Replace Feeder or Not?

eastcreek · Sep 21, 2004

List,

Here's what I'm telling the other electricians is possible without performing a change-out of the existing feeder. Please tell me if you see a problem with this...

Existing 480/3 feeder is on a 110a breaker. Currently it goes directly into the primary of a 480/208 75kva transformer. The secondary feeds a 225 amp main in the first of 2 full panels. The load is 55amps per phase. (Only computers.)

I'm suggesting we replace the 225 amp main breaker with a 75 amp main. Pull the transformer and install a 30kva instead. This will allow us to put in a 6 space 480 volt panel on the primary side of the transformer. From it we need to put in a 3 pole 50 and 20 breakers to feed a computer room hvac unit and get it off of the computer room feeder which is getting close to overloaded.

Some folks here say we would violate a "nameplate rating" rule. I've asked which section in the code that could be found but still haven't gotten an answer. Others think this could cause tripped breaker problems in the future.

Any opinions out there?

Mike Reisen
All Air & Electric
Arlington, Texas

bob · Sep 21, 2004

Re: Replace Feeder or Not?

Your post does not say anything about added load.
If the load is the same, why chaange out tht feeder?
Why replace the 75 kva transformer. Is space a problem?
Don't understand why you want to replace the 225 amp main with a 75 amp breaker.
You can still add a 480 volt panel if space is not a problem and feed the existing transformer.
I do not see anything you listed that would violate a name plate rating.

eastcreek · Sep 21, 2004

Re: Replace Feeder or Not?

Bob,

The hvac (Heating Ventilation and Air Conditioning) equipment will be the added load. It needs to be removed from a different feeder.

Mike

charlie b · Sep 21, 2004

Re: Replace Feeder or Not?

If the computer load is 55 amps on a 3-phase 208/120V system, that equates to 19.8 KVA. So a 30 KVA transformer would certainly be big enough. It sounds like you are trying to reduce the ?connected load,? and thereby the ?demand load,? on the 480 volt panel (call it ?Panel 1?) that presently feeds the 75 KVA transformer. With that done, you can then justify transferring load from a different 480 volt panel (call it ?Panel 2?). I infer that you are thinking that you must replace the 75KVA with a 30 KVA, or else you won?t be able to add load to ?Panel 1.? In other words, the existing load on ?Panel 1? is close enough to its limit that you need to subtract the 45 KVA (difference between the transformers) in order to add more load.

Is that the problem that you are trying to solve?

If so, there is nothing wrong with the concept. But to make it work, you need to evaluate the entire load picture. Article 408.13 (2002 version) tells us you must start by calculating the load on Panel 1 using the method of Article 220. You must include the HVAC loads that you intend to move to Panel 1 from Panel 2. Then you decide on the minimum feeder size. Finally, the rating of Panel 1 must be at least as high as the ampacity of that minimum feeder size. That is the only related context I can find for a ?nameplate rating? rule. If the existing feeder and the rating of Panel 1 are high enough, there is no problem with "nameplate ratings."

I must admit, however, that I am confused by your description of a ?6 space 480 volt panel on the primary side of the transformer.? It sounds like you intend to use all 6 spaces for 2 new breakers. How would this be new ?Panel 3? be connected? Would you be using the same 110 amp breaker on Panel 1 to feed both the new 30 KVA transformer and the new Panel 3? Or would the new 30 KVA transformer be fed from Panel 3, which in turn will be fed from Panel 1? In other words, when you say "6 spaces," do you mean "6 single phase, or equivalently two 3-phase spaces," or do you mean "6 spaces, each being a space for a 3-pole breaker"?

eastcreek · Sep 21, 2004

Re: Replace Feeder or Not?

Wow...this is getting confusing...

Currently there is no panel ahead of the 75 kva transformer. The feeder goes directly into the transformer. We need to place the 480 volt hvac equipment on a 6 space panel that has yet to be installed ahead of this transformer. This will keep the havc equipment on a backed-up generator feeder as it is now. So, we will be adding to the load on "feeder 1". I feel we need to downsize the transformer so that no one ever puts more load on the secondary than what curently exists. (Running a new feeder is a option, but as usual a very elaborate plan would be required.)

This is the problem we are tring to solve. We must take the hvac equipment off of "feeder 2". This feeder is for the main computer room.

The "nameplate rating" issue came up when I stated let's pull the 225amp main out of the 1st panel off the secondary of the above transformer, and install a 75 amp main in the 1st of the 2 225 amp rated panels. I contend it's ok to feed a panel with a current load LOWER than what it's rated at.

Yes, all 6 spaces will be for the hvac equipment. One 3-pole 50 and one 3-pole 20.

At this point I still think it's an ok install except for the fact nothing will be able to be added to "Panels 1 and 3" and it may be a bit easier to trip feeder 1's breaker.

charlie b · Sep 21, 2004

Re: Replace Feeder or Not?

Originally posted by eastcreek: Currently there is no panel ahead of the 75 kva transformer. The feeder goes directly into the transformer.

I must not have made my question clear. The feeder goes directly to the transformer from where? You said there was a 110 amp breaker. What panel is that breaker in? Are you keeping that 110 amp breaker in place, and using it to feed both the new Panel 3 and the new 30 KVA transformer (i.e., double-lugs on the 110 amp breaker)? I hope not, but what I am missing about your intended installation?

I contend it's ok to feed a panel with a current load LOWER than what it's rated at.

I agree. 408.16(A) and (B) simply require that the overcurrent device(s) be rated ?not greater? than the rating of the panel.

eastcreek · Sep 21, 2004

Re: Replace Feeder or Not?

Charlie,

The 110 amp breaker originates from the buildings main electrical room about 300' from where the current 75kva transformer is.

Yes, this breaker will stay in place...and yes, we intend for the feeder to feed "panel 3" (a feed through 6 spaces panel..assuming it's available) and the downsized 30kva transformer.

Mike

charlie b · Sep 21, 2004

Re: Replace Feeder or Not?

Once again, I might be misunderstanding your intended installation. But it sounds to me like you are walking into a code violation, and what is worse, a very unsafe system. But let me first see if I understand.

What I think I read is the following:

There is a 110 amp breaker in a panel some distance away.

There is an existing feeder from this panel to the primary of the 75 KVA transformer.

You will disconnect the primary of the existing 75 KVA transformer from this feeder.

You will disconnect the secondary of the existing 75 KVA transformer, and remove it from the area.

You will connect a new ?Panel 3? to the existing feeder. ?Panel 3? has no main breaker, and it has feed-through lugs.

You will connect a new (short) feeder from the feed-through lugs of ?Panel 3? to the primary of a new 30 KVA transformer.

You will connect the secondary of the new 30 KVA transformer to an existing 225 amp panel (I didn?t give this one a name in my earlier post). You will replace the 225 amp main breaker in this panel with a 75 amp main breaker.

Do I have this right? If not, then tell me what I have wrong.

But if this is what you have in mind, you can?t do it. The only protection that you are offering for the primary side of the new 30 KVA transformer is the 110 amp breaker. That is because there is no main breaker on ?Panel 3,? and no feeder breaker on ?Panel 3? to supply the 30 KVA transformer. The current rating of the primary (480V) side of a 30 KVA transformer is 36.1 amps. The maximum allowed rating of a primary overcurrent device is 250%. That equates to 90.2 amps. You are allowed to upsize to the next higher standard rating of 100 amps. Therefore, the existing 110 amp breaker would not be a code violation.

Now to step away from the NEC and talk about design. You won?t be able to work on the 30 KVA transformer or on ?Panel 3,? without turning both off. That is not a good design. I would suggest getting a 9-slot panel instead of a 6 slot panel, and feeding the 30 KVA transformer with a breaker (i.e., instead of via feed-through lugs).

jim dungar · Sep 21, 2004

Re: Replace Feeder or Not?

It seems this discussion has gotten convoluted.

If you change the transformer to a 30KVA you must provide it with the correct primary overcurrent protection.

Your question appears to be: can a panel with 225A bussing be protected by a 75A breaker. The answer is YES.

It is an old wife's tale that you have to use specific "main breakers" based on the number of circuits in a panel.

eastcreek · Sep 22, 2004

Re: Replace Feeder or Not?

Thanks guys,

Yes you've got it. And....I like your idea of the 9 slots. It makes sense to provide overcurrent protection for the new transformer while we are in the process of installing Panel 3. (duh....)

I'm thinking will use a 45 amp and go with the smaller allowable breaker size. Once we place the hvac equipment with the 2 breakers (3 pole 50 and 20) required for it in place, we will have a full feeder load. (55 existing + 45 hvac)

After all this now there's concern about moving the hvac equipment off of the computer room feeder and placing it on the feeder for a large call center instead. Some folks think we will get tripped breakers. The other option is a $18,500 new feeder!

charlie b · Sep 22, 2004

Re: Replace Feeder or Not?

Originally posted by eastcreek: The other option is a $18,500 new feeder!

How much money does the owner stand to lose, if there is a trip, or several trips? Perhaps $18,500 is a relatively inexpensive ?insurance policy.? But that is the owner?s call.

Replace Feeder or Not?

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