3 phase Tranformers

theshockdoc · May 13, 2005

In general. When sizing conductors for transformers. Are we sizing per the transformers ampacity or the allowable ocpd per 430.3(B) Ex. If the primary ocpd is allowed to go to 250% max. Is this determine our conductor size or the base ampacity of the trans. primary?

jim dungar · May 13, 2005

Re: 3 phase Tranformers

Conductors are sized based on their loads.
Branch circuit overcurrent protective devices are sized based on the wire.
Transformer OCPDs are sized based on the transformer.

templdl · May 13, 2005

Re: 3 phase Tranformers

I look at that 250% as protecting the upstream distribution system from a transformer failure, such as should there be a winding failure that results in a fault.
When one considers that the secondary protection provides secondary overload protection it would also provide primary overload protection.
Now consider the 25' tap rule where you are allowed to protect a feeder that is up to 25' long if it terminates on correctly sized OCPD. Note how the 250% rule is applied there.
I can't place my hand on a reference but as I recall that 25' can be measured from the pri. OCPD to the sec. OCPD and not simple to the transformer itself.

charlie b · May 13, 2005

Re: 3 phase Tranformers

Originally posted by jim dungar: Conductors are sized based on their loads. Branch circuit overcurrent protective devices are sized based on the wire. Transformer OCPDs are sized based on the transformer.

All well and good. But it does not quite answer the question that I think theshockdoc is asking. I say that because the same question has been nagging at me.

Consider the following scenario. I hope I have all the numbers right. It involves a 480 ? 120/208Y transformer.

We calculate the load on a panel to be 36 KVA. At 208 volts this equates to 100 amps. Reflected to the primary side, this equates to 43 amps.

We select a 45 KVA transformer. Rated primary current = 54 amps. Secondary rated current = 125 amps.

We select the primary OCPD at 125 amps (just under 250% of XFMR rating).

We select the secondary OCPD at 150 amps (just under 125% of XFMR rating).

For the primary feeder, we select a #6 copper. Ampacity at 60C is 55 amps. This exceeds the calculated load, and is just slightly over the rated primary current.

Now we postulate a high-impedance fault on the secondary side. We could instead postulate that the owner installs more load. In either case, let us say that the load, as seen by the primary side, has increased to 65 amps. This is an 18 % overload on the transformer.

The secondary OCPD will not trip, as this load is exactly equal to the selected OCPD rating.

The primary OCPD will not trip, as this load is far less than 250% of the primary rated current.

Therefore, no protective device will act to terminate this event.

The transformer will not be at serious risk, as it can handle an 18% overload for an extended time, with no greater impact than a reduction in its useful lifetime.

But the primary feeder conductors will see 18% more than their rated ampacity. They will not be able to maintain this condition indefinitely. They will overheat, and may even cause a fire.

Back to the original question: Why is it acceptable to leave the primary conductors with insufficient protection?

steve66 · May 13, 2005

Re: 3 phase Tranformers

Posted by Jim Dungar:

Conductors are sized based on their loads.

Jim's posts are usually very accurate, but I have to disagree with this. The conductors have to be sized for the overcurrent protection selected, except as permitted by the tap rules.

Charlie: You're scenero with #6 wire on the primary protected by a 125 amp breaker is only allowed per 240.21(B)(3). That rule includes wire on the primary and secondary that isn't protected at its ampacity. Therefore, in you're example, the conductors would be limited to 25', and they would have to be protected from damage.

I guess the authors felt that was enough to allow the possibility of a small overload.

Steve

P.S. Jim's statement applies to many items like motors, but I think that is only allowed per table 240.4(G). Transformers (except for secondary ties) are not listed in that table.

[ May 13, 2005, 12:49 PM: Message edited by: steve66 ]

jim dungar · May 13, 2005

Re: 3 phase Tranformers

Charlie,

Before I spend time pondering your question, are you sure about the #6 wire? If the transformer primary current is 54 amps and it is a continuous load then we need to chose a #4 conductor (1.25 x 54 = 67.5A).

bob · May 13, 2005

Re: 3 phase Tranformers

We select a 45 KVA transformer. Rated primary current = 54 amps. Secondary rated current = 125 amps. We select the primary OCPD at 125 amps (just under 250% of XFMR rating).
We select the secondary OCPD at 150 amps (just under 125% of XFMR rating).
For the primary feeder, we select a #6 copper. Ampacity at 60C is 55 amps. This exceeds the calculated load, and is just slightly over the rated primary current.

Charlie
When sizing the primary conductor you would need to use 125% x 54 amps = 68 amps and requires
#4 cu. If you use the 75C conductor you could stay with the #6.

charlie b · May 13, 2005

Re: 3 phase Tranformers

Originally posted by steve66: Jim's posts are usually very accurate, but I have to disagree with this. The conductors have to be sized for the overcurrent protection selected, except as permitted by the tap rules.

Isn?t that going at it backwards? I would size the conductors for the load, and then pick an overcurrent device that protects the conductor. My problem with transformers is that the OCPD is much higher than the ampacity of the conductors, and cannot protect them.

Originally posted by jim dungar: If the transformer primary current is 54 amps and it is a continuous load then we need to chose a #4 conductor (1.25 x 54 = 67.5A).

In my scenario, there was no presumption of a continuous load. I just wanted to create a fictitious situation in which a conductor would be placed at risk of destruction, and in which it had inadequate protection.

Originally posted by bob:When sizing the primary conductor you would need to use 125% x 54 amps = 68 amps and requires #4 cu.

Why the 125% factor (unless the load is continuous, which in my scenario, is not a given)?

steve66 · May 13, 2005

Re: 3 phase Tranformers

Isn?t that going at it backwards?

It would be for motors and other loads, but I don't think it is for transformers.

I would size the conductors for the load, and then pick an overcurrent device that protects the conductor.

But if you did that, your OCP device would be too small for the transformer inrush current.

My problem with transformers is that the OCPD is much higher than the ampacity of the conductors, and cannot protect them.

You can only do that per the tap rules in 240.

I am going to go one step farther, and say it's possible your #6 wire on the primary is only allowed by 240.21(B)(3). I don't think any of the other tap rules allow this. Why? Because you are supplying a tap on the secondary from a tap on the primary. In general, this is prohibited by 240.21 (1st paragraph), but it is specifically allowed by 240.21(B)(3).

jim dungar · May 13, 2005

Re: 3 phase Tranformers

Even the NEC recognizes this issue. The 2005 handbook comment under 450.3 FPN 1 says it is possible that Article 450 transformer OCP may satify Article 240 conductor OCP, but it might not. The comment to 450.3(B) clearly states that the transformer OCPD and the primary conductor OCP are two different issues. The conductors must be protected at their ampacity.

There is nothing, that I can find, in Article 450 that overrides the requirements of Article 240. 240.4 says protection must be provided per 310.15 except conditions listed in 240.4(A) to (G). 240.4(F) is for transformer secondary conductors, not the primary. 240.4(G) has "exceptions" for specific applications, like motors, but not for transformers.

I believe the primary conductors must be increased in size to be protected by the primary OCP unless one of the tap rules 240.21(B)is applicable

bob · May 13, 2005

Re: 3 phase Tranformers

CB

Why the 125% factor (unless the load is continuous, which in my scenario, is not a given)?

The extra 25% has nothing to do with continuous load. Its added to allow for the transformer inrush current.

john m. caloggero · May 13, 2005

Re: 3 phase Tranformers

Gentlemen: Perhaps the main points are being overlooked, and this is not uncommon when dealing with xformers and their associated conductors. When teaching OCP of transformers and conductors, a clear distinction must be made between the rule for protecting the TRANSFORMER, and rules for protecting CONDUCTORS. They are two sets of distinct conditions and rules. (1) The load must be calculated. (2) The wire size to handle the load. (3) Over current protection to protect the conductors at their ampacity. (4) A choice must be made whether the transformer installation is going to have PRIMARY protection only or PRIMARY AND SECONDARY OCP. (5) Next where is the location of the OCP devices as it pertains to the transformer?(6) Sometimes there is an advantage in using 250% primary and 125% secondary protection. (7) Sometimes you can kill two birds with one stone, protect the conductors and the transformer with the same overcurrent devices. So in summary in order to give a clear specific answer, one must know how the installer intends to design the system. There is a difference if the primary side is supplied by service conductors or a feeder. Most of the diagrams and commentary on transformers in the NFPA Handbook were done by me over the past 20 yrs.

kiloamp7 · May 13, 2005

Re: 3 phase Tranformers

For whatever it is worth, for plain jane applications of 600V. & below xfmrs:
Size pri. OCPD at an absolute minimum of 125% of rated FLA. Use reguler thermal-magnetic C/B or time-delay fuses.
Size primary conductors so that they are properly protected by that pri. OCPD per Art. 240.
There should be no mysterious excitation inrush tripping, it is a good practical design without wasteful overkill, & it meets NEC.
This general rule applies virtually regardless of the type load on the sec. or the rating or location of the OCPD for the sec. wires.

Generally there is nothing to be gained by going to 200% or 250% on pri. OCPD, but if you do, you'll need heep big wire & raceway for the pri.

[ May 14, 2005, 05:01 PM: Message edited by: kiloamp7 ]

charlie b · May 16, 2005

Re: 3 phase Tranformers

What I am reading now is what I had thought to be the truth. But I had heard other opinions, so I was not certain.

Short summary:

If you size the primary OCPD device at 250% of the rated primary current, then you must select primary conductors that are able to handle the same 250% of rated primary current.

That is the same as saying that the OCPD must be able to protect not only the transformer, but also the primary conductors.

templdl · May 16, 2005

Re: 3 phase Tranformers

Are you stating that you couldn't apply the 25' tap rule? One should consider that the secondary OCPD if sized correctly would protect the pri. conductors where you could apply the 25' tap rule.
Wouldn't Pri.OCPD, when sized at 125% as the sec. OCPD, be essentially redundant overload protection? Can the primary conductors be overloaded without a secondary load?
One would look at primary protection a taking the transformer off line should the transformer fail, i.e. short circuit.
If you can apply that 25' tap rule you don't need to increase the is of the conductors but would be able to apply the 250%.

steve66 · May 16, 2005

Re: 3 phase Tranformers

Charlie:

You can reduce the conductor size, but only for 25' per 240.21(B)(3).

And the 25' applies to the sum of:
(1) any primary wiring not protected at its ampacity.
(2) any secondary wiring not protected at its ampacity.

In your example, say we have 8' of #6 wire between the 125A breaker and the transformer. Then the secondary could only run 17' (25 minus 8) before hitting the overcurrent protection.

Steve

templdl · May 16, 2005

Re: 3 phase Tranformers

Then refer to the first reply that I posted to this string. It appears as though you are saying the same thing that I pointed out there.

jimmie · May 18, 2005

Re: 3 phase Tranformers

Thank you for all of your input. Reading the 2005 NEC commentary with regards to 450.3(B)helps. We're still technically using the 1999 code where I'm at. Thanks again. AKA: The Shock Doc

3 phase Tranformers

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