300 KVA XFMR, Disconnect?

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tryinghard

tryinghard

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I have a project with a dry type transformer outside, it is 450' + from the serving 500A breaker. It is a 480 208Y/120. The drawings show a 1000A disconnect on the secondary side of this transformer feeding a 1000A distribution panel. I have been studying "99" Article 450 & 450-3.

Q1: Do I even need this disconnect on the secondary side?

Q2: Would I ever need disconnecting means on the primary side?
 
Re: 300 KVA XFMR, Disconnect?

Q1: Do I even need this disconnect on the secondary side?
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Yes, 450 covers overcurrent protection (OCP) for the transformer. But you must also protect the secondary conductors per the tap rules in 240.

Q2: Would I ever need disconnecting means on the primary side?
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I think the one 500' away is OK if it can be locked out. If the transformer has to be replaced, or minor maintance needs to be done, that breaker can be turned off and locked out.

I don't see any benefit to adding a second primary breaker at the transformer. I wouldn't think anyone would need to shut it off that often.

Steve
 
Re: 300 KVA XFMR, Disconnect?

Steve,

Got it; I notice in 240-3(f): "multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary over-current protective device." So my case with delta-wye requires over-current protection.

Also 450-6 requires both ends of the secondary to have over-current protection.
I do not see any code requiring disconnection means, (like air-conditioner, appliances, or motors), at the transformer though.

Thanks for your help
 
Re: 300 KVA XFMR, Disconnect?

Also 450-6 requires both ends of the secondary to have over-current protection
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I'm a little confused by that. 450-6 applies to secondary ties, which would only apply if you are connecting two transformers together.

Your OCP on the secondary side probably has to be at the transformer to meet the tap rules in 240. For example, if you are using 240.21(6), your OCP would have to be within 25' of the transformer.

Steve
 
Re: 300 KVA XFMR, Disconnect?

If this transformer was 400' from the primary source and 100' to the secondary destination and was a delta/delta and in compliance with 450-3 with OCP, would I ever need transformer protection other than the source feeder breaker/fuses?

If I understand the above scenario I would not have any disconnect at the transformer location, primary or secondary, is this correct?
 
Re: 300 KVA XFMR, Disconnect?

tryhard, You keep using the term "disconnect" - maybe you should be using the term "overcurrent protection".
Generally, disconnect means are not required for the 480V. pri. or the 208Y/120V. sec. It is a design issue.
But there's serious code rules for OCP of the xfmr pri., the wires feeding the pri., the wires leaving the sec., & for the panel board on the sec.

Your xfmr is delta pri. & wye sec. so OCP protection rules for delta-delta are moot.
 
Re: 300 KVA XFMR, Disconnect?

Originally posted by kiloamp7:
Your xfmr is delta pri. & wye sec. so OCP protection rules for delta-delta are moot.
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I appreciate your concern for my confusion but let me try to restate my real concern.

I understand the difference between "Disconnection Means" and OCP. This particular project I have (which caused my post) is drawn and is referencing the OCP as a disconnect (which will be a breaker disconnect, hence the OCP).

I also understand secondary ties, as defined by 450-6, do not apply in my case because I am not connecting more than one XFMR together.

This posted question originally arose from our Estimator who questioned if this OCP is even needed (which I know it is because it is a delta/wye rather than a delta/delta). This question then evolved to question: if this OCP is not needed would any disconnecting means be needed (like that which is required for motors 430-101 or air conditioners 440-11 as example)?

Article 450 does not require any disconnecting means, probably due to the fact the OCP can be relied on for this function. My case about the delta/wye is resolved, but my concern about a possible delta/delta is essential for a most common scenario, so you see my question is integral not moot.

My question still remains as written in the 5th post of this posting above.
 
Re: 300 KVA XFMR, Disconnect?

Hello Tryinghard,
Perhaps I can clear up your problem. I understand that the xformer is customer owned and is located outdoors and supplies a building. I assume that the xformer is supplied from the utility source, therefore, it is a service and a service disconnect and overload protection is required at the terminating end of the service conductors. This will be the overcurrent protection of the xformer.

The size of the overcurrent device is based on Art. 450, however the conductors on the primary if there are any are service conductors and must be protected in accordance with Art. 230-90. The secondary conductors must also be protected by OCD sized in accordance with Art.240 tap rules for transformers.

Primary current is 361 amps. 1.25 x 361 = 451; next std size is 500A. I recommend 450 amps for the xformer. Conductors on the load-side of the service disconnect are feeders and must have an ampacity of 450A or more. The specs call for a 1000 amp disconnect switch on the secondary, however, since I assume the secondary conductors are outdoors, Section 240-21(C)can be used. If 1000 amps of OCP is used, the secondary conductors must have an ampacity of 1000 amps.

Section 225-31 thru 225-40 are applicable for a feeder supplying a building or structure.
 
Re: 300 KVA XFMR, Disconnect?

Tell us if the 480V. pri. comes from a utility service or if the 480V. pri. comes from a user-owned 480V. feeder.
 
Re: 300 KVA XFMR, Disconnect?

This XFMR is user-owned and fed from a 500A breaker 450' away which is out of sight; it is not supplied from the utility service. The secondary feed is 100' to an MDP inside a building, which has a 1000A main breaker. I know the feeders are correct and I see this case is ?as per code?.

I do notice 240-21(C)(1) say's the same thing as 240-3(f). As example, if the above application remains the same except the XFMR turns into a delta/delta and the OCP at the XFMR secondary is eliminated my question is:
Would any disconnecting means be needed (like that which is required for motors 430-101 or air conditioners 440-11 as example)?
 
Re: 300 KVA XFMR, Disconnect?

In light of 240-3(f) & 240-21(C)(1): if this XFMR was 480-240/120 it appears this would NOT be considered to be protected by the primary OCP (not 3-wire single voltage) and I would need secondary OCP as per 225-31 & 32.

Would the main breaker in the MDP suffice, sized correctly of course?
 
Re: 300 KVA XFMR, Disconnect?

Hi Tryinghard,
With the additional info provided, I can provide the following: 500 Amp CB is ok for the primary overcurrent protection. No disconnect is required within sight of the transformer. The transformer is located outdoors, so Section 240-21(C)(4) is applicable. Since the secondary conductors terminiate in a 1000A CB, the conductors must have an ampacity of 1000 amps. A disconnecting means is required at the building in accordance with 240-21(c)(4)(d). Section 240-3(f) is not applicable to your installation. I hope this answers your questions.
 
Re: 300 KVA XFMR, Disconnect?

John,
I wonder about 240-21(c)(4); it is one of four conditions that allow the secondary to rely on primary OCP, without other OCP at the secondary. I can see how (4) is the correct condition with my case but it states "all of the following conditions [(a)-(d) must be met]"

Please bear with me as I write my concerns/questions/ & understandings as I try to decipher 240-21(c)(4)(a) through (d) one condition at a time:
(a) Got it
(b) "The conductors terminate at a single [overcurrent device]". I see this as the 1000A main breaker in the MDP.
(c) "overcurrent device...is an integral part of a disconnecting means". I see this as possibly the 1000A main breaker in the MDP?
(d) If (c) is true then the disconnecting means here is the breaker (OCP) in the MDP. I am also in compliance with 225-32 for the feeder. :confused:

Any input/opinions would be greatly appreciated.
 
Re: 300 KVA XFMR, Disconnect?

Well, I most often see on drawings OCP at the secondary of a transformer in situations like I have described above: 300 KVA 480-120/208 user owned and fed, outside u/g primary 450' from 500A breaker, outside u/g secondary 100' to 1000A main breaker in a MDP inside a building mechanical room at the point of entrance of the conductors. I have tried to describe the application clearly; my question was always about disconnection means/OCP not sizing.

What I have learned here is: I have met the conditions (a)-(d) in 240-21(c)(4), and I have met the conditions of 225-32. So the 1000A main breaker in the MDP will be the secondary OCP as well as the disconnect, therefore I do not need OCP for the secondary at the transformer at any distance from it. In other words the transformer will be all by it?s self at it?s location, no other disconnect/OCP.

You guys may see this clearer but to me it?s somewhat of a surprise. Is this correct?
 
Re: 300 KVA XFMR, Disconnect?

I do not doubt that you frequently see a sec. O/C device near the xfmr, but NEC (which is not a design manual) does allow the "unlimited length outdoor sec. run" per 240.21(C)4.

NEC did not always recognize this "unprotected outdoor tap".
I'd have to dig out a 1990 or so NEC to see when they first allowed it.
 
Re: 300 KVA XFMR, Disconnect?

From a design point of view my case seems to be redundant, in other words the engineer drew this lazily and created another point to maintain. From an engineering standpoint the feeders and transformer are all protected with the 500A and the 1000A breakers. From a value point of view, my case is requiring a 1000A enclosed circuit breaker (similar to a SWB in size) at the transformer secondary, this is around $5,000.00 +/- installed more than the compared correct application.

This is a school project, so our tax dollars paid for this engineering as well as the exorbitant enclosed circuit breaker, ugh!?!.
 
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