCIE connection are tolerated without issue, sustained power draw at that kind of level would likely cause damage. The pins and connectors are the most likely failure points – he didn’t seem concerned about the traces on the board as they had enough copper in the power plane to withstand the current.

As we all know with hardware failures in PCs, this is something that could in theory happen during a single gaming session, or it might instead take months and months of gaming to wear down componentry. Or it might never affect your system at all. I tend to lean on the side of worrying less about this power draw concern than many in the community are pushing me to. Following the likes of ASUS, MSI and Gigabyte over the last 5-10 years, I have seen all the major motherboard vendors focus on quality of componentry, and specifically on power delivery, in a wide range of product families. That doesn’t mean that any one particular motherboard is not prepared for the overclocked power draw on a Radeon RX 480, or that lower cost or older motherboards that some gamers are still using in their PCs might not be built to withstand it. We just don’t know yet; not enough hardware is in enough hands.

For our part, we are going to be plugging the Radeon RX 480 into a couple of older platforms and running it in some “bad case” scenarios…just to see what happens.

The easy fix to this whole ordeal is for AMD to have used an 8-pin power connector on the RX 480. The PCI Express spec allows an 8-pin connection to draw 150 watts on its own, leaving the power from the PCI Express connection on the motherboard for overhead. This is how NVIDIA designed the GTX 970 (two 6-pin connections) and how AMD designed the R9 380 (one 8-pin connection): both cards have 150+ watt TDPs with power supply overhead available to them. The new GeForce GTX 1070 has an identical TDP to the RX 480 (150 watts) but uses an 8-pin power connection to relieve any concerns at stock performance or while overclocking.

The Witcher 3 power draw, R9 380, Click to Enlarge

Notice in the graph above that AMD designed the card to only draw 50-55 watts through the +12V on the motherboard connection, letting the 8-pin power cable handle the rest of the load.

AMD probably didn’t want to include a second 6-pin or upgrade to an 8-pin connection because of the impression it would give for a mainstream gaming card. Having two power connectors or an 8-pin might tell uninformed buyers that the RX 480 isn’t power efficient enough, that it might not work with an underpowered system and power supply, etc. My hope is that AMD’s partners paid attention to this data and are over building their power delivery to alleviate any concerns. We will know very soon.

As is usually the case in this market, there are some asking for a recall on the RX 480 or saying that AMD is going to be liable for some kind of class action lawsuit. I don’t agree with either sentiment. AMD is clearly pushing the envelope with the power delivery system on the Radeon RX 480 and we have shown that they are definitely drawing power outside PCI Express specifications even at stock clock speeds and power settings. I haven’t run into any stability concerns yet nor have I heard of any other reviewers indicating as much. As the RX 480s start to reach consumers hands today and later this week, I will be paying close attention to community reports while doing our own longer term testing at PC Perspective.

For its part, AMD has acknowledged the issue publicly during its launch day AMA on Reddit and to me through email. They have stated they are looking into the issue and may even be able to come up with a software-based fix. I hope that's the case and I'll be curious if there are any performance changes that occur because of it. Consider this space reserved after a more thorough discussion with the Radeon team.

« PreviousNext »