I'm looking to build a PCB with two distinct planes, one for AC (120V) and another for DC (5V) . Where can I find a standard for the clearance required between the two planes?

 

Firstly, it's not a question of whether it is AC or DC, it is a question of whether it is connected to mains live (which also includes mains neutral), or whether it is connected to earth and can be touched by the user.
Where you can find the standard, depends on which country you wish to sell or use the product you are designing.
There will be a minimum standard for clearance (shortest distance between two points), creepage (distance over a surface) and also a standard for the minimum voltage the insulation between them has to withstand.

 

"Best-Practices" dictate NO "Mains-Connected" traces on the Board.
NO Mains-Connected-Transformers on the Board.
.
.
.

 

Where can I find a standard for the clearance required between the two planes?

There are different requirements for different things. Medical is more restrictive.
Look at the opto-isolators built for Primary to Secondary isolation. They are built for that job.
Look at any power supply. You will see a 5 to 7mm gap. Some time they slot the PCB to increase the isolation.
link

 

Google "electrical creepage and clearance distances"

 

"Best-Practices" dictate NO "Mains-Connected" traces on the Board.
NO Mains-Connected-Transformers on the Board.
.
.
.

That means that no modern appliance which includes a switched-mode power supply is made using "best practice"

 

There will be a minimum standard for clearance (shortest distance between two points), creepage (distance over a surface) and also a standard for the minimum voltage the insulation between them has to withstand.

There's the key! Distance of separation varies with voltages. As for a "Standard", I don't know of any off hand. It's been thousands of years since I dealt with IPC 6010, 6011 and/or 6012. You MIGHT find your answer therein. But you might also have to pay to use those standards, and I haven't seen mine since my cousin borrowed it then his wife took it in the divorce.

 

There's the key! Distance of separation varies with voltages. As for a "Standard", I don't know of any off hand. It's been thousands of years since I dealt with IPC 6010, 6011 and/or 6012. You MIGHT find your answer therein. But you might also have to pay to use those standards, and I haven't seen mine since my cousin borrowed it then his wife took it in the divorce.

I hope his wife is finding them useful. I have copies of the British and European standards, but the mention of 120V AC by the TS suggests that they might not be so useful.
However, the creepage and clearance distances tend to be the same the world over, and the BS and EN standards do have values for 120V.

 

Give this a look:
https://resources.altium.com/p/high-voltage-pcb-design-creepage-and-clearance-distance

 

There should be no issue combining the AC line with low voltage circuitry. Simple spacing may be adequate, but I like when I see cutouts milled into the board to isolate these domains.

 

PCB trace and space does have standards. The PCB might deviate from the standard default for specific needs.
Enhanced design can be used for reducing crosstalk usually not described as AC/DC.
The supply might have a current level where spacing and copper mass might be inadequate, prone to hazards.

Some cases too narrow. Using a precise low ohm measurement is sometimes needed when loss affects performance,
After that parasitic noise at the pins and connectors can be reduced by capacitors, layer and copper trace layout.
Guard rings and air wiring are sometimes needed on signal traces for isolation. Other traces may not be critical.
Testing a board under load with small magnetic field oscilloscope probe, but not just voltage probe.
There can be times when the board must be scrapped, so milling a trace wider might indicate that spacing was significant.
Limiting the supply current to a sub circuit if high current is the problem. I tend to use a separate board for filter and regulator.
The regulator is usually where the supply noise is significantly reduced having test points and good layout makes it easier.
I use larger electrolytic capacitors 68000 uF for applications like PS for instrumentation that need lower noise.
Some of these ideas are not probable in production. Staying with standard spacing can improve quality.