Background I am not a qualified electrician nor do I work in the electrical profession - I'm a software engineer. My dad is however an electrical engineer and when I were younger I used to help him build electrical control panels, some of which controlled 30kW three-phase motors. This included both reviewing the drawings and wiring the actual panels. (Except for the high current cables, I simply wasn't strong enough to loop a 30mm^2 wire through a current transformer!) Needless to say my advice here is offered "as is" and without warranty or liability. I've given you information about my qualifications and experience so that you can make an informed decision on whether or not to follow my advice. General Wiring High current connections should be crimped and not soldered, especially when there is vibration or movement in the wires. Pliers are not crimpers and the flat tools with a simple crushing end are not crimpers. The only type of crimpers you should be using for the red, yellow and blue connectors are ratchet crimpers. After crimping a connector on a wire, give it a good pull - If it can't take a decent tug, you should re-do it. Red crimps are for 0.5mm^2 to 1.5mm^2 wire, blue crimps are for 1.5mm^2 to 2.5mm^2 wire and yellow crimps are for 4mm^2 to 6mm^2 wire. Always use the correct coloured crimp for your wire size. Bootlace ferrule crimps should be used for screw terminals rather than bare or tinned wire - Again use the correct ratchet crimper and not pilers. These are also useful where you need to connect two wires into the power supply, where two ring crimps may not fit. Strain relief should be provided at both ends of a moving wire - The connections should never be subject to movement. I personally like to use silicone wire with heatshrink around the areas where I'll be fixing it. I then use cable ties to attach these areas to the frame. Don't run high voltage wires along side low voltage wires - Ideally the heatbed should have separate drag chains for the power wires and the thermistor wires. Don't run high current wires along side low current wires - Ideally the thermistor wires and heater wires would be separated for the hotends, although another option is to use shielded cable for the thermistor. This is even more important when using a thermocouple, where the voltages are minute. SSR The Omron G3NA-210B-UTU SSR is to be derated by 30% when not mounted on a heatsink which gives a good margin when used in a 240V region but is right on the limit when used in a 110V region. Additionally, this rating is given when the SSR is mounted on a thermally conductive surface like the side of a control panel. As such, mounting the SSR directly onto the plastic panel may be a fire hazard in 110V regions: "If a material with high thermal resistance, such as wood, is used, heat generated by the G3NA may occasionally cause fire or burning. When installing the G3NA directly into a control panel so that the panel can be used as a heat sink, use a panel material with low thermal resistance, such as aluminum or steel." The heatsink tab on the back of the SSR is not earthed and metal screws are used - The screws should therefore be earthed. Earthing Your earth wiring should be continuous where possible, an example of this is where the guide suggests that the earth feed from the IEC connector is taken into the earth terminal of the power supply, but the earth to the rest of the machine is then taken from the mounting hole on the side of the power supply - These should both connect into the earth terminal. Another example is the heatbed earth where the guide shows them both connecting to different earth screws - They should use the same earthing point and not rely on the conductivity of the heatbed bracket. If you have many earths to connect up, it is often better to have a single earth stud and use a series of ring crimps to connect everything to this point - A good place to put an earth stud would be on the aluminium frame. Spring washers should be used on the earth screws - Firstly to prevent them from coming loose and secondly to ensure that there is good connectivity with the underlying metal. This is especially important when earthing the aluminium frame and the coated heatbed. The coatings should be scratched off even when using a spring washer. Always confirm earth continuity with a multimeter for any exposed metal service, including bolts. Unrelated To Electricals Although this is common practice in the 3D printing world, the Z-axis lead screw looks to be axially supported by only the motor's radial bearings - As such these bearings must take the entire weight of the heatbed, the part being printed and any forces due to acceleration / deceleration. This is not good practice and also results in an undefined amount of z-axis slop. Lead screws and ball screws are meant to be both axially and radially supported at the driven end and radially supported at the floating end. (Using preloaded back-to-back angular contact bearings on the driven end.) Just because it's common practice (Probably due to designers often reusing sections from other machines) doesn't mean it's a good idea.