Telecom Wire Guage Question??

[jhawkins]

Need Electrical Engineering help on this one, it is out of my expertise as a Telecom Engineer.

We are installing a Candeo AC/DC Rectifier System for a Nortel PBX. The panel power is 215/400 three phase (This is overseas, but we must meet NEC standards as it is considered U.S. soil) One 30 Amp breaker is installed in the panel for each rectifier unit. Six rectifiers will be installed in the Candeo rack which will be operating 24/7. Each rectifier is nominally rated at 14.5 amps @ 208VAC continuous. We are running 6 Phase and 6 Neutral feeds in 1 1/4" liquid tight conduit to the box on top of the Candeo Rack and 1/2" liquid tight with 1 ea phase and neutral from the box to each rectifier terminal. We plan to use THHN #12 AWG wire per the 90degC column on Table 310-16.

Is this acceptable per NEC? If Not, please explain why not, with references.

 

[charlie b]

Re: Telecom Wire Guage Question??

I am not certain I am correctly interpreting your description. What I think you are saying is that
</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">(1) You are intending to protect a #12 conductor with a 30 amp breaker.
  (2) You are intending to run 12 conductors in a single conduit from the power panel to the top of the rectifier bank.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">Item (1) violates NEC article 240.4(D). You cannot protect a #12 with anything larger than 20 amps.

Item (2) will not work either. A 14.5 amp continuous load must be treated as though it were a 18.1 amp load (i.e., upsize by 25%). A #12 is good for 30 amps at 90C. But you must derate by 50% because you are running 12 current-carrying conductors in a single conduit, per Table 310.15(B)(2)(a). So the #12 is only good for 15 amps, and you need 18. A #10 would be good for 20 amps (i.e., 50% of 40 amps). That would suffice. There is room in a 1.25 inch conduit for a dozen #10 wires.

 

[dereckbc]

Re: Telecom Wire Guage Question??

Rectifiers are continious loads, so you have to use full load current, plus 125% to the first rectifier, and 100% to the rest. Then derate as Charlie indicated.

[ February 25, 2004, 11:54 AM: Message edited by: dereckbc ]

 

[jhawkins]

Re: Telecom Wire Guage Question??

I wasn't aware the breaker and other factors determined the wire guage requirement.

Thanks for the Help. We will get some #10 AWG wire.