250.32 (B)(2) Buildings or Structures, Supplied by SDS (Separately Derived System)

[unsaint34]

250.30 Grounding Separately Derived Alternating Current System
250.32 Buildings or Structures Supplied by a Feeder(s) or Branch circuits(s).
250.32 (B)(2) Supplied by Separately Derived System
250.32 (B)(2)(a) with OCPD
250.32 (B)(2)(b) without OCPD

I don't understand the purpose of 250.32(B)(2). I am thinking, when a separate structure is supplied by a SDS, 250.30 covers the structure's system automatically, regardless of whether utility service or a feeder supplies the source of the SDS.

Let's assume the source of the SDS is a transformer.

Since it's a separately derived system, the separate structure's disconnect will have a system bonding jumper, right? 250.32 (B)(2)(b) says that is correct (it tells me to follow 250.30), but only when the transformer secondary has no OCPD. What does the transformer secondary OCPD has anything to do with whether or not to have a system bonding jumper? 250.32 (B)(2)(a) seems to say that I don't even need to worry about the system bonding jumper if the transformer secondary has OCPD.

I'd appreciate any help/feedback.

 

[david luchini]

Since it's a separately derived system, the separate structure's disconnect will have a system bonding jumper, right?

Not necessarily. 250.30(A) allows the system bonding jumper on an SDS to be installed anywhere from the source to the first disconnecting means or overcurrent device.

250.32 (B)(2)(b) says that is correct (it tells me to follow 250.30), but only when the transformer secondary has no OCPD. What does the transformer secondary OCPD has anything to do with whether or not to have a system bonding jumper?

The system bonding jumper cannot be installed "after" the secondary OCPD. If the secondary OCPD is located at the transformer, then the system bonding jumper must be installed at the transformer.

250.32 (B)(2)(a) seems to say that I don't even need to worry about the system bonding jumper if the transformer secondary has OCPD.

I'd appreciate any help/feedback.

250.32(B)(2)(a) doesn't tell you not to worry about the system bonding jumper (as it must be installed per 250.30(A)(1)). 250.32(B)(2)(a) tells you that from the OCPD at the transformer, you must run an equipment grounding conductor to the structure disconnect, and that the egc must be bonded to the grounding electrode(s) at the structure.

 

[unsaint34]

Not necessarily. 250.30(A) allows the system bonding jumper on an SDS to be installed anywhere from the source to the first disconnecting means or overcurrent device.

I see that rules regarding the location of the SBJ (system bonding jumper) is different than the location of MBJ (main bonding jumper).

As far as the SBJ goes, there is no specific requirement that it has to be in the service disconnect. 250.30(A)(1)(a) & (b) clearly give the location options of either at the source or at the disconnecting means.

However, MBJ of a grounded system has to be "within the enclosure for each service disconnect.," according to 250.24(B), although 250.24(A)(1) states grounding electrode conductor connection (which sounds to me a MBJ) can be made at any point from the service point to the disconnect. How should I interpret this? Don't these two articles contradict each other? MBJ has to be in the service disconnect enclosure but it can also be at the service point?

 

[unsaint34]

The system bonding jumper cannot be installed "after" the secondary OCPD. If the secondary OCPD is located at the transformer, then the system bonding jumper must be installed at the transformer.

That makes sense to me intuitively. But is there a code article for that?

What if the transformer did NOT have 2ndary OCPD? In that case can I make the SBJ either at the transformer or at the disconnect? I will attempt to attach two pictures from Stallcup book regarding 250.32(B)(2)(a) & (b). According to the pictures, the SBJ was always made at the transformer regardless of the 2ndary OCPD.

 

[Smart $]

That makes sense to me intuitively. But is there a code article for that?

What if the transformer did NOT have 2ndary OCPD? In that case can I make the SBJ either at the transformer or at the disconnect? I will attempt to attach two pictures from Stallcup book regarding 250.32(B)(2)(a) & (b). According to the pictures, the SBJ was always made at the transformer regardless of the 2ndary OCPD.

250.30(A)(1).If no secondary disconnct or OCP, SBJ must be at transformer. Didn't look at pictures.

 

[unsaint34]

So far I learned SBJ has to be either before or at the first OCPD according to 250.30(A)(1).

Now, to my original question. I think I should word my question better... A separate building is supplied by a feeder. The feeder is supplying a isolation transformer which is supplying the building. The article 250.32 (B)(2) (Supplied by Separately Derived System) is exactly for this situation, right?

My question is this... Since the separate building is supplied by a SDS, shouldn't 250.30 cover the building system grounding? Why does 250.32 (B)(2) reiterate grounding rules? I mean, would the 250.32 (B)(2)(a) & (b) rules not apply to any other SDS?



"250.32 (B)(2) Supplied by Separately Derived System.
(a) With Overcurrent Protection. If overcurrent protection is provided where the conductors originate, the installation shall comply with 250.32(B)(1).
(b) Without Overcurrent Protection. If overcurrent protection is not provided where the conductors originate, the installation shall comply with 250.30(A). If installed, the supply-side bonding jumper shall be connected to the building or structure disconnecting means and to the grounding electrode(s)."

 

[Smart $]

So far I learned SBJ has to be either before or at the first OCPD according to 250.30(A)(1).

Now, to my original question. I think I should word my question better... A separate building is supplied by a feeder. The feeder is supplying a isolation transformer which is supplying the building. The article 250.32 (B)(2) (Supplied by Separately Derived System) is exactly for this situation, right?

No. The supplying conductors in the case described are transformer secondary conductors if the transformer is outside and the disconnecting means is at the building. The transformer secondary conductors are not a feeder. Only if the disconnecting means is located prior to supplying the building would the building be supplied by a feeder. That's essentially what 250.32(B)(2) says.

Other scenarios exist, where the building is supplied directly by a feeder from another building or structure... no transformer involved.

My question is this... Since the separate building is supplied by a SDS, shouldn't 250.30 cover the building system grounding? Why does 250.32 (B)(2) reiterate grounding rules? I mean, would the 250.32 (B)(2)(a) & (b) rules not apply to any other SDS?

250.30 covers GEC. GEC and building and structure GES's are covered under Article 250 Part III. The requirements intermesh as they apply. 250.32 rules apply to any SDS.