When to use 250.24(C) and 250.32(B)

[Salvador Santos]

Good day!

I have some questions regarding the Articles 250.24(C) and 250.32(B). I'm on an ongoing project that is required to build an independent power source to a military camp. We will have 6x1.25MVA Gensets, 380V, 3Ph feeding 6x1.25MVA Step up Transformer 380V Delta/15kV Wye then to substations Stepdown transformer 15kV Delta/380V wye. Our substation will have its own grounding system composed of 3 Grounding rods bonded together by 95mm2 grounding electrode. Our buildings which will have their own MDP will be fed by the substation by phase conductors, neutral conductor and grounding electrode.

Our designers sized the feeder grounding electrode based on 250.32(B), following the size of the ground cables from Table 250.11. In my understanding it should fall under Article 250.24(C) and the grounding electrode sizes should be based on Table 250.66.

My question are:
1. Where does the feeders for each building falls?
2. Should we not consider our cable us service feeders?
3. When do we consider our MDP and Sub DP as equipment or service equipment.
4. Generally, I'm just confused between 250.24(C) and 250.32(B)

Thanks a lot in advance.

 

[don_resqcapt19]

In my opinion, you do not have a "service" and nothing in 250.24 applies to your installation.

Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

 

[Smart $]

If you want to wire per NEC and unless you work for the utility company, grounding is all according to 250.30 and 250.32 for less than 1kV. For the 15kV stuff, see Part X.

 

[Salvador Santos]

Thanks for the reply.

I'll be working as a QC Engineer for the project and this will be my first time to encounter such situation and I don't want to proceed not knowing what to do.I want to add more questions.

1. Is the Grounded Electrode Conductor same as the Neutral Conductor?
2. I have notice that the sizing of the Neutral conductors are the same with the phase conductors? Is this ok?
3. As per my understanding the cable that runs from the grounding bus of the substation's switchboard going to the MDP grounding bus of the buildings will be considered as EGC and will be sized as per Table 250.112?

Again! Thanks for the help.

 

[don_resqcapt19]

Thanks for the reply.

I'll be working as a QC Engineer for the project and this will be my first time to encounter such situation and I don't want to proceed not knowing what to do.I want to add more questions.

1. Is the Grounded Electrode Conductor same as the Neutral Conductor?

No. The neutral conductor is a circuit conductor for line to neutral loads. The grounding electrode conductor connects the electrical system to a grounding electrode.

2. I have notice that the sizing of the Neutral conductors are the same with the phase conductors? Is this ok?

They are often the same size as the phase conductors, but are permitted to be smaller based on the neutral load calculation. The minimum size for a feeder neutral would be the same as an EGC that has been sized per 250.122.

3. As per my understanding the cable that runs from the grounding bus of the substation's switchboard going to the MDP grounding bus of the buildings will be considered as EGC and will be sized as per Table 250.112?

Again! Thanks for the help.

Correct.

 

[Salvador Santos]

Thank you so much Don for the replies.

I still have some questions in my mind and I hope you don't get tired of me.

There is a minimum of 5 buildings being supplied per substations and the minimum or the nearest distance of the buildings from the substation is 20 meters. As per our design, our feeder cables are composed of 3 phase cables, a neutral and an EGC. My questions are:

1. Do we really need to run EGC from substation switchboard to each building if in each building we have separate grounding system?
2. Can we localize per building's MDP OCPD the ground fault path by bonding the ground bus to the neutral bus (Just in case there is no need for the EGC to run from substation to building's MDP)?
3. I'm just thinking aloud, if both answers for 1 & 2 are yes, shouldn't the neutral cable per MDPs serves as a grounded electrode conductor from buildings to substations since the neutral cables will be bonded to the grounding electrode.

Again thanks for the help.

 

[Smart $]

Thank you so much Don for the replies.

I still have some questions in my mind and I hope you don't get tired of me.

There is a minimum of 5 buildings being supplied per substations and the minimum or the nearest distance of the buildings from the substation is 20 meters. As per our design, our feeder cables are composed of 3 phase cables, a neutral and an EGC. My questions are:

1. Do we really need to run EGC from substation switchboard to each building if in each building we have separate grounding system?
2. Can we localize per building's MDP OCPD the ground fault path by bonding the ground bus to the neutral bus (Just in case there is no need for the EGC to run from substation to building's MDP)?
3. I'm just thinking aloud, if both answers for 1 & 2 are yes, shouldn't the neutral cable per MDPs serves as a grounded electrode conductor from buildings to substations since the neutral cables will be bonded to the grounding electrode.

Again thanks for the help.

Don must have got tired of you

If you were a utility, you could do as you propose.

Otherwise, 1) yes... additionally, neutral and grounding in each building remain separate, 2) no... as a result of previous answer, and 3) under the NEC each building or structure has its own grounding electrode system (GES); the NEC does not cover interbonding of GES's.

 

[Salvador Santos]

Thank you Smart $.

I guess I'm to blame with Don's impatience.. I'm a Filipino whose working now under an American Project. I use to have a senior QC Electrical that handles these things but now, I'm the one handling everything:?. I'm not well versed with NEC 2008, I used to handle interior distribution only. So please be patient in me cause I'm just a newbie when it comes to exterior power distribution.:ashamed1:

 

[Smart $]

Thank you Smart $.

I guess I'm to blame with Don's impatience.. I'm a Filipino whose working now under an American Project. I use to have a senior QC Electrical that handles these things but now, I'm the one handling everything:?. I'm not well versed with NEC 2008, I used to handle interior distribution only. So please be patient in me cause I'm just a newbie when it comes to exterior power distribution.:ashamed1:

Not a problem... and I was just kidding about Don. He's usually very responsive, but as with all of us, other issues sometimes take priority. :angel:

 

[don_resqcapt19]

Out of town without my computer this weekend.

 

[Salvador Santos]

:lol:hahaha. Then I guess I have nothing to worry about. Thanks a lot to both of you and Smart $. Your supports were very helpful and very much appreciated. I'm trying to read more and understand the NEC. It's just English is my second language and I can easily get confused. So please bear with me if I ask for more questions later on.