Multiple single-phase loads on a 208 3-wire branch circuit

[Ben Hartman]

I have a situation where a building only has a 208/120 feed.
I need to power a rack of ac to dc rectifier modules that each require >200vac to work.
The outputs of these rectifiers are all tied together and share the dc load, so the load balance should be pretty close.
If I balance the rectifiers around pairs of phases,
1. What problems might this cause?
2. Would the currents in each phase roughly balance out to the total net draw of the system? I know that the current draw of the whole system isn't simply the sum of each module. I'm just thinking about the overcurrent protection issues. Do I size the circuit breaker differently?

Thanks for any thoughts.

 

[waross]

If you use a breaker for each load, base the breaker sizing on the rated current.
If you use one three phase breaker for three equal loads, one on each phase, base the breaker sizing on 1.73 of the rated current of the individual loads.
eg For rated current of 13 Amps continuous, you would use wire rated at 125% of 13 Amps or 16.25 Amps.
Probably # 12 AWG. The wire would be protected by a 20 Amp breaker.
For three 14 Amp loads on one three phase breaker, the current will be 1.73 x 13 Amps or 22.5 Amps.
Wire at 125% would be rated for 28 Amps. Probably #10AWG. The wire would be protected by a 30 Amp breaker.




Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Ben Hartman]

Thank you for helping me to clarify this in my mind.
So given my need for a total of 40 modules of 208 load, I think I'm best off coming from the main distribution with 3 2-wire (+ground) 208v circuits, each driving a third of the modules, and not a single 3-phase circuit that would have to be sized up (1.73).
Right?
(I'll hand this suggestion over to the MEP to further vet.)

Best, Ben

 

[HCBFlash]

Consider these items as well:

-40 is not divisible by 3 (into a whole number)

- 3separate branch circuit breakers are each separate trip points or disconnects which need to be secure.

- if your "modules" need individual over current protection, having 3 smaller breakers gains you no protection.

- a single 3 wire plus ground circuit may well cost less in both time and materials to install.

- a single 3-pole breaker takes half the panelboard branch space, and has lower maintenance costs than the 6-pole option.



.


Me wrong? I'm just fine-tuning my sarcasm!