250.24 and 250.30 Confused

[VIC1958]

Can someone please help me deciphering the difference between these two section as it relates to where the GEC bonds to the neutral in a residential application. 250.24 states from what I see, the GEC can be bonded to the neutral at several areas, service drop, meter can or at the panel board. In 250.30(A)(1) says the GEC needs to be bonded to the neutral at the first system disconnection means. I know of certain municipalities that follow 250.30 and I know of others that state the GEC must bond to the neutral lug in the meter can no mater what. Thanks for any input.

 

[roger]

250.24 is pertaining to the service, 250.30 is pertaining to a Separately Derived System

Roger

 

[VIC1958]

Thanks Roger, I guess my confusion starts with the definition of a Separately Derived System.... I understand batteries, solar, wind, etc. But how do transformers play into it, if the premise wiring system is being fed by a transformer. Pardon my ignorance.

 

[kwired]

Thanks Roger, I guess my confusion starts with the definition of a Separately Derived System.... I understand batteries, solar, wind, etc. But how do transformers play into it, if the premise wiring system is being fed by a transformer. Pardon my ignorance.

If the secondary of the transformer is not connected to any part of the primary (before any bonding for equipment grounding purposes is done) then it is separately derived. In other words it does not depend on any of the primary portion to derive its output voltage.

An autotransformer has a current carrying connection that is common to both primary and secondary coils, and this would not be a separately derived system.

Most general use transformers are separately derived systems.

 

[VIC1958]

So it can be assumed most of the time that utility owned transformers feeding residential units are a separately derived system, generally speaking. Also, that the dwellings grounding electrode/GEC can be bonded to the neutral at any point from the transformer and the first point of disconnection at the house?

 

[kwired]

So it can be assumed most of the time that utility owned transformers feeding residential units are a separately derived system, generally speaking. Also, that the dwellings grounding electrode/GEC can be bonded to the neutral at any point from the transformer and the first point of disconnection at the house?

They are separately derived from the primary distribution. For NEC applications though the transformer itself is not covered if it is on the supply side of the "service point" and what you first encounter in a utility supplied system that is covered by NEC is usually called "Service conductors" whether it be service lateral, service drop....

GEC must be bonded to service equipment somewhere on the premises side of the service point, and at very least at each structure served if there are multiple structures involved in the distribution of the customers portion of the service conductors.

 

[roger]

So it can be assumed most of the time that utility owned transformers feeding residential units are a separately derived system, generally speaking. Also, that the dwellings grounding electrode/GEC can be bonded to the neutral at any point from the transformer and the first point of disconnection at the house?

Vic, these illustrations may help clear it up.

Service Grounding 250.24

Separately Derived System

Roger

 

[david]

Referencing NFPA 70/2008
I might not understand what you?re asking but I think you are confusing the allowable connections of the Grounding electrode conductor to the grounded conductor (neutral) in 250.24 (A)

With the mandated bonding location of the grounded service conductor (neutral) found in 250.24 (C)

 

[kwired]

I see the picture of the service installation has ungrounded conductors entering the weatherhead, but no ungrounded conductors anywhere else

Vic, these illustrations may help clear it up.

Service Grounding 250.24

Separately Derived System

Roger

 

[shortcircuit2]

Yes that could be dangerous with those short ungrounded conductors...

In Mike's photo of an AC service it implies that there are only 3 options for a connection point of the GEC. Let me point out that there could be a trough between the meter and the service disconnect(s) where the GEC could also connect to the grounded conductor.

 

[Smart $]

Can someone please help me deciphering the difference between these two section as it relates to where the GEC bonds to the neutral in a residential application. 250.24 states from what I see, the GEC can be bonded to the neutral at several areas, service drop, meter can or at the panel board. In 250.30(A)(1) says the GEC needs to be bonded to the neutral at the first system disconnection means. I know of certain municipalities that follow 250.30 and I know of others that state the GEC must bond to the neutral lug in the meter can no mater what. Thanks for any input.

I think you are confusing, just a tad, between the GEC connection(s) and the main or system bonding jumper...

Regarding a service, the main bonding jumper (MBJ) must be located in the service disconnecting means. The MBJ is the connection between the grounded service conductor and the equipment grounding system (EGS, to which EGC's are connected). At the location of the MBJ, any GEC can be connected to either side of the MBJ, i.e. the grounded (neutral) conductor or the EGS bus.

A separately derive system (SDS) is similar, but the equivalent of an MBJ for service is called a system bonding jumper (SBJ). The SBJ can be either in the source or first disconnecting means enclosure. The GEC(s) can be connected to either end of the SBJ wherever the SBJ is located... but IIRC it(they) can only be connected to the grounded (neutral) conductor at or ahead of the SBJ... not on load-side of the SBJ.

 

[kwired]

Yes that could be dangerous with those short ungrounded conductors...

In Mike's photo of an AC service it implies that there are only 3 options for a connection point of the GEC. Let me point out that there could be a trough between the meter and the service disconnect(s) where the GEC could also connect to the grounded conductor.

There could be a gutter or splice box ahead of the meter also and the GEC could land there as well.