Circuit Breaker Panels after Transformer
Circuit Breaker Panels after Transformer
(OP)
I have a transformer to bring the 3-phase voltage of 480VAC down to 400VAC (for feeding european equipment in the US). From the secondary side of the transformer i want to feed 25 3-phase loads (=75 poles)trough a circuit breaker panel. According to NEC408, i can handle only up to 42 poles (=14 3-phase loads) in one circuit breaker panel. Here my questions:
-am i allowed to connect two circuit breaker panels in parallel to the transformer, and where has the main circuit breaker to be in this case? Do both of them need a main circuit breaker?
-am i allowed to connect the two circuit breaker panels behind each other - one circuit breaker of the first panel to protect the secondary panel?
I appreciate any help!
-am i allowed to connect two circuit breaker panels in parallel to the transformer, and where has the main circuit breaker to be in this case? Do both of them need a main circuit breaker?
-am i allowed to connect the two circuit breaker panels behind each other - one circuit breaker of the first panel to protect the secondary panel?
I appreciate any help!






RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer
A better way, in my opinion, to have more than 42 poles is to use two panelboards in parallel ( a double section panelboard), as long bussing for both are rated for the total load, they can be fed by a common main breaker. See NEC 408.16(A),Exception No. 1 (2002 Edition).
The 42 pole restriction is for total number of poles in one cabinet.See NEC 408.15
Alternative way is to have one larger panel with a subfeed breaker feeding a smaller rated panel with additional poles as you described.
RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer
Don
RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer
You can also use two main-breaker panelboards from one transformer, but you will need to read the NEC on transformer protection carefully. If the panel mains are to be the transformer secondary protection, and the primary protection is >125% of the primary rating, the sum of the panelboard mains must be less than 125% of the transformer secondary. If you want larger main breakers, the primary breaker must be <125% of the primary rating, which can cause false trips on energization.
RE: Circuit Breaker Panels after Transformer
-i need on all loads the neutral (coming from the transformer)
-i still have a problem understanding the terminology in the NEC. My cabinet will be used to feed industrial equipment (machinery). All loads are 3 phases + neutral + PE, protected with between 15 to 35Amps. Is it then, a lighting and appliance branch circuit panelboard (408.14 A) or a power panelboard (408.14 B)?
-in case of using a 2-section panelboard, is the main breaker external or internal. Has somebody a link to a manafacturer to get a better impression?
Thanks for your patience...
RE: Circuit Breaker Panels after Transformer
I'd suggest providing a main secondary breaker in a separate enclosure for transformer protection and two panelboards,connected together with gutter taps or sub-feed lugs.
It might be simpler to just call your local GE or Cutler-Hammer sales engineer and tell them what you are trying to do. They do this for a living.
RE: Circuit Breaker Panels after Transformer
Thanks to all of you for your help. I appreciate all your effort!!!
This case is closed.
RE: Circuit Breaker Panels after Transformer
The transformer primary protection may be increased to 250% of the rated primary current of the transformer, if the secondary side protection is not more than 125% of the rated secondary current of the transformer. The both panels come with 42 poles, which will result in 84 poles total. The 100A sub panel does not have to have the main circuit breaker if the panel is within sight of the panel feeder circuit breaker.
RE: Circuit Breaker Panels after Transformer
Anyways, the point I am trying to get across is that their catalouges don't let you design the bkr panel from scratch. I mean, give me a catalouge that tells me that "bkr type A"
will mount on "busbar guts B" in "tub style C", etc., and let me make the mistakes. Go to any other manufacturer like FPE, ABB, Allen Bradley, and the like because their catalouges give you lots of design info. You can't go wrong with a good catalouge - ask any salesman or designer.
Save yourself a headache.
RE: Circuit Breaker Panels after Transformer
Lighting and stable static predictable loads are one thing but the dynamics change drastically when u r supplying motor loads. You may find that if u have motor loads the capacity of your system can be greatly reduced. Do a proper load calculation according to NEC or ESA or wherever you are. Then determine if yuor transfomer has enough capacity. Don't put the cart before the horse.
RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer
I know you really meant to say '... handle the rated secondary current and fault level of the transformer'.
It would be kinda interesting to watch how a non-fault-rated busbar behaves when subjected to a close-in fault. I'd watch it from a several hundred yards away though.
RE: Circuit Breaker Panels after Transformer
ScottyUK — Wouldn’t a suitably unbraced bus section dynamically assume a “sinusoidal” shape in its own right?
RE: Circuit Breaker Panels after Transformer
I was just trying to give the guy some practical advise. What are you 2 doing to help?
RE: Circuit Breaker Panels after Transformer
Making sure that a serious point is highlighted?
No offence was intended, but feel free to take it if you wish...
RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer
RE: Circuit Breaker Panels after Transformer