Juniper MX240 Hardware Manual page 197

DC Power Circuit Breaker Specifications
Each DC power supply has a single DC input (–48 VDC and return) that requires a
dedicated circuit breaker. We recommend that you use a dedicated facility circuit
breaker rated for 40 A (–48 VDC) minimum, or as permitted by local code. Doing so
enables you to operate the router in any configuration without upgrading the power
infrastructure.
If you plan to operate a DC-powered router at less than the maximum configuration
and do not provision a 40 A (–48 VDC) circuit breaker, we recommend that you
provision a dedicated facility circuit breaker for each DC power supply rated for at
least 125% of the continuous current that the system draws at –48 VDC.
1.38 A + 0.4 A + 3.1 A + 1.9 A + 3(7.6 A) =
1.38 A + 0.4 A + 3.1 A + 1.9 A + 22.8 A =
29.58 A @ 48 VDC = 1,420 W DC
Input current from a DC source other than –48 VDC (based on maximum
configuration; applies to DC power supply only):
( 54 VDC input) * (input current X) = ( 48 VDC input) * (input current Y)
54 * X = 48 * 29.58 A
X = 48 * 29.58 A/54 = 26.3 A
System thermal output (based on maximally configured DC-powered router):
Watts DC * 3.41 = BTU/hr
1,420 * 3.41 = 4,842 BTU/hr
Appendix D: Power Guidelines, Requirements, and Specifications
DC Power Specifications and Requirements
169