Transformer Secondary Protection

[richanton]

I have a question on a change I want to make. I have two(2) 300 kva xfmrs, each feeding an 800A panelboard with 800A main breakers. Each panelboard is within 10' of the transformer. Each transformer is currently running at about 100kva of load. The primary of each transformer is a 400 amp breaker molded case breaker.

I want to eliminate one transformer and feed the second panelboard from the same transformer.

My question is the proper protection. What I would like to do is keep the 800 amp cable to the first panel and the 800 amp main breaker and then tap from the incoming terminals of the panelboard to a second panelboard with a one per phase 500MCM cable, changing the main breaker on the second panelboard to 400 amps. (This is due to the limitation of only being able to hold 3-500MCM on the panels incoming terminals.)

So does that meet NEC 450 requirements, if not, how can I make it meet NEC.

 

[georgeshall]

If you are taking 2 sets of 500’s from the transformer secondary, without overcurrent protection at the transformer secondary, the panelboard main circuit breaker trip rating (800A) must not be greater than the ampacity of the conductors (760A). See Art. 240.21.C where it is stated that “the provisions of 240.4(B) shall not be permitted for transformer secondary conductors.”

To answer your question, I would say that tapping the secondary conductors at the first panelboard would not be allowed. See Art. 240.21.C.(1)-(6). However, you can route the conductors for the second panelboard from the secondary of the transformer, or you can just subfeed the 2nd panelboard from a breaker in the 1st panelboard.

Just my 2 cents. Hopefully someone much older and wiser will chime in.

 

[gepman]

georgeshall brings up an interesting point with the

Art. 240.21.C where it is stated that "the provisions of 240.4(B) shall not be permitted for transformer secondary conductors."

I didn't realize this but I almost never utilize 240.4(B) anyway.


I agree with georgeshall but I add that per the NEC articles he quotes your existing system does not meet code (your secondary conductors have less ampacity than the secondary protection).

I also don't see how his first suggestion (routing the conductors from the second panelboard from the secondary of the transformer will solve this problem unless you change each panelboard main circuit breaker size so that it less than the conductor ampacity for each panelboard. Even then it might not meet note 3 of Table 450.3(B) which requires the transformer secondary protection to be grouped in one location. The maximum rating of the secondary protection is 125% of full load rating or 900A for a 300kVA 240V secondary or 1047A (can be rounded up) for 208V secondary.

If you had a delta:delta single voltage transformer then you could change the primary to 350A. Primary protection alone is acceptable for particular delta:delta transformers.

I think subfeeding the second panel as suggested by {b]georgeshall[/b] (you still need to solve the secondary conductor ampacity problem perhaps by changing the main circuit breaker to 700A) is the best solution.

 

[alehman]

You can't tap a tap (especially an illegal tap). The feeder from the transformer to the first 800A panel is considered a tap. The single set of 500's you propose from the first panel to the 2nd would be tapping the tap - not allowed.

The exclusion of transformer secondary conductors was new in the 2005 NEC. An inspector educated me on that one the hard way. Fortunately in my case it was a small feeder that was inexpensive to replace. I don't understand the reasoning behind it.

 

[gepman]

alehman

A tap of a tap is a good way of looking at it. If the exclusion was introduced in 2005 as you say then perhaps georgeshall's installation was previous to the exclusion and therefore is acceptable as is but he still couldn't make the new tap.

Perhaps the reasoning for the exclusion is that most small (<800A) transformers feed panelboards and the actual load is not known (since panelboard load's can change easily). That is the reason that I never use the provisions of 240.4(B) on feeders, the circumstances of use can change easily (i.e. a 800A feeder ought to have 800A ampacity).

The only time I use 240.4(B) is in a situation in a branch circuit where the load is very unlikely to change (it seems that things are always changing in industrial situations) and there is a large economic consideration in the cost difference of conductors. Most of the time there isn't that much difference in cost (i.e. going from 500MCM to 600MCM).

 

[richanton]

Thanks guys for thr responses.

Based on my interpetation of all this, it looks like if I connect 2 sets of 600MCM to the transformer secondary and have one set go to each panel, and replace each panel's 800A main breaker with a 400A main breaker, only then does it actually meet NEC.

 

[alehman]

I think that is correct. The length of the feeders must comply with the tap rules and the overcurrent protection must also satisfy the rules for transformers.