knightridar:
The power supply is 24V,21A (max), 500 watts.I am powering the arduino and relay with 24 to 12v dc converters, and 24 to 5 volt dc converters respectively.
The solenoids are 24V DC, and power ranges from 10-12 watts on each one.
That would mean they have a max. amperage of .5 amps.
Eight solenoids at 500mA per (if that's what they actually draw) is 4A. That should be no problem for a 21A supply. Just to know, I'd power up one each of the devices fed by the 5V converter and measure the current they each draw, then you'll have some numbers to calculate total current draw on the converter when all are on. What is the wattage on the two DC-DC converters?
I know things can get messy in the early stages but I'd think about putting some kind of temporary insulator, maybe a piece of paperboard, like from a cereal box, between the relays / ACS712s / terminal strips. It's too easy for parts to get together in ways you don't want them to.
I'd also suggest to you to stage the relay turn-ons. That is, instead of energizing all eight at once and creating a big power supply spike (transient voltage), space out and reduce the power supply spikes by putting a short delay, maybe half a second between each turn-on.