System Bonding Jumper - SDS

[W@ttson]

For a separately derived system designated generator which is a roll up generator (assuming floating neutral) and plugs into a generator tap box, is the first disconnect the breaker on the gen set or the distribution panel downstream of the tap box?

Ultimately, the question would be where does the system bonding jumper have to go?
1. Does it have to be in the gen set breaker area?
2. Can it be at the tap box?
3. Can it be at the distribution panel downstream of the tap box?

 

[augie47]

There are some variations.. see 250.30

 

[W@ttson]

Yes, here is the section:
(1) System Bonding Jumper.



An unspliced system bonding jumper shall comply with 250.28(A) through (D). This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices, in accordance with 250.30(A)(1)(a) or (A)(1)(b). The system bonding jumper shall remain within the enclosure where it originates. If the source is located outside the building or structure supplied, a system bonding jumper shall be installed at the grounding electrode connection in compliance with 250.30(C).
Exception No. 1:
For systems installed in accordance with 450.6, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.
Exception No. 2:
If a building or structure is supplied by a feeder from an outdoor separately derived system, a system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor. If a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.
Exception No. 3:
The size of the system bonding jumper for a system that supplies a Class 1, Class 2, or Class 3 circuit, and is derived from a transformer rated not more than 1000 volt-amperes, shall not be smaller than the derived ungrounded conductors and shall not be smaller than 14 AWG copper or 12 AWG aluminum.

• (a)
  Installed at the Source. The system bonding jumper shall connect the grounded conductor to the supply-side bonding jumper and the normally non-current-carrying metal enclosure.
• (b)
  Installed at the First Disconnecting Means. The system bonding jumper shall connect the grounded conductor to the supply-side bonding jumper, the disconnecting means enclosure, and the equipment grounding conductor(s).

Exception:
Separately derived systems consisting of multiple sources of the same type that are connected in parallel shall be permitted to have the system bonding jumper installed at the paralleling switchgear, switchboard, or other paralleling connection point instead of at the disconnecting means located at each separate source.

 

[W@ttson]

The question I guess really is whether the breaker on the gen set is considered the first disconnecting means or if the distribution panel downstream is considered the first disconnecting means.

I would like to consider the load terminals of the generator as where the source begins. First disconnect after that would be the first disconnect. But I may not be right on that.

 

[infinity]

Is there a transfer switch? You said that the generator neutral is floating so is this set up as a non-SDS?

 

[Little Bill]

The neutral must be switched if it is a SDS. And if it is switched, it must be bonded.

 

[W@ttson]

Is there a transfer switch? You said that the generator neutral is floating so is this set up as a non-SDS?

No neutral. The ATS is a 3 pole switch. No neutral from the utility, no neutral from the generator. Only 3 phase loads at this point of the distribution (a transformer further down derives a neutral for hotel loads).

 

[W@ttson]

The neutral must be switched if it is a SDS. And if it is switched, it must be bonded.

There are no neutrals, so essentially their is nothing to switch. Since no conductors from the utility are in contact with any conductors from the separate source (except for grounding and bonding), by definition it is a Separately derived system.

This being a Roll up generator setup, the generators getting rolled up need to be designated to be either Bonded Neutral or Floating Neutral as per the plaque requirement that is to be put on the Tap box. The plaque calls for Floating Neutral generators so all of the grounding/bonding is left on the facility end.

 

[W@ttson]

So to make the one line a little clearer the setup is like this:

Source 1 (Utility) -> ATS -> Distribution Panel
Source 2 (Roll up generator) -> Tapbox -> ATS -> Distribution panel.

The Tapbox does not have OCPD. When on gen, the first disconnecting means is the distribution panel except for the OCPD on the gen.

 

[wwhitney]

There are no neutrals, so essentially their is nothing to switch. Since no conductors from the utility are in contact with any conductors from the separate source (except for grounding and bonding), by definition it is a Separately derived system.

OK, so if there is no neutral, then being a grounded system is optional. Do you have a corner grounded delta, in which case you would need a System Bonding Jumper, or do you have an ungrounded delta, in which case there would be no System Bonding Jumper?

Cheers, Wayne

 

[W@ttson]

OK, so if there is no neutral, then being a grounded system is optional. Do you have a corner grounded delta, in which case you would need a System Bonding Jumper, or do you have an ungrounded delta, in which case there would be no System Bonding Jumper?

Cheers, Wayne

Sorry, I can see where I was confusing.

The utility is technically a 4w, 3PH service. So they give us a neutral. We do all of the Main bonding jumper and ground electrode system items.

We just dont need a neutral going to the ATS.

I left out the Main disconnect on the utility side for simplicity in the one line discussion above. Technically the Source 1 is like this:

Source 1 (Utility) -> Main Disconnect (here we do the MBJ etc) -> (we only send 3ph + EGC to ATS) ATS -> Distribution Panel

 

[wwhitney]

But the generator is just 3W3PH? That seems potentially problematic, as you would be changing the voltage system grounding when switching from utility to generator.

If you use a 4W3PH generator, with no N-G bond at the generator, and then connect the generator neutral to the utility neutral (either by bringing the utility neutral to the ATS, or the generator neutral to the utility main disconnect), then you won't have an SDS, and the MBJ in the main disconnect will provide the N-G bond whether on utility or generator power. The ATS just needs to be 3 pole.

Cheers, Wayne

 

[augie47]

I agree with Wayne that even though you are not using the neutral at the ATS or distribution panel your premises service is still a grounded service (3 ph 4w) so you cannot bond the neutral to ground other than at the service. I would think his solution of installing a neutral to the TS or generator and not consider it an SDS.
The alternative might be to run the generator as a 3p3w with GF indicator and not utilize the neutral nor bind it at the generator. We will see if Wayne sees a problem with that,

 

[wwhitney]

The alternative might be to run the generator as a 3p3w with GF indicator and not utilize the neutral nor bind it at the generator. We will see if Wayne sees a problem with that,

I'm no expert, but the reason I called that problematic is that switching from utility to generator would switch from an L-G voltage that is stable and defined as 1/sqrt(3) times the L-L voltage to an L-G voltage is unstable and undefined. Some of the loads might not like that (VFDs?); if they are all happy with either L-G voltage, then I expect it would be OK.

Another option is to use a 4W3PH generator and install it with an SBJ and GEC as typical for an SDS. But then as the loads don't use a neutral, the ATS only has to be 3 pole. I think this is what the OP has in mind. So then the question is, in the one-line:

Source 2 (Roll up generator) -> Tapbox -> ATS -> Distribution panel.

where is the first system disconnecting means? The SBJ and GECs can be there or at the generator.

Cheers, Wayne

 

[W@ttson]

But the generator is just 3W3PH? That seems potentially problematic, as you would be changing the voltage system grounding when switching from utility to generator.

If you use a 4W3PH generator, with no N-G bond at the generator, and then connect the generator neutral to the utility neutral (either by bringing the utility neutral to the ATS, or the generator neutral to the utility main disconnect), then you won't have an SDS, and the MBJ in the main disconnect will provide the N-G bond whether on utility or generator power. The ATS just needs to be 3 pole.

Cheers, Wayne

The generator is 4w 3ph. The generator is sending the 3 phases and the neutral to the tap box. The neutral does not leave the tap box. It does not go to the ATS and therefore, it is an SDS.

I ultimately want to do the system bonding jumper connection at the Tap box. I have plenty of space in there. I can run my GEC to there too easily. Then when a floating neutral generator comes and rolls up the N-G bond occurs at the tap box, blind to the generator.

My issue is just confirming that I can do the N-G bond in the Tap box.


The ATS is 3 pole, without even a terminal for the N.

 

[W@ttson]

I agree with Wayne that even though you are not using the neutral at the ATS or distribution panel your premises service is still a grounded service (3 ph 4w) so you cannot bond the neutral to ground other than at the service. I would think his solution of installing a neutral to the TS or generator and not consider it an SDS.
The alternative might be to run the generator as a 3p3w with GF indicator and not utilize the neutral nor bind it at the generator. We will see if Wayne sees a problem with that,

I disagree. There is no difference between having a four pole transfer switch transferring the neutral such that the Gen neutral and utility neutral never touch, or just not bringing the generator neutral to the ATS nor the utility neutral to the ATS.

The systems are separate, only ground connections touch.

Here is a thought experiment. Suppose you had a generator installed 4w, 3PH. Your utility is 4wire 3PH. You have a 4 pole ATS. The neutral is switched and you do not have multiple N-G that see each other. At the generator now you are required to install a system bonding jumper, run a GEC, and drive ground rods or connect to an approved electrode.

How is the above different than what I described where the N's of the two sources never are physically in the same place?

=====================================================

I could pose my same question as follows:
Supposed I have a SDS generator and a 4 pole ATS. I have a terminal cabinet between the generator CB output and a large CB disconnect after the terminal block.

Can I place the system bonding jumper for my SDS generator in that terminal cabinet which is between the output SB of the gen and the Circuit breaker disconnect.

 

[W@ttson]

I'm no expert, but the reason I called that problematic is that switching from utility to generator would switch from an L-G voltage that is stable and defined as 1/sqrt(3) times the L-L voltage to an L-G voltage is unstable and undefined. Some of the loads might not like that (VFDs?); if they are all happy with either L-G voltage, then I expect it would be OK.

Another option is to use a 4W3PH generator and install it with an SBJ and GEC as typical for an SDS. But then as the loads don't use a neutral, the ATS only has to be 3 pole. I think this is what the OP has in mind. So then the question is, in the one-line:



where is the first system disconnecting means? The SBJ and GECs can be there or at the generator.

Cheers, Wayne

Yes. This is it!

That is exactly the situation. The utility is wired as a 4w3PH and the gen is wired as a 4w3ph. We just dont use the neutrals. The neutrals never meet.

I just want to know if I can do the system bonding jumper for the generator in that tap box.

 

[wwhitney]

I just want to know if I can do the system bonding jumper for the generator in that tap box.

In practice that would work fine.

Per the section you quoted, your two choices for where to put the SBJ are at "the source" or at the "first disconnecting means". So can you construe the tap box to be one of those?

Not sufficiently familiar with a "tap box" to know the details, e.g. if it would have a disconnecting means.

Cheers, Wayne

 

[augie47]

Another option is to use a 4W3PH generator and install it with an SBJ and GEC as typical for an SDS. But then as the loads don't use a neutral, the ATS only has to be 3 pole. I think this is what the OP has in mind. So then the question is, in the one-line:

That makes sense

 

[W@ttson]

In practice that would work fine.

Per the section you quoted, your two choices for where to put the SBJ are at "the source" or at the "first disconnecting means". So can you construe the tap box to be one of those?

Not sufficiently familiar with a "tap box" to know the details, e.g. if it would have a disconnecting means.

Cheers, Wayne

no disconnecting means in the tap box. This is a sample:

Generator connection box / generator docking station

UL listed generator docking station that range from 400A to 1200A. Male camlocks are accessible through a dropdown panel at the bottom and are color coded.

temporarypowersupply.com

Just a box where the cam locks plug into. Just bus bar in there. Do you think that is still OK?