Safe i7-8700 CPU Core Current (Amps)

augiem

Senior member
Dec 20, 1999
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#1
Does anyone know the safe maximum current (amperage) limit of a Core i7-8700? I cannot seem to find this information in the Intel datasheets. I'd also like to know the default value.

I'm trying to undervolt an i7-8700 processor to reduce heat because of case size and constraints environmental factors. I'm trying to find the best balance between speed and heat.

Motherboard: ASRock Fatal1ty Z370 Gaming-ITX/ac
CPU: Intel Core i7-8700
All settings default
Stress tests: Prime 95 FFT, Blend

I've found a relative VCore undervolt of -50mV to be stable, but it still produces too much heat. The BIOS has additional settings which allow me to set a TDP limit and a maximum current (CPU Core Current Limit).

Setting the TDP limit to 125W brings the heat levels under control, but I'm concerned that the CPU current draw is being greatly increased to achieve this. The CPU current is being handled automatically by the motherboard. I believe it's increasing the current to compensate for the lower VCore in order to achieve the same frequency. Unfortunately, the BIOS doesn't tell you what the default value for current is.

If, on the other hand, I set CPU Core Current Limit to 135 without a TDP limit, it will cap the TDP around 126W under load. That would signify to me that it's drawing at least 135A above when the TDP is set to 125W and current is handled automatically.

Setting the current limit to 135A causes the FFT test to run at the same frequency as when using the TDP limit = 125W. However, the Blend test runs at a higher frequency when using TDP limit vs current limit, indicating to me that the motherboard is allowing the CPU to draw even more current than 135A when handled automatically. Therein lies my concern. Is 135A-150A+ safe for this CPU? I don't want to be shooting myself in the foot undervolting to reduce heat only to be killing the CPU by pumping too much current.
 
Apr 27, 2000
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#2
Current is something that is hard(er) to monitor/control vs voltage and clockspeed. The main things you need to worry about are total power draw and heat. If you are reducing heat, you are (overall) increasing the life of your CPU. Your motherboard will balance current load more based on chipset/socket tolerances than anything else. You have very little control in that department (as you have observed).

I believe that you can monitor various current levels with software tools like HWiNFO64, though the readouts may not be accurate.

Bottom line: voltage + heat kill CPUs.
 
Mar 10, 2004
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#3
XTU might allow you to set a current limit with core controls.
 

augiem

Senior member
Dec 20, 1999
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#4
Thanks for the replies guys!

I ended up finding what I was looking for in the datasheet, page 110:
ICCMAX, 65W Hexa-core GT2 = 133A

So 133A is the max. I'm pretty sure it's going past that based on my tests above. :confused:

I believe that you can monitor various current levels with software tools like HWiNFO64, though the readouts may not be accurate.
HWInfo64 is what I'm using to monitor this info and unfortunately it doesn't show current. Trying to calculate it from the CPU Package Power or IA Cores Power and the VCore doesn't seem to yield results that would agree with what's happening with the CPU Core Current Limit above.

XTU might allow you to set a current limit with core controls.
I'll check it out. Thanks! Oddly, the release notes for the latest version does not list Coffee Lake compatibility.
 
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augiem

Senior member
Dec 20, 1999
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#5
XTU proved to be very useful indeed. I was able to see the default settings set by the motherboard for the TDP throttle limit and current throttle limit and when either type of throttling kicked in. I was pretty surprised by what I saw.

Default current throttle limit: 255.500A

After some testing, I was able to determine that during stress testing using the default settings, the CPU is pulling 147A -- 14A over the published ICCMax value. I determined this by progressively changing the current throttle limit until it stopped throttling. Unexpectedly however, undervolting didn't change this cutoff point.

So now the question is, if the ICCMax is 133A, why is the chip pulling 147A on default settings?
 

dahorns

Senior member
Sep 13, 2013
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#6
Cant help you on the current issue. But as far as heat goes, i set my vcore to 1.185 and mine it now runs super cool (for coffee lake). Under stress test I max out around 67 degress using an h7.
 

Yuriman

Diamond Member
Jun 25, 2004
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#7
Can't answer "why", but I highly doubt it's something you need to worry about. Modern CPUs are near indestructible, and generally long out-live both their usefulness, and any other component in the system.
 

augiem

Senior member
Dec 20, 1999
746
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#8
Cant help you on the current issue. But as far as heat goes, i set my vcore to 1.185 and mine it now runs super cool (for coffee lake). Under stress test I max out around 67 degress using an h7.
How are you stress testing? I've gotten very different numbers depending on the software and test used. P95 Blend maxes around 70C while FFT can get to 100C, both 100% CPU utilization.

Are you using a fixed vcore? When using turbo, fixed vcore doesn't work and I have to use a vcore offset instead. Max without instant blue screen when stress testing I can get away with is -0.050V. Board only supports setting up to -0.100V.

Can't answer "why", but I highly doubt it's something you need to worry about. Modern CPUs are near indestructible, and generally long out-live both their usefulness, and any other component in the system.
I know. I just like to understand things. All I can assume is that ICCMax is calculated at the max vcore voltage of 1.52V and that it can handle more at the typical 1.2xV.
 
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dahorns

Senior member
Sep 13, 2013
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#11
How are you stress testing? I've gotten very different numbers depending on the software and test used. P95 Blend maxes around 70C while FFT can get to 100C, both 100% CPU utilization.

Are you using a fixed vcore? When using turbo, fixed vcore doesn't work and I have to use a vcore offset instead. Max without instant blue screen when stress testing I can get away with is -0.050V. Board only supports setting up to -0.100V.



I know. I just like to understand things. All I can assume is that ICCMax is calculated at the max vcore voltage of 1.52V and that it can handle more at the typical 1.2xV.
Fixed vcore, fixed ratios at 45 for each core. I get 1470 avg in cb 15. Stress testing using prime fft, intel burn test, among others. Works great, zero stability issues.
 

The Stilt

Golden Member
Dec 5, 2015
1,709
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#12
VRM will be the limiting factor, however at 125W power limit that won't be a concern. The VRM will overheat (and throttle) instead of suddenly bursting into flames at those current levels.
 

augiem

Senior member
Dec 20, 1999
746
0
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#13
Fixed vcore, fixed ratios at 45 for each core. I get 1470 avg in cb 15. Stress testing using prime fft, intel burn test, among others. Works great, zero stability issues.
Sounds nice. Those temps are really good for that OC in FFT.But I'm assuming you're using a massive tower heatsink to achieve that. My problem with this build is that I'm limited to 45mm max on the heatsink, so I'm using a Noctua NH-L9i, which is just 37mm tall including the fan. The entire heat issue is because I'm building for this case. Plus it's going to be used in a tropical country and they tend to leave the AC off. Anyway, I'm pretty happy with the settings I have dialed in for now.
 

dahorns

Senior member
Sep 13, 2013
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#14
Sounds nice. Those temps are really good for that OC in FFT.But I'm assuming you're using a massive tower heatsink to achieve that. My problem with this build is that I'm limited to 45mm max on the heatsink, so I'm using a Noctua NH-L9i, which is just 37mm tall including the fan. The entire heat issue is because I'm building for this case. Plus it's going to be used in a tropical country and they tend to leave the AC off. Anyway, I'm pretty happy with the settings I have dialed in for now.
Ha. I dont consider it massive since I usr a mATX case, but certainly the Cryorig H7 isnt going to fit in your case. Good luck with your build.
 

bobhumplick

Junior Member
Jun 29, 2018
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#15
no desktop cpu today, either amd or intel, sticks to the tdp all the time. there are certain power limits that it uses pl1 pl2 pl3 and pl4.

intel was better at sticking to tdp til they went to 6 cores and up.

but under turbo speeds or avx it may go over tdp for awhile. if its a 65w chip i would set it as high as it will go. you will be heat throttled long before it becoms a problem. now if its an i7 that might be a different story but for i3 and i5 just let it go man.
 

whm1974

Diamond Member
Jul 24, 2016
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#16
I would simply run at stock speeds and replace the stock HSF with a decent aftermarket one that fits in your case.
 


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