CompactLogix Platform - Calculating Power Consumption...
It is very confusing, isn't it?
Hopefully this will clear some of it up for you...
Note: I am referring here specifically to the older but still supported 1769 Modular Controllers and in particular I am going to focus on the popular 1769-L32E. The reason I am doing so is twofold...
1. The attached screenshot in Post#1 is from...
Publication 1769-UM011-EN-P -
1769 CompactLogix Controllers User Manual (Page 83)
...which refers specifically to the older 1769 L3x Modular Controllers.
2. You have also specifically mentioned as an example a Controller with a current rating of "0.66A at 5V DC". A rating of 660 mA @ 5V DC matches only the 1769-L32E or 1769-L35E Controllers.
So moving on...
The 1769-L32E Controller has a current draw rating of 660 mA @ 5V DC and 90 mA @ 24V DC. We know that it has to able to draw that power from somewhere. That somewhere would obviously have to be the Power Supply. So, and regardless of what one might be reading or misreading, one should know that the Controller is and must be included in the total power consumption calculation.
So, all components that may draw current from the Power Supply should be included in the calculation. That means, both the Controller for Bank 1 plus any Modules which may also be added to this same Bank must be included in the power consumption calculation for the Power Supply in Bank 1. Any Modules which may be added to subsequent Banks, if they exist (Bank 2 or 3) must be included in the calculations for the Power Supply in those Banks.
The general rule is that no matter which Power Supply is used we are only allowed a maximum of 2.0 A @ 5V DC on either side of the Power Supply and 1.0 A @ 24V DC. Let's say we are using a 1769-PA2 which provides 2.0 A (2000 mA) @ 5V DC and 0.8 A (800 mA) @ 24V DC.
The Controller must reside in Slot 0 so it is always on the left side of the Power Supply from the perspective of looking in from the front. The 1769-L32E will automatically consume 660 mA @ 5V DC and 90 mA @ 24V DC from the left side of the Power Supply. This leaves us with a remaining 1340 mA @ 5V DC and 710 mA @ 24V DC. This is before any other Modules have been added.
As the Controller must reside in Slot 0, and it has a distance rating of no more than 4 Slots from the Power Supply; you can only add a maximum of 3 Modules between the Controller and the Power Supply on the left side. So a maximum of 3 Modules may consume some or all of remaining 1340 mA @ 5V DC. Let's say we add 2 x 1769-IQ32T @ 170 mA each @ 5V DC and 1 x 1769-OB32T @ 220 mA @ 5V DC. This has now consumed a further 560 mA @ 5V DC, leaving a remaining 780 mA @ 5V DC at the Power Supply. The total current consumption on the left side is now 1220 mA which is well under the permissible 2000 mA (2.0 A). As no more Modules can be added to the left side of the Power Supply the only way that the remaining 780 mA @ 5V DC may be consumed is by adding Modules to the right side of the Power Supply. If the remaining current is adequate enough a maximum of 8 Modules may be added to the right side of the Power Supply.
Note: These Modules do not draw any current @ 24V DC. It is only the 5V DC that we are concerned with here.
Regardless of documentation, the above is a given. So now let's look at how you are supposed to decipher the confusing information shown on Page 83...
On Page 83, or in Post#1, we can see from the page title that the table is supposed to guide the user on how to "Calculate
System Power Consumption". "System" should infer that all components that may draw current from the Power Supply be included here. But they are in fact not. The table is actually
only referring to Controller current consumption. This is the first piece of misleading information.
Next, the title for the table itself says...
Table 19 -
I/O Module Power Consumption Calculation Table
This is the second piece of misleading information. The table is not referring, at all, to Discrete I/O, Analog I/O or Speciality Modules. Again, it is only referring to Controller current consumption.
Within the columns of the table we see references to "Number of Modules" and "Module Current Requirements". Again, these are misleading or confusing descriptions. The only "Module" they are referring to here is the Controller itself. If we use their somewhat confusing terminology of "Module" for a Controller and then fill in the table we will see that for the "Number of Modules" they are actually referring to the "Number of Controllers" that will be used in the System. This, of course, can only be 1 Controller in Slot 0 for the 1769 Modular Systems. So the table is completed by filling in...
Number of Modules: 1 (beside 1769-L32E)
Calculated Current:
at 5V DC: 660 mA
at 24V DC: 90 mA
Total at 5V DC: 660 mA
Total at 24V DC: 90 mA
So now, having completed this very confusing but somewhat pointless table, we have apparently calculated the "
System Power Consumption", even though we haven't even mentioned an actual Module yet.
The current ratings shown in that table are just the Controller specific current requirements of which you can find in...
Publication 1769-TD005K-EN-P -
CompactLogix Controllers Specifications (Page 54)
They have nothing to do with the current consumption of I/O Modules. You do not add the number of Discrete I/O, Analog I/O or Speciality Modules to this table using the ratings shown. The modules have their own specific ratings which you can look up in...
Publication 1769-TD006_-EN-P -
1769 Compact I/O Modules Specifications
The current draw ratings in that document, for the intended Modules, should be used along with the Controller rating to calculate the total "System" power consumption.
What they have failed to provide here is a clear opening description of what is to be included in the power consumption calculation and then the means to add the actual Modules after the Controller in the table so that one can calculate the actual current draw for the "System".
More to come...