allowing for expansion

Its seems that every new machine we recieve has only a couple of spare inputs or outputs available. As we all know once the machine is in place management wants add a few things to "make it better"?
Is there a general rule of thumb that we should request a specific number of spare input/outputs? maybe a percentage?

Thanks
Bob

Typical specs say 10 to 20 percent.

This can easily be misunderstood...for instance

is that slots, i o modules or memory?
does a slot count as 16 spares?
should all the spares be wired to terminal strips?
should extra power capacity be added

if you, as the specifier, do not carefully think out the spare support you might need, you could either end up paying for lots of stuff you will never use, or , (if the market is tight and your projects are decided on low bidder) you could end up with empty slots, no spare DIN rail, and no spare power to drive any of it.

anyway, 10 to 20 percent is the simple answer.

I see this all the time, and it is usually a waste of money. If this is for a SCADA system where there may be unknown expansion requirements, you can justify spare slots and spare terminals, but that is about it.

Nowadays it is generally so simple to add remote I/O or network to another PLC that it makes more sense to simply specify a communications network on the PLC so the new items can be added later. When expansion time comes just add another panel close to the new I/O points and drop a communications line.

For I/O, I usually see %20 or 2 slots. I often see %20 free panel space. I try to plan it so you can add another starter or two, a rack, and some fuses.

I've had a few who will waive the requirement to get the panel down from 3 door down to two.

I just built a small machine with a micrologix 1200 and used every input on the base unit. I just left room to the right on the DIN rail to add I/O modules, but I'm not going to buy one unless I need it.

Working for machine builders, I always have 20% spares INs and OUTs BEFORE we start building the system. I end-up most of the times using 50% of those extras and after that I'm left with maybee around 10% free I/Os. Thats what the End-User gets.

For your own sake: provide a few extra i/o ready wired to terminals. If you have not used up all the i/o in the available modules, simply wire the remaining unused i/o to terminals.

For the customers sake: Provide so much unused slot space that a few extra modules can be installed.

If any larger expansion is required, it is probably the best to plan for putting the expansion in a new panel. This panel can be made by a panelbuilder a lot faster and cheaper than if you had to squeeze the expansion into an existing panel. Commisioning this expansion panel will also be faster and cheaper.

About program memory: If the available memory goes below 20%, you will start finding yourself limiting the things you would like to do, and you will start wasting time and money trying to shoehorn the program into the available space. It is probably cheaper to go for the next size in PLC memory (OK, this does not apply for PLC5, its just TOO expensive)