XMER PRIMARY&SECONDARY PROTECTION

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I need some help defining the need for primary and/or secondary overcurrent protection of a transformer. Here's my scenerio:

High Voltage panel feeds a 480 to 208/120 transformer which in turns of course is used to feed multiple Low Voltage panels. All swithgear is located in the same room. Normally a typical design would include a circuit breaker in the HV panel to feed the Xmer and a fused disconnect on the secondary side of the Xmer as well. Lately (and mainly as a way of cutting $$) I've been asked to examine the need to have both the primary and secondary OCPD's especially since all equipment is in the same room therefore one could easily cut power to the Xmer from the primary CB.

If the Xmer power can be cut by means of the CB could I eliminate the secondary fused disconnect?

Thanks.
 

ronaldrc

Senior Member
Location
Tennessee
Re: XMER PRIMARY&SECONDARY PROTECTION

If you have a delta primary and a wye secondary with a neutral you have to have your secondary overcurrent protection in the secondary also.

See 240.4 (F) Protection of conductors.

Ronald :)
 

steve66

Senior Member
Re: XMER PRIMARY&SECONDARY PROTECTION

You may be able to use the tap rules in 240.21 to eliminate the need to protect the secondary conductors. Try reading 240.21(C).

You may still need a main circuit breaker to protect the panelboards. See 408.14 and 408.16 to see if you need a MCB on each panel.

Steve
 

ronaldrc

Senior Member
Location
Tennessee
Re: XMER PRIMARY&SECONDARY PROTECTION

Linda and Steve its not the lenght its the voltage ratio between the primary and secondary with 120 volts you would have a ratio of 1 to 4 you can exceed the primary current of the primary breaker twice.

That is if you protect the transformer like you should with the primary breaker.

Ronald :)
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Re: XMER PRIMARY&SECONDARY PROTECTION

First, if you use a primary OCPD of not greater than 125% of the rated primary current, then the secondary of the transformer does not require protection. However even though you don't have to provide protection for the secondary of the transformer, you do have to provide protection for the secondary conductors. If the length of the conductor pemits you could use 240.21(C)(2) or (6). Note that the length in these rules is the total end to end length of the conductor.

Ron,
Why do you say the length doesn't matter? Are you looking at the rule in 240.21(C)(1)? If so that rule does not apply to this installation.
Don
 

jim dungar

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Location
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Retired Electrical Engineer - Power Systems
Re: XMER PRIMARY&SECONDARY PROTECTION

Transformers 600V or less never, ever require secondary protective devices. So we do not have to look at Art 450 any more (of course 450-3(b)still applies)

However, all lighting and branch circuit panelboards require a main device at their supply(actually up to two), see Art 408-16.

Now, for the important stuff,
All secondary conductors must be protected per Art 240.4(F). Art 240.21(C) dictates the location of the protective device for the secondary conductors. 240.21(C)(1) is not applicable, it actully restates 240.4(F). 240.21(C)(2), and (3) allow the secondary conductors to be protected by the main device in a panel (or even multiple mains) based on length and grouping. 240.21(C)(6) is also applicable except multiple protective devices are not allowed.
 

ronaldrc

Senior Member
Location
Tennessee
Re: XMER PRIMARY&SECONDARY PROTECTION

Don and Jim Lindas' question was rather she needs overcurrent protection in the secondary not where in the secondary.

Ronald :)
 

jim dungar

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Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: XMER PRIMARY&SECONDARY PROTECTION

Linda's transformer does not need any secondary protection, her primary protection will be limited to 125% primary FLA.

Linda's panels will each need to have main devices if they are lighting and branch circuit panels (which they probably are).

Linda's secondary conductors need to be protected, however I do not have enough info to give a definitive answer, so some possible answers are:

If the panels are located not more than 10' from the transformer terminals, then the panel mains will provide the required protection in all installations. Art 240.21(C)(2)

If this is an industrial installation the length to the panels can be increased to 25'. The panels need to be grouped together and the sum of the devices cannot exceed the transformer secondary FLA. Art 240.21(C)(3)
 

ronaldrc

Senior Member
Location
Tennessee
Re: XMER PRIMARY&SECONDARY PROTECTION

Jim I didn't say she needs to protect the windings but that was the point since you brought it up lets talk about that.

Since she didn't mention amperage lets assume she is talking a 120/208 , 200 amp. 3 phase secondary wye with a neutral.

A 100 amp. breaker will protect the primary or any load on the secondary at 208 thats fine. But at 120 volts to the neutral the secondary can be loaded to 400 amps. before the primary breaker would see 100 amps. and would smoke both windings of the transformer.


Ronald :)
 

jim dungar

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Location
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Retired Electrical Engineer - Power Systems
Re: XMER PRIMARY&SECONDARY PROTECTION

Ronald,

How could the secondary be loaded to 400A? The transformer has a line to line voltage ratio of 1:.4333 (208/480) so the maximum line current ratio would be 1:2.3.

A typical transformer for your example would be a 75KVA 480V to 208Y/120. Primary amps would be 90A and therefore would be protected by your primary 100A device. The secondary windings would be rated for 208A each.

Art 450.3(B) does not require protection of the transformer secondary windings.

Art 240.4(F) does require protection of the conductors connected to this transformer secondary.

Art 240.21(C)(2) allows the conductors to be sized and protected at a minimum to serve the load. It is allowable to connect 10' of 1000A conductors and a 1000A OCPD to the secondary of the 75KVA transformer under discussion.

Art 240.21(C)(3) allows an industrial location to have 25' of 208A conductors protected by multiple OCPDs totaling less than 208A.

Art 240.21(C)(6) allows 25' of conductors rated at a minimum of 76.9A ([100/3]*[480/208])terminating in a single equally sized device.
 

ronaldrc

Senior Member
Location
Tennessee
Re: XMER PRIMARY&SECONDARY PROTECTION

Hi Jim
Sorry about the delay yesterday got tied up and didn't have time to respond back like I wanted to.

Jim I read the code and I know where I need to use a disc. to protect the secondary conductors and where I don't.

But I have a problem with the reason.And I would like to talk to someone that knows what there doing and most of the regulars on here do Bennie,Bob, both Charlie,
Don,Ed, Jim and Roger that the ones I know best.

Please look in the 2002 handbook at 240.4(F) and read it, it tells you can not use a primary OCD to protect the secondary conductors.

Now look at the forth para. in the highlighted explaination of why you can't and think about the neutral and how it balances and then tell me if you agree with this?


Ronald :)

[ April 08, 2004, 05:14 PM: Message edited by: ronaldrc ]
 

jim dungar

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Location
Wisconsin
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Retired Electrical Engineer - Power Systems
Re: XMER PRIMARY&SECONDARY PROTECTION

Ronald,

In your transformer posting, yes your calculations are correct for the 25KVA unit.

The reason for the concern of a 2 wire L-N overload not being protected by the primary is valid because a transformer will produce as much current as required (limited by it's magnetic circuit) until it "melts" or the primary protective device operates. In the handbook, and your example, the transformer has two primary windings connected in series and protected at 50A. The secondary has two 104A windings connected in series creating a 2:1 transformer with a center tap (480-120/240). But if only one winding is loaded (L-N) then what we really have is a 4:1 ratio (480 to 120V) transformer which would try to provide a secondary current of 200A (50 x 4) before the primary saw any overload.

The secondary conductor protection required by Art 240.4(F) is further defined under the "tap rules" in Art 240.21(C).
 

ronaldrc

Senior Member
Location
Tennessee
Re: XMER PRIMARY&SECONDARY PROTECTION

Hello Jim

I understand what you are saying about the one 120 volt winding being used by itself and being overloaded before the primary sees an overload.

I think the chances of that ever happening would be very slim, but if it can be done someone will somewhere.

Thanks :)
 

jschultz

Member
Re: XMER PRIMARY&SECONDARY PROTECTION

Originally posted by jim dungar:
However, all lighting and branch circuit panelboards require a main device at their supply(actually up to two), see Art 408-16.
What about exception #1?
 

jschultz

Member
Re: XMER PRIMARY&SECONDARY PROTECTION

seems kind of like this discussion is making it really hard for people to follow.

She is most likely going to need overcurrent protection on the secondary side. Unless she has one of the odd transformers mentioned in 240. Whether it is for transformer protection, conductor protection, located in our out of the panelboard, is not the question.

Secondary protection is or is not required is the question.

Now if she wants to knwo where it can be located then we can answer that, and which sections of the code require it. yada yada.

SO my answer is yes, you need secondary overcurrent protection.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: XMER PRIMARY&SECONDARY PROTECTION

Jschultz,

408.16(A) exception #1 is almost superfluous. It doesn't change the requirement to provide a main device it only says it can be the same one that protects the panelboard feeder. In my opinion this gives permission to have multiple panelboards protected by one "main" rather than one per panel.

If Linda has several panelboards with main breakers which are located within 25ft of cable of the transformer, then a single secondary device is not required per 240.21(C)(3). In this instance, the sum of the ratings of the panelboard mains can not exceed the output rating of the transformer

There is absolutely no 450.3(B)requirement for any transformer 600V or less to ever have secondary overcurrent protection.
240.4(F) does require conductors on the multi-voltage secondary of a transformer to have overcurrent protection. 240.21 lists the requirements for the location and sizing of this required protection. A grouping of panelboard main devices would meet 240.4(F) if they are located per 240.21(C).
 
Re: XMER PRIMARY&SECONDARY PROTECTION

Jim,

If secondary protection of a xmer less than 600 v is never required, then when would you ever apply method 2 primary/secondary protection of Table 450.3(B)??

Thanks for all the help - everyone.

Linda
 
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