Protection for Transformer Secondary conductors

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jkim780

Senior Member
Just curious, NEC 240.21(C) allows conductors to be connected to a xfmr secondary without overcurrent protection. So, if there is a fault in secondary conductors, what is going to protection them? what is the reason behind this?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
say you have a single phase 480/120V xfmr with 5A fuses on the primary side.

if the current in the secondary conductors exceeded 20A, the primary fuse(s) would blow protecting the secondary conductors. if 20A is adequate prtection for the secondary conductors, you are set.

this only applies to single phase 2 wire and 3phase delta-delta xfmrs.
 

jkim780

Senior Member
petersonra,

Doesn't taht only apply to xfmr protection (xfmr wing)? What if you have a fault within 25ft of secondary conductors when you installed them per 240.21(C)(6)? What is going to protect 25ft of secondary conductors? Why they are allowing these conductors without protection?
 

jtester

Senior Member
Location
Las Cruces N.M.
I think the Code recognizes there will be some distance between the transformer and the OCPD, unless the transformer comes with one internal to it. The question then becomes how far can you go without significantly increasing the likelihood of a problem?

Under various conditions the powers that be have decided different distances, but have recognized there has to be some distance allowed.

Jim T
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
jkim780 said:
petersonra,

Doesn't taht only apply to xfmr protection (xfmr wing)? What if you have a fault within 25ft of secondary conductors when you installed them per 240.21(C)(6)? What is going to protect 25ft of secondary conductors? Why they are allowing these conductors without protection?

Lets look at a hypothetical situation; take a 480-208V delta/delta transformer w/ primary conductors of #2AWG cu, and secondary conductors of #4/0AWG.

Primary protection of the transformer is a thermal-mag breaker rated for 125A. The system has an available fault current of 65kA.

Worst case, looking at the short circuit contribution, for a fault on the secondary side, at the end of the 25ft (closest to the 208V bus) the maximum fault current through the transformer and through the cable to the fault would be roughly 5000 A. By looking at the cable damage curve for this cable, the cable can withstand 5000 A for 6.8secs. In that case the primary side breaker would have tripped nearly instantaneously. Keeping in mind that 5000 A secondary is about 2000 A primary. (transformers act like short circuit chokes)

If the fault current was less on the primary, the primary breaker might take slightly longer to trip, but in all cases (except equipment failure) it would trip sooner then the cable damage curve. So, excpet for the portion of the faulted cable, the remainder could be salvaged and reused.
In both cases the primary breaker also trips prior to transformer damage.
Therefore, the cable is protected despite that no secondary protection is applied.
 

jkim780

Senior Member
jtester said:
I think the Code recognizes there will be some distance between the transformer and the OCPD, unless the transformer comes with one internal to it. The question then becomes how far can you go without significantly increasing the likelihood of a problem?

Under various conditions the powers that be have decided different distances, but have recognized there has to be some distance allowed.

Jim T

So that cable length is a some random number that they picked out just like 25 ohms in ground rod..
 

jkim780

Senior Member
kingpb said:
Lets look at a hypothetical situation; take a 480-208V delta/delta transformer w/ primary conductors of #2AWG cu, and secondary conductors of #4/0AWG.

Primary protection of the transformer is a thermal-mag breaker rated for 125A. The system has an available fault current of 65kA.

Worst case, looking at the short circuit contribution, for a fault on the secondary side, at the end of the 25ft (closest to the 208V bus) the maximum fault current through the transformer and through the cable to the fault would be roughly 5000 A. By looking at the cable damage curve for this cable, the cable can withstand 5000 A for 6.8secs. In that case the primary side breaker would have tripped nearly instantaneously. Keeping in mind that 5000 A secondary is about 2000 A primary. (transformers act like short circuit chokes)

If the fault current was less on the primary, the primary breaker might take slightly longer to trip, but in all cases (except equipment failure) it would trip sooner then the cable damage curve. So, excpet for the portion of the faulted cable, the remainder could be salvaged and reused.
In both cases the primary breaker also trips prior to transformer damage.
Therefore, the cable is protected despite that no secondary protection is applied.

kingpb,

will the xfmr primary breaker protect the secondary conductors regardless of primary condutors length?
 

jkim780

Senior Member
This has been confusing for me for sometime, so please correct me if I am wrong.

If you have a circuit breaker protecting the xfmr primary conductor (sized 125% or next higher standard rating), that circuit breaker can also provide protection for the xfmr (primary & secondary winding per table 450.3(B) primary only protection) and the xfmr secondary conductors.

I guess the part make me confused is 240.4(F) which permits the secondary circuits conductors from a transformer to be protected by overcurrent devices in the primary circuit conductors of transformer only in two following cease
1) 2-wire pri and 2-wire sec.
2) 3-ph, delta/delta with a 3-wire, single voltage secondary.

When you have a 480 delta to 120/208 wye 3-ph, 4-wire xfmr, the above two rules don't apply, so the primary circuit breaker can't protect secondary conductors. Does it? Please help me understand.
 
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jtester

Senior Member
Location
Las Cruces N.M.
jkim780 said:
So that cable length is a some random number that they picked out just like 25 ohms in ground rod..

I don't know how they decided on the numbers, I doubt they were random. As in the case of the 25 ohms, we may be just too young to have been around when it was first figured out, and I'm 55.

Jim T
 

coulter

Senior Member
jkim780 said:
... if there is a fault in secondary conductors, what is going to protection them?
jkim -
The answer is "there is none."

Let's think about this a little bit. How do you get a fault in the secondary conductors? Backhoe? Drill rig? Once they get hit with a backhoe, they are faulted.

Now, What is there to protect if you do get a fault? Certainly not the conductors. They're finished. All you want to do is get the fire put out and stop any further equipment damage. The primary OCPD will do that.

Now, what about protecting the secondary conductors from overload? The secondary OCPD will do that no matter how far it is from the xfmr

carl
 

jkim780

Senior Member
coulter said:
jkim -
The answer is "there is none."
Let's think about this a little bit. How do you get a fault in the secondary conductors? Backhoe? Drill rig? Once they get hit with a backhoe, they are faulted.
????
There are plenty ways to get a fault. How about bad connection or bad insulation for starter?

coulter said:
Now, What is there to protect if you do get a fault? Certainly not the conductors. They're finished. All you want to do is get the fire put out and stop any further equipment damage. The primary OCPD will do that.
How about people's life or property? You have to remove a ground fault quickly form metal parts of electrical equipment, otherwise it will remain energized providing the potential for electric shock and fire.

coulter said:
Now, what about protecting the secondary conductors from overload? The secondary OCPD will do that no matter how far it is from the xfmr
There is no secondary OCPD if a xfmr is installed per 240.21(C). My question was why the code allows the xfmr secondary conductors without overcurrent protection?
 

jkim780

Senior Member
jtester said:
JKIM

Where would you propose to put the secondary conductor OCPD in the case of a transformer installation?

Jim T

jtester,

I see your point. But that was not the question that I had in mind. My concern was If 240.21(C) was conflict with 240.4(F) or not. See below for my question. Thanks.

This has been confusing for me for sometime, so please correct me if I am wrong.

If you have a circuit breaker protecting the xfmr primary conductor (sized 125% or next higher standard rating), that circuit breaker can also provide protection for the xfmr (primary & secondary winding per table 450.3(B) primary only protection) and the xfmr secondary conductors.

I guess the part make me confused is 240.4(F) which permits the secondary circuits conductors from a transformer to be protected by overcurrent devices in the primary circuit conductors of transformer only in two following cease
1) 2-wire pri and 2-wire sec.
2) 3-ph, delta/delta with a 3-wire, single voltage secondary.

When you have a 480 delta to 120/208 wye 3-ph, 4-wire xfmr, the above two rules don't apply, so the primary circuit breaker can't protect secondary conductors. Please help me understand.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
jkim780 said:
kingpb, will the xfmr primary breaker protect the secondary conductors regardless of primary condutors length?

Longer cable lengths reduce fault current levels becasue of the additional impedance in the cable. so adding cable is not the issue.

I am working on modeling this situation, trying to determine exactly where the "length" rules come from, and will post as soon as I have time to complete it.
 

coulter

Senior Member
jkim -

jkim780 said:
There are plenty ways to get a fault. How about bad connection or bad insulation for starter? ...

Examine what you mean when you say, "fault". Bad insulation turns into a fault. A bad connection turns into a fault. If your definition of "fault" is anywhere near mine, then faulted conductors are finished - there is nothing in good working order to save.

jkim780 said:
How about people's life or property? You have to remove a ground fault quickly form metal parts of electrical equipment, otherwise it will remain energized providing the potential for electric shock and fire.
Of course. I didn't see that in your op. If that is what you want to protect against, then you should be designing in GFP. Maybe you would want zero-sequence CT at the xfmr secondary. This is different than dealing with faulted conductors.

jkim780 said:
There is no secondary OCPD if a xfmr is installed per 240.21(C). My question was why the code allows the xfmr secondary conductors without overcurrent protection?

Lets start with 90.1(A), (B), (C). The NEC readily admits, it is not absolute safety, it is not necessarily efficient, it is not a design guide.

One of our posters made a particularly insightful comment - I'll paraphrase, "Saying that an installation is built per code means that if it were one bit worse, it would be illegal." One of my comments was, "No I didn't build it per code, I couldn't stand that level of un-reliability."

NEC is minimums - Might be okay for your house - generally it is way too light for industrial. For example, a recent poster was looking for advice on replacing burned up conductors on a 2MW (?) gen. He was really looking to minimize the copper because the cost of copper is high. I never got a chance to answer. I wanted to advise him, that minimums were not a good idea - minimums burned up once.

Now let's take a look at 240.21 tap rules and 450.3 xfmr OCP. Especially see 450.3, FPN 1 (NEC 2002). This stuff is some serious power, commercial buildings, industrial facilities, process power and control.

IMO when you are dealing with designs and installations concerned with tap rules and xfmrs, it is up to the designer to make it safe and reliable. The NEC won't save you or make you safe.

My design and build philosophy is to:
1. Make it so it will do the intended job.
2. Make it safe for the intended user.
3. Keep it contained so it doesn't crap up the environment.
4. Make it reliable to keep the life cycle costs down.

My theory is that if you do those four things, you are well past code.

jkim780 said:
My question was why the code allows ... (pick any subject you want)
The NEC is not a design guide. The AHJs are not designers. One size does not fit all installations. When an installation gets to this level (tap rules and xfmrs), the customer and the designer figure out what is important.

For example, you started with conductor protection and faults. Then you switched to ground fault and personnel protection. Well what is the application? Commercial highrise, or industrial process? One general answer does not fit all.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
jkim,
There is no secondary OCPD if a xfmr is installed per 240.21(C).
Lets be careful here. 240.21 has nothing to do with OCPD for the transfomer. Those rules are found in 450.3. The rules in 240.21 only apply to the conductors and the rules in 450.3 only apply to the transformer. The installer must satisify both sets of rules.
Don
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
jkim,
My concern was If 240.21(C) was conflict with 240.4(F) or not. See below for my question.
There is no conflict. 240.21(C) provides the requirements to protect the secondary conductors where they are not protected by the primary OCPD. Yes there is some length of unprotected conductors, but according to the code the risk is acceptable.
Don
 

coulter

Senior Member
jkim780 said:
...When you have a 480 delta to 120/208 wye 3-ph, 4-wire xfmr, the above two rules don't apply, so the primary circuit breaker can't protect secondary conductors...
jkim
Three issues here - you have to look at all.

1. "...the primary circuit breaker can't protect secondary conductors..." Add "from overload". Yes that is true.

Also true are:

2. The primary circuit breaker will protect the personnel and equipment from secondary conductor fault.

3. The secondary OCPD will protect the secondary conductors, the transformer, and the primary conductors from overload and from faults down stream from the sec OCPD.

What protects the secondary conductors from a secondary conductor fault? As I said earlier - whoops, I'm repeating myself - you will hve to go back and read it.

jkim - A question for you. How would you induce an overload on the secondary conductors between the xfmr and the secondary CB and still have some conductors worth protecting?

As for the length of the tap conductors, IMO the code panels used an ouija(sp) board or dart board to pick the numbers. Do I think the numbers are bad? No - not good, not bad, just are. The selections are okay - they work and are not onerous(sp)
 
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