Engineering Documentation

Appendix A – Discontinued Products
Stand-alone Control Unit (SCU)
192
| 231 125-3002
Multiple SCUs on One Power Source
Table Number of SCUs Allowed on a Single Three-Wire Circuit shows the number of
SCUs allowed on a single three-wire (ACH, an ACN, and earth ground) circuit if local
code permits.
Number of SCUs Allowed on a Single Three-Wire Circuit.
Circuit Breaker Size
1
Maximum Va
les for Concentrated
Loads
Maximum Values for Evenly
Spaced Loads
Length
2
SCU Length
2
SCU
15 amp (No.14 AWG THHN) 75 ft (22.87 m) 7/10 100 ft (30.48 m) 7/10
20 amp (No.12 AWG THHN) 115 ft (35.06 m) 7/10 130 ft (40.63 m) 7/10
1)
Assumes minimum voltage of 102 Vac at circuit breaker and 5 Vac maximum voltage drop (97
Vac at loads). See Class 1 power trunk information in the Wire Specification Tables section of
Chapter 1.
2)
Conduit length from MBC/MEC/RBC/SCU to MBC/MEC/RBC/SCU.
Metal Oxide Varistors (MOVs)
For SCUs, MOVs must be used across the DO terminals when connected to loads in
all cabinets. MOVs are factory-installed on all DOs in SCUs. See the Controlling
Transients [➙ 23] section in Chapter 1 for MOV part numbers.
When installing MOVs across the DO relay contacts on termination boards, keeping
MOV leads as short as possible makes the MOV more effective at reducing spikes
from field wiring or controlled devices. Remove and reinstall any MOVs with leads
longer than 1 to 1-1/2 inches (25.4 mm to 38.1 mm).
Line Voltage Receptacle
MOVs are factory installed in the line voltage receptacle of Rev. 8 and later SCU
enclosures.
Fig. 153: SCU Receptacle with MOVs.