You've hit on a very common and practical problem in electrical work! Here's the breakdown of how to handle this situation, along with code considerations

Understanding the Code Requirement
NEC 250.122

This section dictates the size of the equipment grounding conductor (EGC) based on the overcurrent protective device (OCPD) protecting the circuit. In your case, the 100A OCPD forces you to run a #8 AWG copper EGC.

The Problem: Terminating to Smaller Devices

The challenge is that the #8 AWG EGC is too large to directly terminate to the grounding terminals of devices protected by the 30A and 60A OCPDs.

The Solution: Splicing/Pigtailing is Allowed and Common NEC Permits Splicing of EGCs

The NEC does permit the splicing or tapping of EGCs, unlike the restrictions on tapping ungrounded conductors in many scenarios. There's no requirement for upstream OCPD on the EGC splice itself.

Proper Splice Methods are Key

The code mandates that any splices must be made with listed connectors or by listed splicing means. Typical wire nuts, crimp connectors, or split bolts are all acceptable, as long as they are appropriately sized and listed for the conductors being joined.

How to Execute the Solution

Here's how you'd handle the termination in a junction box:1. Run the #8 AWG EGC through the junction box/enclosure. This is your main grounding conductor.2. For each of the smaller circuits (30A and 60A)



Determine the Required EGC Size

Using NEC 250.122, find the minimum EGC size for a 30A circuit and a 60A circuit. Let's say a 30A circuit needs a #12 AWG EGC and a 60A circuit needs a #10 AWG EGC.

Pigtail/Splice



For the 30A circuit

Use a wire nut or crimp connector listed for connecting a #8 AWG to a #12 AWG. Connect the #12 AWG to the #8 AWG. This creates a "pigtail."

For the 60A circuit

Use a wire nut or crimp connector listed for connecting a #8 AWG to a #10 AWG. Connect the #10 AWG to the #8 AWG.

Terminate

Connect the pigtail (#12 or #10) to the grounding terminal of the receptacle, light fixture, or other equipment on that specific circuit.3.

Terminate the #8 AWG EGC

The #8 EGC must also be terminated to a grounding point in the panel and in the junction box/enclosure (if metal) using appropriate connectors.

Important Considerations Listed Connectors

Always use connectors that are listed (e.g., UL listed) for the types and sizes of conductors you are joining. This ensures a reliable and safe connection.

Tight Connections

Make sure all connections are tight and secure. Loose connections can lead to arcing and heat, which are fire hazards.

Enclosure Grounding

If the junction box or enclosure is metallic, it must also be bonded to the EGC system. The #8 AWG EGC is the perfect way to do this.

Continuous Run

Ideally, the #8 AWG EGC should run continuously from the source (panel) to the furthest point of the circuit. This provides the most reliable grounding path. If you must splice the #8 AWG, ensure the splice is accessible and made using listed methods.

Non-Metallic Raceway

Since you are using a non-metallic raceway, the EGC is crucial for providing a low-impedance fault current path back to the source.

Accessibility

Although unlikely to require maintence, if there are multiple splices in a j-box it is best to leave enough length so they can be undone in the future if needed.

In Summary

Splicing/pigtailing EGCs to accommodate different termination sizes is not only permitted by the NEC but is also a common and practical practice in electrical work. As long as you use listed connectors, make tight connections, and adhere to the code requirements, this method provides a safe and effective grounding system. Flag for review