Is there a general rule of thumb for spec’ing as-I power supply
requirements?

I have a single run as-I network (using a dp/as-I interface) with
around 15-20 slave nodes (most 4in/4out).

I am finding it a devil of a job to get load data for the nodes

Hi hrpr99, its a matter of checking each manufacturers data sheet, our sensor block nodes are typicaly 50mA however with 4 sensors expect upto 280mA on the Yellow AS-i cable. Our outputs are switched powered via the Black Power connection, this load is supplied from a normal 24VDC supply. You must consult the data sheet of each device in question.  Other members may be able to offer a general guidline, Sorry I am not in a position to do so.

When the first 2DI/2DO slaves were introduced the theoretical maximum load per slave was limited to 200mA. Therefore the max 31 slaves of an early network could add up to 6.2A if all inputs and outputs operated simulataneuosly at maximum permitted load. This was unlikely and 4A yellow cable power supplies were typical. Not all sensors require max load and not all outputs use maximum design curent. For example a Kuhnke Airbox uses hit and drop technology to hold an output valve open at 15mA even though the available rating is 45mA. If black cable power is used ( aux power 24VDC)then insignificant yellow cable load is placed on the outputs.

Since then the number of products introduced with ASI connectivity and the yellow cable/black cable combinations means that it is difficult to generalise, but......

Rough guide for you as you describe might be 4A because you can easily buy a standard product with this rating, but Derek is right and you should really check out each slave and do the maths. Remember each will be rated for max loads that may not apply to the sensors or output loads you are using on your application