About power ratings – Rockwell Automation DeviceNet Media Design Installation Guide User Manual
Page 26
Publication DNET-UM072C-EN-P - July 2004
1-16 Get Started
Use a power supply that has current limit protection as described in national
codes such as NEC, Article 725.
If you use a single power supply, add the current requirements of all devices
drawing power from the network. This is the minimum name-plate current rating
that the power supply should have. We recommend that you use the
Allen-Bradley 24V dc power supply (catalog number 1787-DNPS) to comply
with the Open DeviceNet Vendor Association (ODVA) power supply
specifications and NEC/CECode Class 2 characteristics (if applicable).
About power ratings
Although the round thick cable and Class 1 flat cable are both rated to 8A, the
cable system can support a total load of more than 8A. For example, a 16A
power supply located somewhere in the middle of the cable system can supply
8A to both sides of the PowerTap™. It can handle very large loads as long as
no more than 8A is drawn through any single segment of the trunk line. However,
cable resistance may limit your application to less than 8A.
Drop lines, thick or thin, are rated to a maximum of 3A, depending on length.
The maximum current decreases as the drop line length increases.
You may also determine the maximum current in amps (I) by using:
I = 15/L, where L is the drop line length in feet
I = 4.57/L, where L is the drop line length in meters
IMPORTANT
The dc output of all supplies must be isolated from the ac
side of the power supply and the power supply case.
Drop line length
Allowable current
1.5m (5 ft)
3A
2m (6.6 ft)
2A
3m (10 ft)
1.5A
4.5m (15 ft)
1A
6m (20 ft)
0.75A
To determine the required power supply
current:
1. Add the current requirements of all
devices drawing power from the
network.
For example:
6.3A
2. Add an additional 10% to this total
to allow for current surge.
e.g. 6.3A x 10% = 6.93A
3. Make sure the total of 2 is less
than the minimum name-plate
current of the power supply you are
using.
e.g. 6.3A < 8A and NEC/CECode