Re: Adding a Reverse Loop to the Bus

Steve Haas

Ric Z inquires:

<<In Alan's web site the possible length of a bus element is 27-30' from the
booster. Does this length also include the reverse loop wiring? For
instance if my bus length to the reverse loop is 20' and the reverse loop is
12', have I exceeded the 27' limit? If so I would have to move the booster
closer to the loop or add another curcuit breaker.>>

Alan has done a fantastic job of putting a lot of good
information together in a common location. I agree with 99.9 percent of
what his pages contain.

However, I disagree with him on several topics, one of which
is the maximum length of bus runs.

For example, the NCE Power Pro System Manual has the
following recommendations:

Runs to twenty-five feet: #16
Runs to fifty feet: #14
Runs over fifty feet: #12

Knowing that longer runs will inevitably create incremental
problems, most DCC manufacturers have understated the length of wire run
requiring a specific size wire. There is no need for the rest of us to
hobble ourselves further by placing additional constraints on the length of
a bus of a given size.

One local layout where I support the DCC system has #12 AWG
buses running almost eighty feet. All boosters and DCC circuit breakers are
clustered in one corner of a 28 x 42 foot room (owner's choice). From that
corner, to the opposite corner of the building are two bus runs; one serves
the port area, the other serves a reversing block in the same area. These
blocks are fed with 12 AWG wires approximately 80 feet long with no issues.

We haven't consciously twisted the wire pairs for each
district, but all the track bus wiring headed in one direction from the
command and booster complex are bundled together, with each pair split off
from the bundle as it reaches it's booster sub-district. We have been
adding choke/filters/snubbers at the end of the long track district runs
before they are split into the track feeders for individual blocks.

Bottom line - Alan's recommendations are a bit conservative
and can be pushed. However, the more you push them, the more you should be
prepared to implement the techniques used to avoid some of the problems of
long bus runs, namely twisting the pairs (three turns per foot), and adding
a snubber at the end of the bus.

<<Also, should a long bus always be terminated?>>

Always is a pretty strong term. Many folks exceed the
normal paramters and never have any problems with the operation of their DCC
systems. On the other hand, terminators are cheap and easy to implement,
and while their presence may or may not help the quality of the signal on
any given layout, they'll never harm a system or its performance by their

If in doubt, add the terminators. The cost of the
components is negligible, they're easy to put together and install, and
they'll never hurt.

Bet regards,

Steve Haas
Snoqualmie, WA

Also, should a long bus always be terminsted?


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