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?
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
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.
jkim780 said:So that cable length is a some random number that they picked out just like 25 ohms in ground rod..
jkim -jkim780 said:... if there is a fault in secondary conductors, what is going to protection them?
????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.
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 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.
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?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
jtester said:JKIM
Where would you propose to put the secondary conductor OCPD in the case of a transformer installation?
Jim T
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.
jkim780 said:kingpb, will the xfmr primary breaker protect the secondary conductors regardless of primary condutors length?
jkim780 said:There are plenty ways to get a fault. How about bad connection or bad insulation for starter? ...
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: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.
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?
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.jkim780 said:My question was why the code allows ... (pick any subject you want)
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.There is no secondary OCPD if a xfmr is installed per 240.21(C).
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.My concern was If 240.21(C) was conflict with 240.4(F) or not. See below for my question.
jkimjkim780 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...
Don -don_resqcapt19 said:...according to the code the risk is acceptable.