Transformer Neutrals
Transformer Neutrals
(OP)
Can anyone provide a reference/starting point in the NEC to the tying of (2) identically sized transformers neutrals together on the secondary. These transformers distribute power through independent panels located in one enclosure and power common IT equipment. I have researched and don't see a direct correlation in the NEC.






RE: Transformer Neutrals
Key code requirement is not to ground the neutral of a service or a separately derived system at more than one point and no where downstream of its main disconnect. (You need to search for this on you own). Also avoid current through grounding path during normal operation.
If the transformers are paralleled and there is a ground fault protection on its secondary mains, you need to watch out for proper GF current sensing. It has more to do with engineering than the Code.
Perhaps, you can clarify your system/concern with more detail.
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformer Neutrals
I do understand the engineering impacts ie. neutral currents due to phase imbalance, and necessity for like characteristics for the transformers should their load become entirely shared (paralleled).
I was curious if the NEC prohibited the transformer neutrals from be tied together on a common neutral bus.
I have read and understand the Key code requirement you mention above.
Hope this clarifies my previous post..
RE: Transformer Neutrals
In general, the transformers can't/shouldn't be operated in parallel.
RE: Transformer Neutrals
In theory - Two transformers with identical voltage ratings will be connected in parallel to serve a load. The following rating & impedance data are provided:
Transformer 1: 1000kVA, Z = 4.5%
Transformer 2: 2000kVA, Z = 6.0%
The maximum load that can be served (kVA) by the bank without overloading either transformer is:
Convert T1 impedance to T2 Base: .045j (2000/1000) = .090j
Then use equation: Load T1/Load T2 = ZT2/ZT1 or (1000kVA*.090j)/.060 = Load T2 = 1500
Load Total-Paralled = Load T1 + Load T2 = 1000kVA + 1500kVA = 2500kVA
Could secondary neutrals be connected to a common point while accomplishing the above? if the connection point is isolated from ground?
RE: Transformer Neutrals
RE: Transformer Neutrals
Cheers,
Dave
RE: Transformer Neutrals
Matt
RE: Transformer Neutrals
You are describing two totally different systems in two different posts!.
Parallel transformer operation is a totally different story has nothing to do with dual bus system you described in the earlier post.
[quote]I was curious if the NEC prohibited the transformer neutrals from be tied together on a common neutral bus.[/q]
The answer is Yes. For the reason I mentioned in my second paragraph of my earlier response. Plus there is no reasons to interconnect two separately derived system conductors unless they are being operated in parallel with a proper design.
I have done only data centers in last decade, and I understand your description, however you cannot interconnect the two neutrals in load centers (RDCs)as you describe. Why would you want to do that to begin with?
Isolation to ground at the RDC has no bearing. Both transformers are already grounded at their secondaries.
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformer Neutrals
My 3rd post was for discussion in response to the post from dpc about paralleling.
Reason for my initial post: I discovered and error in an installation. Not my design but my problem. I was looking for valid reason to avoid systems re-work. Proper engineering dictates re-work will commence tomorrow....
Thanks again for the input.
RE: Transformer Neutrals
% impedance or transformer impedances describe the current under short circuit conditions when the entire circuit is comprised of the resistance and inductive reactance of the transformer.
Similar transformers with equal impedance often (but not always) have the same X/R ratios. If the X/R ratios are different The sum of the transformer currents may be more than the load current. This may mean overloaded transformers.
Bill
--------------------
"Why not the best?"
Jimmy Carter