Question regarding an 8 pin CPU power supply extension cable

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Jaga848

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Hello!

I'm about to build my first PC, and I have (through various sources) been informed that the 8 pin CPU cable on my PSU won't be long enough to reach my 8 pin mobo connector through the back of the case. I do not want to route the cable under my GPU, so I'd like to ask - what are the cons of using an extension cable for the 8 pin connector? I've read various post ranging from people saying it's completely fine to people saying that it's very bad and can melt, damaging the other components.

What's your opinion?
 
Solution
RetiredChief
Yes you can do this. You asked for the “Con” so here goes.

There will be two additional losses that will subtract from the +12 V. (A) the I2R loss of the wire and (B) the I2R loss at the connecter (Pin to socket resistance)
For Ex using a 95 W CPU and 8 Pin connector and 12” extension. (8 Pin = 4 x 2 wires in series) equates to approx. 2 Amps per pair

(A) Wire loss. 16 gauge wire R = .004 Ω/ft x 1 ft x 2 wires x 2 Amps = 0.016 V
PS for 22 ga wire the loss would be 0.064 Volts (0.016 Ω per ft) While NOT a safety hazard as 22 ga wire can handle up to 7 Amps HOWEVER the IR loss is 4 x greater than 16 ga.

(B) Connector IR loss. Harder to define as dependent on material, tension and degree of oxidation generally this is much greater...
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boosted1g

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Jun 12, 2013
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With anything electrical.

Extension cords/cables must match the gauge (thickness) of the original cable.
If talking other things then PCs where you are extending it by feet and not inches then you should increase it a size.
So if your PSU has 16 gauge wire for 8 pin then a 16 gauge extension cable is just fine.

The safety issue comes in when you have a 16 gauge wire and you plug in an extension cable with say 22 gauge wiring on it.
Now the wire is to thin to support the current being pulled by the motherboard and thus you get increased resistance and end up heating the copper enough to melt the plastic if not start a fire.
 
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RetiredChief

RetiredChief

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Feb 22, 2007
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Yes you can do this. You asked for the “Con” so here goes.

There will be two additional losses that will subtract from the +12 V. (A) the I2R loss of the wire and (B) the I2R loss at the connecter (Pin to socket resistance)
For Ex using a 95 W CPU and 8 Pin connector and 12” extension. (8 Pin = 4 x 2 wires in series) equates to approx. 2 Amps per pair

(A) Wire loss. 16 gauge wire R = .004 Ω/ft x 1 ft x 2 wires x 2 Amps = 0.016 V
PS for 22 ga wire the loss would be 0.064 Volts (0.016 Ω per ft) While NOT a safety hazard as 22 ga wire can handle up to 7 Amps HOWEVER the IR loss is 4 x greater than 16 ga.

(B) Connector IR loss. Harder to define as dependent on material, tension and degree of oxidation generally this is much greater than the resistance of the wire (12->24 “). This is also the main reason they went with an 8 pin connector vs the 4 pin for CPUs 65 W and under. Wild guess .02 ohms per contact so loss = .02 x 2 x 2 Amps = .08 Volts

(C) Total loss = A + B = approx. 0.1 volt NO Biggy so +12V at load would be =11.9V For 95 watt load, for @ idle the loss would be practically zero.

Side comment: Connector melting is due to connector pin/socket resistance, not wire ga.
When a 4 pin connector is used for a 95 w CPU. 4 amps sqed x .02 ohms = .32 w. Have you ever touched a ½ watt resistor? But if that contact resistance was just 0.0625 ohms than the Power dissipated at the contact would be 1 Watt

PS it did not take superscript so I2R is I squared R
 
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J

Jaga848

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I will get this power supply, as I'm working with a tight budget:

http://www.lc-power.com/en/product/netzteile/standard/lc600h-12-v231/

My case is an MS cyclops V midi tower.

Okay, so basically what I got so far is that using an extension is not recommended, unless necessary. The loss will be only 0.1V, and what causes it to melt is socket resistance. I'm still kind of confused on whether the wire gauge matters. :/
 
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RetiredChief

RetiredChief

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Feb 22, 2007
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I'd prefer the 16 ga because of the lower ohms/ft.
The 22 ga wire WILL work as it can handle a MAX current of 7 Amps, 16 Ga max I = 22 Amps. Looking at a HIGH OC of a 95 watt CPU @ 135 Watts.
135 Watts / 12 = 11.25 amps. IF all connector pins and wires have the same resistance then this equates to a Current of 11.25 Amps / 4 = 2.81 Amps. per wire
For 95 Watt CPU Current would be 1.98 Amps and for 65 W CPU I = 1.35 Amps (These are at MAX loads )

As I stated the reason they went with an 8 pin connector (Vs 4 pin ) was not the wire ga, it was the power dissipation on the Pin/socket. ie a 4 pin CPU power using 16 ga wire can handle up to 44 Amps (Upto a 528 Watts load); 4 pin using 22 ga wire could handle a MAX of 168 Watt Load, The connector (having a higher resistivity) CAN NOT.
Bottom Line go with the 16 ga, but if all you find is 22 ga you would be safe.
PS Gold plated pins would be best as gold Does not oxidize as fast as tin and it is the oxidation over time that one must worry about.

As to PSU - Could not fine a through review; Looks like a middle of the road. Two +12 V rails, each limited to 20 Amps (240 Watts) - 480 watts total. And before you ask; if you only use 15 Amps on +12V1 you can NOT use 25 Amps on +12V2.
 
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J

Jaga848

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Thank you for your thorough explanation, I have a lot more to learn. By the way, the system that I'll be building shouldn't be too power hungry. My specs will be:

Gigabyte B250-HD35 mobo
i5 7400 3.00GHz
Zotac GTX 1050 ti OC edition
8 gigs of Kingston Fury 2400MHz DDR4

I won't be OCing anything (even though the graphics card says so :'D)
 
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