Emerson MultiFlex RTU, Installation And Operation Manual

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6 • MultiFlex RTU Operator’s Guide 026-1706 Rev 3
Rule 3: The secondary of the center-tapped
transformer must not be grounded on any
side.
Verify that neither side of the transformer
secondary is connected to earth ground before
powering the old-style MultiFlex board. A
grounded secondary will damage the MultiFlex
board.
In addition, the old-style MultiFlex RTU boards
can be powered by one of the 50VA or 75VA
non-center-tapped transformers listed in Table 2-
1 on page 3 . Figure 2-6 shows how to wire the
transformers to the MultiFlex boards.
2.3.3. Wire Types and Maximum Dis -
tances
For powering I/O boards, use only the listed wire
types from Table 2-3 . Three-conductor non-
shielded cables are the recommended wire for
connecting between the center-tapped
transformer and the I/O boards. Shielded cable
should not be used for power wiring. The center
tap should be wired with the third conductor to
earth ground at the transformer.
The wire length from the transformer and the
number of boards connected to the same wire
determines the type wire gauge used. In most
cases, the distance between the I/O boards and
the transformer that supplies power to them is
not enough to be concerned with. But it is very
important not exceed this maximum wire length
or the boards will malfunction.
Use these formulas to determine if the wire
gauge you are using fits within specification:
Sensors requiring 24VAC can be powered from
the same transformer powering the input board
as long as the resulting total load of both the
input board(s) and the sensor(s) connected to the
transformer does not exceed the transformer’s
VA rating.
Power Wiring Types
14 AWG Belden 9495
18 AWG Belden 9493
Table 2-3 - Power Wiring Types
14 AWG:
Feet = 0.25/(VA/24) x 0.00252
18 AWG:
Feet = 0.25/(VA/24) x 0.0064
(VA is the total VA rating of the I/O boards)
For example, if you had an 80 VA load:
14 AWG: 29 ft. (rounded down)
18 AWG: 11 ft.
Table 2-4 - Power Wire Lengths