48 • Transformers 026-1701 Rev 0 01-05-98
Matching Transformers to Boards
Power Requirements
Each board used has specific power requirements.
These requirements determine how many boards may be
wired to each transformer. Power requirements for each
board on the RMCC network are listed in Table 13.
Transformers for powering the input and output boards
should be wired according to Figure 36 and Figure 37 de-
pending on the number and type of boards being powered.
To select a power transformer for a board or a series of
boards:
1. Determine what the total VA is for the boards
that will be powered by the transformer.
EX: Two 8IOs (18.0 VA each), and one 4AO
(10.0 VA) boards are to be powered by one
transformer
2. Use a transformer that has a power rating
higher than the total calculated VA (See
“Specifications” on page 47.).
EX: Three board transformer (56 VA) is suf-
ficient
56 VA is greater than 46 VA
Fusing Specifications
Follow NEC guidelines regarding fusing of the trans-
formers. The following overcurrent protection devices may
be used to fuse the primary circuit:
Transformer Wiring Diagrams
The following diagrams show how to wire transformers
for various situations. Be sure that the diagram matches the
situation before using it to wire a transformer.
16AI 8RO 4AO 8DO ARTC/8IO 485 Alarm
Amps
0.25 0.75 0.5 1.5 0.75 0.1 / 0.05
VA
5.0 15.0 10.0 18 18 12
V AC
24 24 24 24 24 120 / 208
Center Tap
Used
YES YES YES YES NO N/A
Table 13 - Power Requirements
2( 18VA) 1( 10VA)×+× 46VA=
Do not fuse transformer circuits using 32-volt auto-
motive fuses.
Type 115 V AC
Power
230 V AC
Power
3 Board
1 A, 250 V, Slow
Blow
Bussmann AGC-1, or
Littelfuse 312001
0.5 A,250 V, Slow
Blow
Bussmann AGC-1/2,
or Littelfuse 312.500
6 Board
2 A, 250 V, Slow
Blow
Bussmann AGC-2, or
Littelfuse 312002
1 A, 250 V, Slow
Blow
Bussmann AGC-1, or
Littelfuse 312001
10 Board
4 A, 250 V, Slow
Blow
Bussmann AGC-4, or
Littelfuse 312004
2 A, 250 V, Slow
Blow
Bussmann AGC-2, or
Littelfuse 312002
Table 14 - Transformer Fuse Types
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