System bonding jumper required at the source?

[Jpflex]

NEC 250.30 A1 a system bonding jumper shall be made at any point of the separately derived system from the source to the first disconnecting means OR IT SHALL BE MADE AT THE SOURCE OF A SEPARATELY DERIVED SYSTEM THAT HAS NO DISCONNECTING MEANS OR OVERCURRENT DEVICE


what is meant by “no overcurrent protection or disconnecting means?” A separately derived transformer system must have over current at the primary source windings or both primary and secondary windings so this statement makes no sense unless they mean only secondary conductors?

And I’m not sure what they mean about not having a disconnect do they mean disconnect upstream of transformer or below as in the distribution panel? The code apparently leaves this to interpretation?

Also separately derived systems located outdoors away from structure requires a grounding electrode conductor installation at the outdoor source? Doesn’t this grounding electrode connection at the outdoor separately derived outdoor system force the system bonding jumper to be also located at outdoor source? Thanks

 

[don_resqcapt19]

There are cases where the primary OCPD can protect the transformer primary, transformer secondary and the secondary conductors. This can be done where the transformer is a single phase two wire to two wire transformer, or a three phase delta, three wire to three wire transformer. This would require the primary OCPD to be rated at 125% or less of the primary current and the secondary conductors to have an ampacity at least equal to the rating of the primary OCPD times the turns ratio.

 

[Jpflex]

There are cases where the primary OCPD can protect the transformer primary, transformer secondary and the secondary conductors. This can be done where the transformer is a single phase two wire to two wire transformer, or a three phase delta, three wire to three wire transformer. This would require the primary OCPD to be rated at 125% or less of the primary current and the secondary conductors to have an ampacity at least equal to the rating of the primary OCPD times the turns ratio.

Why does the code say 3 phase delta 3 wire to 3 wire single phase voltage?

 

[augie47]

Why does the code say 3 phase delta 3 wire to 3 wire single phase voltage?

Where does the Code say that ?

 

[petersonra]

Why does the code say 3 phase delta 3 wire to 3 wire single phase voltage?

it does not say that.

single phase 2 wire, or 3 wire delta to delta.

 

[don_resqcapt19]

Why does the code say 3 phase delta 3 wire to 3 wire single phase voltage?

I did not say 3 wire to 3 wire single phase, I said "or a three phase delta, three wire to three wire transformer".

With single phase two wire to two wire transformers and three phase delta, 3 wire to 3 wire transformers, the secondary current is reflected to the primary based on the turns ratio, and the primary OCPD can protect the secondary conductors. Where you have different configurations, the secondary current can overload the secondary conductors and windings without causing the primary OCPD to open the circuit.

 

[Jpflex]

Where does the Code say that ?

NEC 240.21 5 C transformer secondary conductors

A set of secondary conductors feeding a single load or each set of conductors feeding separate loads may not have to have OCPD at the secondary

1) conductors supplied by the secondary side of a single phase transformer having 2 wire single voltage secondary or *******A 3 PHASE DELTA HAVING SINGLE VOLTAGE SECONDARY shall be permitted OCPD on primary only

Therefore, my question was why would NEC call a 3 phase delta a single voltage when there are 3 individual voltage 120 degrees apart?

 

[Krusscher]

NEC 240.21 5 C transformer secondary conductors

A set of secondary conductors feeding a single load or each set of conductors feeding separate loads may not have to have OCPD at the secondary

1) conductors supplied by the secondary side of a single phase transformer having 2 wire single voltage secondary or *******A 3 PHASE DELTA HAVING SINGLE VOLTAGE SECONDARY shall be permitted OCPD on primary only

Therefore, my question was why would NEC call a 3 phase delta a single voltage when there are 3 individual voltage 120 degrees apart?

I think it means a 3 phase system that only provide one voltage like not 240/120 or 480/277 but just straight 480 or 240 so no neutral.

 

[augie47]

The reason I asked (post #4) was to encourage you to reread the NEC Section as you has misquoted giving a different meaning.
A 3 phase delta-delta transformer will have a single voltage secondary as opposed to a wye secondary with more than one secondary voltage available.

You: Why does the code say 3 phase delta 3 wire to 3 wire single phase voltage?
NEC: a three-phase, delta-delta connected transformer having a 3-wire (singlevoltage) secondary,

 

[Jpflex]

The reason I asked (post #4) was to encourage you to reread the NEC Section as you has misquoted giving a different meaning.
A 3 phase delta-delta transformer will have a single voltage secondary as opposed to a wye secondary with more than one secondary voltage available.

You: Why does the code say 3 phase delta 3 wire to 3 wire single phase voltage?
NEC: a three-phase, delta-delta connected transformer having a 3-wire (singlevoltage) secondary,

Last week was my first time installing load breakers for the primary connection of a 4800 volt primary, 480 volt secondary delta delta ungrounded system pad mounted transformer rated at 300 KVA. I assisted and worked with our newly hired lineman in doing this.

One thing he did not know was what exactly to do with the small conductors under the first layer of the primary conductor insulation on the load breaker primary side of termination to the transformer? Initially he had me connect them together and to the pad mounted ground rod.

However the lineman (who has a lineman card) later had me disconnect and separated them in an individual bundle of tape without having a connection because he found 800 volts from a secondary phase to the transformer secondary ground rod?

We have not yet figured out what would cause this secondary voltage from a delta delta transformer that should be the same from either line to line voltage or phase voltage at a target of 480 volts secondary?

However I can say that the person who originally ran wires apparently did not run a supply side bonding jumper from this (ungrounded separately derived SYSTEM but cabinet grounded only) outdoor pad mounted transformer to the first point of disconnect.

Also the line man said some feeder wires going to buildings from transformer secondary are too high exceeding well beyond 500 volts plus while the lab building I’m working at is getting too small voltage

The lab I’m working at supplied by the 3 phases is apparently too low and is causing the lab’s single phase panel to only get 50 volts from phase to neutral and 100 volts from phase A to B. There is half the voltage necessary present here

there is another smaller transformer at this lab which is using phase A and C? from the 3 phases of the first pad mounted transformer mentioned but because the voltage on its primary is only 200 volts, I’m getting half the values mentioned at the labs single phase 120/240 panel.

Also the neutral for the single phase panel at the lab is derived from the mid point of secondary windings of this second transformer at the lab and is terminated at the single phase panel neutral bus/single point system bonding jumper as normal and a ground strap connects EGC bus and GEC to a ground rod

Could this set up be causing the 800 volts to ground on the upstream first pad mounted transformer, since the secondary derived neutral of the second smaller single phase transformer 50kva? at the lab is bonded to the single phase panel at this lab via the required EGC bonding jumper and again bonded from single phase panel to the labs 3 phase panel via gutter and EMT connections and again bonded back to the upstream source equipment via an EGC required to be ran with the ungrounded 3 phase feeders from the source?

So the main question is why are voltages values upstream at first source screwy?

 

[augie47]

Phase to ground voltage reading on an ungrounded system can vary greatly depending on circumstances and the type instrument used to meaasure them and, IMO, are somewhat meaningless.