Here's a copy pasta of a comment I made in Reddit when someone asked is there data to support my position:
Yes, the 8 pin connectors are rated for 150W.
6 pin connectors are rated for 75W.
PCI-E delivers 75W through the slot.
That is easy to find and reference info, it's even on wikipedia. Some of the info is fundamental electronics and electrical engineering. I am not an electrical engineer myself, but my dad is and I grew up doing this stuff. You can ask an electrical engineer to fact check me and if I am wrong I would truly like to be respectfully corrected.
https://www.gpumag.com/gpu-power-connectors-explained/ https://en.wikipedia.org/wiki/PCI_Express#Power It takes a bit of digging but you can find the engineering specs if you really want to go that deep. I found a bunch for PCI-E(PCI-E publishes them all,) but still trying to find some PSU engineering spec sheets currently.
Also Buildzoid has an excellent video that talks about this where he specifically says daisy chains should be fine unless we get into 400W territory. That video is 3 years old and was made before we even had 2000 series.
https://www.youtube.com/watch?v=9nM80JmzKvc If you have a Daisy chain with connectors rated at 150W and the cable can handle more you have an issue at the PSU side. The PSU can only safely draw 150W through it's single connection and somehow deliver more than 150W to two 8 pin power connectors at the GPU. Electricity does not magically increase amps and volts to make more watts. Actually because of resistance and voltage drop you get less than you asked for in the form of less voltage and the same(or more) current(amps.) This creates heat at the connectors, unused or inefficient electricity turns into heat.
Also lets remember you're NOT getting 100% efficiency(that's almost impossible) and you're not going to get the full 150W you're asking for. And that he cables should be 16 gauge for pigtails, but is typically 18 gauge which isn't as good.(Lower numbers are thinker wiring.) Also some could be using Aluminum wiring instead of copper which creates more heat. There is apparently enough variability in PSU specs and design that a daisy chain could work fine on PSU and be magic smoke on another. Is that alone a risk worth taking?
If we do the math each PSU end power connection is 150W send and each GPU power connection is 150W request. So if we have a daisy chain on an EVGA FTW3 Ultra we have on the PSU the ability to send 150W x 2 + 75W from PCIE slot(375W total, 25W short of the 400W power draw at max load.) The GPU will be asking for potentially up to 3 X 150W +75W PCIE slot which is 525W total potential power requested. This is obviously not good if the PSU is trying to deliver more power than the connectors and cables are capable of handling. You'll only get that demand during peaks, but it only takes one 500W plus peak to make magic smoke.
When you ask for more power that means more current(amps) and voltage. Wattage is literally volts multiplied by amps (W==v*a). Voltage can be modeled as the "amount" of electricity and current modeled as the "pressure of flow" in a sense. Current is more dangerous as a lot of electricity without "flow pressure" moving it doesn't do much except maybe look cool and tickle a little. You give a little voltage a 1 amp of current and that can kill people in contrast. When your GPU asks for power it's that extra current it's not rated to handle that will create the magic smoke.
If we ask for more than the cable and/or connection can handle we have risk of "magic smoke." The PSU and GPU assume the cables can handle what they want to do. If the cable isn't good enough we get magic smoke at worst case. With a Daisy Chain we are asking for potentially 300W through a cable not at the correct gauge to handle that through a single 150W rated connection at the PSU. Also how does the PSU and GPU know to not push too much power through the Daisy chain cable? It doesn't know it's a daisy chain, it sees three 8 pin power connections and that it's getting power from them. How could it know not to push more than 150W through the daisy chain? (It doesn't, it could easily overload the daisy chain on the assumption that it's a properly rated cable.)
It makes 0 logical or rational sense to use a daisy chain for 2000 or 3000 series. 3000 series is definitely drawing too much power for it to be smart to use a daisy chain. 2000 series seems to be borderlining it depending on specific GPU and PSU.
1000 series had low enough power draw it was permissible. But even then I would not recommend it to be on the safer side. Jayz 2 Cents found his OC and performance was slightly limited on a 1080 something using a daisy chain. His results have not been verified by others repeating the same test, so scientifically it's not exactly solid data(yet.)
Now add in that it kinda depends on your PSU and how well it followed specs like 16 gauge wiring for a daisy chain cable and copper wiring it seems like it's just not worth the risk to use a daisy chain at all, ever.
I've come to question their existence. And now think they should not have ever been made. Daisy chains seem like a recipe for user error to destroy components. When it comes to making and selling a product an uniformed consumer could easily destroy it's prudent to minimize giving them ways to enact disastrous failure. Especially when there's a fire hazard potential.
The rule of thumb should be this: One cable per 8 pin GPU connection. If they're 2 x 6 pin that's actually fine as 6 pin is only 75W and a daisy chain on 2 x 6 pin is 150W(2*75W.) But for newer builders than can be a point of confusion so we should really stick to a "One cable per 8 pin connector" mantra when it comes to GPU power.
Thanks for coming to my Flange Talk. ;)