Circuit Breaker Panels after Transformer 7

[Bin510]

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!

 

[REDDOG]

What are the individual loads and what size are they is KW, voltage, etc.?

 

[Bin510]

I have not sized the transformer yet. The 13 loads on the first circuit breaker panel would reach a total of 70kVA/102A, the 12 loads on the second circuit breaker panel bring it to 41kVA/96A. The voltage on the secondary side of the transformer is 400VAC in a 4 wire system (3phases + neutral + PE).

 

[rbulsara]

Yes, both of your options would be acceptable.

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.

 

[dpc]

I basically agree with rbulsara. But you might want to review Article 408 and decide if your panels would be considered "lighting and appliance branch-circuit" panelboard or a "power panelboard". If all your branch circuits are three-phase with no neutral connection, your panel would be considered a power panelboard, and I do not believe that the 42 pole limitation would apply.

 

[WoodrowJWeen]

The top of a 84 pole panelboard (75 poles plus some spaces)would be physically too high, putting circuit breaker handles over 6'7" (also a code violation). Just simpler and easier to use 2 or more 42 pole panelboards.

 

[resqcapt19]

Note that even if the power panelboards do not require a main, the secondary conductors from the transformer will require overcurrent protection per Article 240 unless the transformer is delta/delta. See 240.4(F).
Don

 

[thwrj]

Install a 2-section panelboard.

 

[alehman]

I agree with dpc and thwrj. A two section panelboard with one main breaker and thru-feed lugs is the simplest solution.

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.

 

[Bin510]

Thanks for all your answers, they are containing very helpfull informations for me. Please provide some more help for the questions below:

-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...

 

[dpc]

Your second post contradicts your first. If 10% of your loads are line-to-neutral, you have a lighting and branch circuit panelboard and are limited to 42 poles.

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.

 

[Bin510]

I will get in touch with Cutler Hammer to caltify the details.
Thanks to all of you for your help. I appreciate all your effort!!!

This case is closed.

 

[jbartos]

Suggestion: I usually prefer to have one larger panel, e.g. 225A and one smaller one, e.g. 100A. It is possible to have 100A CB in 225A panel and a long nipple between the panel. This way many conduits or cables can be brought and installed from either side of each panel. The transformer secondary will be protected by the main 200A circuit breaker in the larger panel, e.g. 225A panel.
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.

 

[skiier]

suggestion to bin510. Avoid Cutler Hamer. They make an excellent product but there catalouge and order procedures really suck. Their bkr cabinets are generally standard sizes but the amount and type of bkrs u can install are limited by the physical size of the type of bkr :i.e.: short cct rating = size. As your short cct rating increases so does the size of the bkr and the amount of bkr's you can install for a certain &quot;tub&quot; reduces accordingly. U have to tell Cutler Hamer what u need and they tell you what the layout will be - and hope and pray that the salesman knows what he is doing and that he is right. I had such an incident with some very expensive bkr panels from Cutler Hamer for a paint line at Toyota Canada, Cambridge, Ontario. You see they have a 5' height restriction for electrical panels in their plant and I had ordered my panels from Cutler Hamer based on this restriction. 3 months after placing my order for my panels - and coincidentally 3 days before delivery - Cutler Hamer calls and tells me they can't give me what I want because they made a mistake on the order. The panels they want to give me are 6' 8&quot;! Huh?
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 &quot;bkr type A&quot;
will mount on &quot;busbar guts B&quot; in &quot;tub style C&quot;, 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.

 

[skiier]

I also agree with reddog. What type of loads are u feeding?
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.

 

[skiier]

Oh yes and keep it simple. Fuse and wire the tf according to your local codes. If possible fuse the primary only. On the secondary install and wire - within applicable code distance - a splitter or switchgear unit or panel combination or whatever . No need to fuse any panel or switchgear where it taps from the splitter or busbar at the transformer secondary because these components will be ordered and installed by you to handle the rated secondary current of the transformer. From a splitter you can install seperate disconnects rated 30A to whatever. A switchgear unit will really clean up your install but using bkr's instead of fuses can greatly increase cost. Oh yes I am assuming reasonable and easy to work with conductor sizes here, say 400A max.If your load exceeds this try to splitt the tf load to 2-400A services or 1-400A c/w 1-200A or whatever combination you have, you get the point. The point here is that smaller components, although more are needed, are a cheaper and easier to work with option than more expensive larger components, where less are needed. Design it from several angles but choose the most cost effective and simple design.

 

[ScottyUK]

Skiier,

I know you really meant to say '... handle the rated secondary current [blue]and fault level[/blue] 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.

 

[busbar]

ScottyUK — Wouldn’t a suitably unbraced bus section dynamically assume a “sinusoidal” shape in its own right?

 

[skiier]

Hey guys I am not trying to splits hairs here. I am assuming the design is engineered properly guys. I also assume that you are smart enough to realize that. Details shouldn't be overlooked but they shouldn't be dwelled on as well.
I was just trying to give the guy some practical advise. What are you 2 doing to help?

 

[ScottyUK]

[blue]&quot;What are you 2 doing to help?&quot;[/blue]

Making sure that a serious point is highlighted?

No offence was intended, but feel free to take it if you wish...