The NMRA has various standards both electrical and mechnical. All of the standard can be found here:
nmra standards and recommended practices
Of these standards, the NMRA has two standards that define what the track voltage is.
1) DC track voltage using NMRA Standard S9
2) DCC track voltage using NMRA Standard S9.1 Section C.
Simple Summary
NMRA S-9 Electrical says:
The minimum DC track voltage is 12VDC got HO and O scale. The maximum DC track voltage is set by the voltage rating of the motor and must not be allowed to overheat.
NMRA S-9.1 DCC electrical Section C says:
The DCC voltage is based on the DC voltage PLUS 2 volts. 12VDC + 2V => 14V DCC For HO and O scale
See below for N and Z scale voltages.
DETAILED DISCUSSION OF NMRA S-9 Electrical.
There is only one section that talks about track voltage. This section says:
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NMRA STANDARDS S-9 Electrical
I. POWER
A. Full throttle voltage available at railhead shall not be less than 12 volts direct current at maximum anticipated load. (1)
NOTES:
(1) When using a power source delivering a wave with greater harmonic content than full wave rectified sine wave, exercise care not to operate in such a manner to exceed the rated current or otherwise overheat the motor.
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Comment
The actual DC voltage values for each scale is not listed in the standard. Only the minimum is shown and it is the 12VDC standard we all know and love that has existed longer than I care to know.
In practice the following happens:
N scale is typically 2V lower or 10VDC. Why? Given the smaller size and mass, between the motor and lights, the whole engine gets hot at 12V.
Z scale is typically 9VDC. Why? Same as above only worse.
O scale is typically 2V higher or 14VDC. Why? The bigger motors draw higher current causing more voltage drop in the wiring such that by the time the voltage reaches the motor, its 12V.
DETAILED DISCUSSION OF NMRA S-9.1 Electrical Standards for DCC.
Section C covers the DCC voltage requirements. It talks about both minimum and maximums for the booster and decoders. The following are sentences are key excerpts of the whole text relating to nominal voltage operation.
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C: Power Transmission and Voltage Limits For Transmitting Power Through the Rails
The baseline method for providing the power to operate locomotives and accessories, which shall be supported by all Digital Command Stations and Digital Decoders, is by full-wave rectification of the bipolar NMRA digital signal within the Digital Decoder.
The RMS value of NMRA digital signal, measured at the track, shall not exceed by more than 2 volts(8) the voltage specified in standard S-9 for the applicable scale.
(8) The additional voltage is to compensate for voltage drop in the Digital Decoder, to ensure that the maximum voltage as specified in the NMRA Electrical Standard (S-9) is available at the motor brushes.
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Comment
This section picks up where the S-9 DC standard left off. All this section says is the DCC track voltage is to be no more than 2V higher than the DC track voltage to compensate for the voltage drop in the decoder for the given scale.
The calculated typical DCC voltages are actually shown in Section C's voltage graphs plot lines showing the typical voltage for each scale. You can read the typical DCC track voltage from the graph. It is the same in both the booster and decoder graphs.
So for the scales shown:
Large Scale = 16VDC + 2V = 18V DCC.
HO & O = 12VDC + 2V => 14V DCC.
N scale = 10VDC + 2V => 12V DCC.
NOTE: NCE's 10Amp booster is designed for O scale. The booster is set at the factory for 16V DCC at the track level. It is higher that 14V given the extremely high current output from the booster to support multiple O scale engines that can have old current hogging motors. 10Amp can drop a lot of voltage in wiring.