It can be difficult if you get conflicting advice from good sources.
I write before that values for VTT and VCCSA must to be equal !!!!
... says Kope
My advice is never over 1.4v vcore and 1.2v VCCSA + VTT, then you'll be alright
What's your VCCSA voltage at? over 1.2v for 24/7 can be very harmful on the C2 version of the 3930k. C1s can take a little more abuse, up to 1.4v 24/7.
... says MrTooShort
VTT CPU Voltage & 2 VTTCPU Voltage: To let users over-volt part of the CPU VTT that affects OC without over-volting the part of VTT that does not affect OC, we split up VTT rails to VTT and Secondary VTT.
VTT over-volting helps with DRAM and BCLK Overclocking. Secondary VTT does not. However, try to maintain at most a 300mv delta between both voltages for stability. 1.40v is fine on these CPUs. 1.35v is sufficient most of the time.
CPU VCCSA Voltage: Two options here. The first is “Manual Mode”, which allows us to set a ‘static’ level of voltage for the System Agent. The second mode is “Offset Mode”, which works on the same principals as Vcore Offset Mode. VCCSA has a direct impact on memory stability, so will need increasing if you are running high density modules and/or overclocking the memory controller. As a general rule VCCSA should not be set higher than CPU Vcore as this will impact system stability. The reason for this is likely due to processor Vcore being used as either a pull-up rail for IO signaling stages, or internal DC coupling.
If overclocking past DDR3-2000, then finding the optimal delta between CPU vcore and VCCSA becomes critical with some memory modules. Hence, we recommend making small changes and monitoring for impact upon stability. It is probably wise not to exceed a voltage delta larger than 0.6V between VDIMM and VCCSA, so if VDIMM is set to 1.65V, then set VCCSA at around 1.05V minimum as a starting point AND also try to keep VCCSA within 0.3V of Vcore if possible.
.......says ASUS Rampage IV Extreme - UEFI Guide for Overclocking
Difficulty for me was that my overclock would crash if VTT and VCCSA were set equally at 1.2v
I've ended up with a graphics benching compatible cpu clock of 4848MHz, but using a VCCSA setting of 1.22v (which UEFI reading says is 1.24v) and VTT setting of 1.3625v (UEFI reading says 1.362v.) That's with 16GB of 2133 Ripjaws X.
Don't know how that would go long term, but I've only used it for 3DMark11 benching sessions.