System Bonding Jumper Size in Relation to Grounding Electrode Conductor Size

[EEric]

My question is with regards to the installation of a generator, but I think the same question could apply to any separately derived system.

My generator is rated at 1000KVA. I sized the system bonding jumper at 250kcmil using table 250.102(C)(1) (x3 parallel 600kcmil phase conductors).

I sized the grounding electrode conductor at 3/0AWG using table 250.66.

It seems odd to me that the system bonding jumper must be sized larger than the grounding electrode conductor given that these two conductors essentially form a series circuit between the neutral of the generator and the grounding ring system. Is there a code provision that I am missing, or a reason for the for the larger system bonding jumper size that I am not considering? Is this just the way it is and should I stop questioning it?

Thank you in advance!

 

[augie47]

They serve different purposes. A grounding electrode limits the voltage imposed by line surges or lightning and is limited by earth resistance whereas the SBJ provides a low impedance path back to the source in the event of a fault.

 

[Dennis Alwon]

My question is with regards to the installation of a generator, but I think the same question could apply to any separately derived system.

My generator is rated at 1000KVA. I sized the system bonding jumper at 250kcmil using table 250.102(C)(1) (x3 parallel 600kcmil phase conductors).

I sized the grounding electrode conductor at 3/0AWG using table 250.66.

It seems odd to me that the system bonding jumper must be sized larger than the grounding electrode conductor given that these two conductors essentially form a series circuit between the neutral of the generator and the grounding ring system. Is there a code provision that I am missing, or a reason for the for the larger system bonding jumper size that I am not considering? Is this just the way it is and should I stop questioning it?

Thank you in advance!

In older versions of the NEC the gec and the bonding jumper were exactly the same and were determined by 250.66. Someone must have done some engineering to show that the bonding jumpers were a different story once you got up into the larger wire sizes. The table 250.102(C) is the same as T. 250.66 except for the last entries.

 

[infinity]

The practical reason is that the GEC to the electrode isn't doing much and in some cases it's next to worthless. If your GES was two ground rods and your GEC to the generator is a #6 how much could that actually be doing? The SBJ on the other hand is instrumental in ensuring that the OCPD opens during a fault.

 

[jaggedben]

...
... [ system bonding jumper and the grounding electrode conductor] ... these two conductors essentially form a series circuit between the neutral of the generator and the grounding ring system.

In addition to what else has been said, this is incorrect. Exceptions notwithstanding, both conductors by default normally connect directly to the neutral.

 

[bdm5066]

@jaggedben -> the highlighted snip is taken from the 2017 NEC. Is that not a series connection from the generator neutral point to the grounding electrode (ground ring in this case)?

 

[infinity]

View attachment 2571037 @jaggedben -> the highlighted snip is taken from the 2017 NEC. Is that not a series connection from the generator neutral point to the grounding electrode (ground ring in this case)?

Do you mean a series connection with the service equipment? Since it's a 4 pole transfer switch the service neutral connection is removed when you're on generator power.

 

[Dennis Alwon]

Since it is a separately derived system it should be treated as a new service in that a gec would be required to either rods or other compliant means of grounding electrodes.

Not sure wat you mean by series connection.

 

[infinity]

I'm guessing by series connection he's talking about both the service and the SDS have GEC's that are connected to the same electrode which is a ground ring.

 

[EEric]

I may not have used "series" correctly, but my initial mention of a series connection was referring to the diagram posted by bdm0566 showing a direct path from the neutral of the generator, through the SBJ, then through the GEC, to the ground ring system.

That being said, I think I do have an understanding now of why the SBJ and GEC are sized differently.

The function EGC is to mitigate any voltage spikes on the generator enclosure, and the function of the SBJ is to ensure a low impedance path for fault current, allowing the OCPD to operate effectively. Sometime in NEC history it was determined that the SBJ requires a larger conductor size compared to the GEC once the phase conductor is larger than 1100kcmil (copper).

 

[Dennis Alwon]

I may not have used "series" correctly, but my initial mention of a series connection was referring to the diagram posted by bdm0566 showing a direct path from the neutral of the generator, through the SBJ, then through the GEC, to the ground ring system.

That being said, I think I do have an understanding now of why the SBJ and GEC are sized differently.

The function EGC is to mitigate any voltage spikes on the generator enclosure, and the function of the SBJ is to ensure a low impedance path for fault current, allowing the OCPD to operate effectively. Sometime in NEC history it was determined that the SBJ requires a larger conductor size compared to the GEC once the phase conductor is larger than 1100kcmil (copper).

I assume you meant gec.

 

[EEric]

I assume you meant gec.

Yep, my bad - should have said "the function of the GEC".

 

[infinity]

I think I do have an understanding now of why the SBJ and GEC are sized differently.

Yes basically the GEC is really not doing much during the normal operation of the system but the SBJ is. Same would apply to a large service say 4000 amps that only has a #6 GEC going to a pair of ground rods yet the MBJ is 750 kcmil.

 

[jaggedben]

View attachment 2571037 @jaggedben -> the highlighted snip is taken from the 2017 NEC. Is that not a series connection from the generator neutral point to the grounding electrode (ground ring in this case)?

My point was that the configuration shown is not required and is in fact only allowed through an exception, even though it's extremely common. The SBJ bonds the neutral to the metal parts. The GEC connects the neutral to earth. Conceptually they start out as separate parallel connections; by default 250.30(A)(5) requires the GEC to connection directly to the neutral (grounded conductor.) Then there's the exception, allowing the GEC to be connected to the neutral through the SBJ when the SBJ is a wire or busbar.

 

[TX+ MASTER#4544]

My question is with regards to the installation of a generator, but I think the same question could apply to any separately derived system.

My generator is rated at 1000KVA. I sized the system bonding jumper at 250kcmil using table 250.102(C)(1) (x3 parallel 600kcmil phase conductors).

I sized the grounding electrode conductor at 3/0AWG using table 250.66.

It seems odd to me that the system bonding jumper must be sized larger than the grounding electrode conductor given that these two conductors essentially form a series circuit between the neutral of the generator and the grounding ring system. Is there a code provision that I am missing, or a reason for the for the larger system bonding jumper size that I am not considering? Is this just the way it is and should I stop questioning it?

Thank you in advance!

2023 NEC

3 600 kcmil conductors x 3 = 1800 kcmil, this exceeds the copper rating of "...over 1100 kcmil.." found in the copper column as noted in T.250.102 (C) (1).

Because the 1100 kcmil exceeds the....over 1100.... The table footnote number 1 says when you exceed that rating (1100 ) the grounded

conductor( neutral ) or bonding jumper shall have an area not less than 12 1/2 % (0.125 ) of the ....equivalent area for parallel supply conductors.

Thus......3 600 kcmil x 3 = 1800 kcmil, 1800 x 0.125 = 225 kcmil. Proceed to Chapter 9 and T.8 Conductors and find the circular mils area greater than 225 kcmil which would be a 250 kcmil conductor. That is also the size of your bonding jumper, too.

You don't need a ground rod.

Check the caption at the top of T.250.102(C), it is for sizing neutral (grounded) and MBJ and system BJ and SSBJ.
It is for conductors as small as 2 AWG or smaller and through size 1100 copper or 1750 aluminum, then you have to apply the 12 1/2 %.

The grounding electrode conductor was selected from T.250.66. There, you max out at a 3/0 copper, there's no 12 1/2% to consider.

A 1,000 KVA generator is an extremely large generator. 1,000KVA x 1,000 = 1,000,000 VA. Assuming the voltage is a standard 4160 voltage and 3 phase of course ......4160 volts x 1.732 = 7205 that's a three phase divider to find the amps.

1,000,000 VA / 7205 = 139 amps. T.450.3(A). For primary protection (OCPD) and not more than 6% impedance and in a unsupervised location and using a fuse rating of 300% would be....139 amps x 300 % = 417 amp fuse.

T.450.3(A) and note number 1 allows for a next size up and T.240.6 (A) List a 450 amp to be permitted.
It is under the 800 amp rule found in240.(B) (1)(2)(3). This is for the primary side only and not the secondary side.

Thanks for reading
Comments accepted
TX + MASTER #4544

My question is with regards to the installation of a generator, but I think the same question could apply to any separately derived system.

My generator is rated at 1000KVA. I sized the system bonding jumper at 250kcmil using table 250.102(C)(1) (x3 parallel 600kcmil phase conductors).

I sized the grounding electrode conductor at 3/0AWG using table 250.66.

It seems odd to me that the system bonding jumper must be sized larger than the grounding electrode conductor given that these two conductors essentially form a series circuit between the neutral of the generator and the grounding ring system. Is there a code provision that I am missing, or a reason for the for the larger system bonding jumper size that I am not considering? Is this just the way it is and should I stop questioning it?

Thank you in advance!