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 Post subject: Panel rating on xfrm secondary? Need help
PostPosted: Mon Jun 30, 2008 7:01 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 483
Location: New England
Ok guys, talk to me like a 3 year old. I'm trying to understand this and decide the best course of action.

I have been performing the the IE calcs for 80 panels in large admin building. The building is service by GE switchgear, which helps tremendously because I can literally dial in the Short Time and Inst trip settings to get all the 480V panels down to 4.0 cals/cm2. I have noticed that there is a real problem with molded case breakers in the under 100A class as the Instantenous is non adjustable and doesn't enter into the 2 cycle trip range until you get a multiple of 20 or better. This may sound strange, but in these cases the low fault current works against me and raises IE. On the 480V side, I often have subpanels fed with 200A branch breakers out of 800A MLO fed from the switchgear. On almost all of these I have to rely on the switchgear breaker to trip on the fault rather than the immediate upstream 200A branch breaker. Yes I could coordinate it, but then I have to suffer 5 sec clearing time on the 200A branch and IE of 12 cal/cm2, but I if leave the 800A switchgear breaker settings low I get IE of 3 cal/cm2 at the panel. I really just mention this for comment.

Now for my question. I have a lot of 30KVA and 45KVA xfrms feeding 208/120 panels. The 30KVAs are fed with 50A on the primary, and the 45KVAs are fed with 70A on the primary. There are a couple of panels that are MLO, and yes I know this is a code violation, but I didn't install it. Besides, based upon previous questions it seems the consensus is that you can't assume the fault is downstream of the MCB as the MCB is in the panel and the fault may be on the line side of the MCB. Now I know that under IEEE since these panels are less than 240V and fed with less than 125KVA xfrms I don't really need to perform an analysis. But I'd like to see how this would match up. I am calculating Isc for the secondary assuming an infinite bus on the primary. I then find arcing current on the secondary. Knowing secondary arcing current I calculate the corresponding primary current, as well as 85% of the primary current, and then look at the clearing time for the primary xfrm breaker. It is not unusual to see 5 to even 10 secs. If you plug these numbers into the calculations you get 12 cal/cm2. Dropping to 2 secs can usually get it under 8 cal/cm2. However, sometimes even at 2 secs I come up with Level 3 (more than 8 cals).

Something doesn't chive. Is the reason we are told an analysis is not required is because the equations DON'T work? If they do work, should I really be putting Level 1 labels (my default minimum) or should I be playing it safe and fudging to Level 2, or go for what the equations actually are saying and go Level 3.

In another building I have a 112.5KVA xfrm feeding a 400A CB panel, but it is MLO. Again even if the panel had a MCB it wouldn't make a difference. This panel is coming up with a 13 cal rating based upon 2 secs on the primary. The actual trip curve says 8 secs. Do I just call it a Level 1 and take advantage of the IEEE exemption.

I would welcome some insightful discussion on how to handle this.

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