Generator Neutrals

[Ed MacLaren]

Re: Generator Neutrals

I don't see a circuit between the white and the green

The white and green are not directly connected to each other.
I see a series circuit, from Point A, in the following order, to Point B.

Through the GFCI, along the black wire, through the load, through the (green) EGC to the generator frame, into the earth, through the earth to the victim's feet, through the victim's body, through the load device enclosure, and through the (gray) neutral back to GFCI and the generator winding.

Keep in mind that the EGC and neutral have been mistakenly interchanged in the load device, as described in the article.

If you wish, substitute a wet concrete floor for the earth.

I changed the diagram a bit to clarify the current path.

Ed

P.S. I type real slow.

[ January 18, 2005, 06:26 PM: Message edited by: Ed MacLaren ]

 

[crossman]

Re: Generator Neutrals

How about if this generator was solidly mounted on a truck using steel mounting hardware, like a welding rig or something... with unbonded neutral. In the scenario with reversed neutral and EGC, 120v would exist between the vehicle and the case of any equipment that was plugged into the generator.

 

[Ed MacLaren]

Re: Generator Neutrals

the fact that full potential would exist between the equipment housing and the generator housing

All of the items that I listed above are in series with each other.

The only place I see full potential is between the "hot" terminal of the load, and the load enclosure.

Ed

 

[crossman]

Re: Generator Neutrals

hold on I am thinking

[ January 18, 2005, 06:17 PM: Message edited by: crossman ]

 

[ddderek]

Re: Generator Neutrals

Originally posted by ddderek:

Originally posted by Ed MacLaren:

but the bonding jumper is not sized large enough.

Why would it need to be #6?

Ed

OK, back to the bonding jumper SIZE... according to 250.28 (D) the main bonding jumper shall not be smaller than sizes shown in Table 250.66 for grounding electrode conductors. (That states for minimum number 8 conductor size) (was looking at alluminum for # 6 ref. earlier) But the point is the Gen bonding jumper is NOT sized according to code for the Seperately derived system being used. So may not be sufficient in and of itself... Thus isolating becomes necesary rather than three pole transfer switch?

And don't foget this question posed about the size of the bonding jumper... Isn't the factory installed bond undersized according to code???

 

[crossman]

Re: Generator Neutrals

Re: full voltage in Ed's diagram

Well, I think this is one of those "approximation" things that was mentioned in another thread... splitting hairs?

So are we talking of the situation of the guy in the circuit like your drawing or are we talking about the situation without the guy completing the circuit?

1) In the former case with a completed circuit through either a person's body or through a voltmeter, the only place I see full voltage is at the actual terminals of the generator. Any current flowing in the cirucit will produce voltage drops (however small) in every part of the cirucit. The largest drop will occur across the largest resistance. Now, we can argue the semantics of what is meant by "full voltage" however.

2) In the case with no completed circuit and no current flow, I see full voltage in a whole lot of places...

any place on the hot wire to any place on the neutral wire

hot wire to load case

gen frame to neutral

gen frame to load case

equipment ground to neutral

equipment ground to load case

To have a voltage drop you must have current flow. This leads us back to that thread about infinite resistance. UGH.

[ January 18, 2005, 06:40 PM: Message edited by: crossman ]

 

[iwire]

Re: Generator Neutrals

Originally posted by ddderek:
Isn't the factory installed bond undersized according to code???

What does the NEC have to do with anything 'factory installed'?

As long as it has a UL tag it has been tested and does what it is supposed to.

Don't forget that most times a bonding jumper must withstand utility fault current levels.

The max fault current from a 5KW portable generator will not be anywhere near the same.

[ January 18, 2005, 06:50 PM: Message edited by: iwire ]

 

[Ed MacLaren]

Re: Generator Neutrals

Isn't the factory installed bond undersized according to code???

I was hoping one of the "NEC guys" would address this question, as I don't know the answer.

Here, (Canada) equipment bonding conductor size is determined by the amp rating of the overcurrent device that will be operated by the equipment bonding conductor, but in no case does it ever have to be larger than the largest ungrounded conductor of the circuit.

Ed

 

[Ed MacLaren]

Re: Generator Neutrals

In the case with no completed circuit and no current flow, I see full voltage in a whole lot of places...

In a series circuit, that has an open, such as would be the case if our victim removed his hand from the load device enclosure, full voltage is available across the open, until he puts his hand back on the box, when current will start to flow, and then the voltage is divided among the series circuit components.

In a series loop, voltage drop across a circuit component is directly proportional to it's resistance, and an "open" in a circuit is a very high resistance.

By the way, when trouble shooting in a series circuit, such as a motor control "off device" loop, using a voltmeter to locate an open, when you find full voltage, you have found the open.

Ed

[ January 18, 2005, 07:16 PM: Message edited by: Ed MacLaren ]

 

[iwire]

Re: Generator Neutrals

Ed we may be getting into a bonding / grounding confusion.

Bonding Jumper. A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.

For a separately derived system neutral to ground conection this is sized in relation to the derived conductors (12.5%)

What you described

Here, (Canada) equipment bonding conductor size is determined by the amp rating of the overcurrent device that will be operated by the equipment bonding conductor, but in no case does it ever have to be larger than the largest ungrounded conductor of the circuit.

We still call a grounding conductor and we size it exactly the same way as you.

 

[crossman]

Re: Generator Neutrals

I think we are in agreement.

In a series circuit, that has an open, such as would be the case if our victim removed his hand from the load device enclosure, full voltage is available across the open , until he puts his hand back on the box, when current will start to flow, and then the voltage is divided among the series circuit components.

Agreed.

Just to make sure, concerning the "open" situation: Full voltage would also exist between any point on the hot wire and any point on the neutral wire. And full voltage would appear between anything that the hot is connected to and anything that the neutral is connected to.

In a series loop, voltage drop across a circuit component is directly proportional to it's resistance, and an "open" in a circuit is a very high resistance.

Agreed

By the way, when trouble shooting in a series circuit, such as a motor control "off device" loop, using a voltmeter to locate an open, when you find full voltage, you have found the open.

Agreed

And on to the other question of system bonding jumper sizing... very interesting! Again I am at home wihtout my code book. Doesn't it say that for main bonding jumpers and system bonding jumpers that we have to use the table for grounding electrode conductors unless we go off the chart on the maximum side, and then we use 12.5% of the phase conductor area? But if we are on the small end of the chart, we must use the chart? Isn't a #8 copper the smallest on the chart?

But I know this isn't right because we don't have to use #8 for system bonding jumpers on small control transformers and stuff. Hmmmm....

 

[ddderek]

Re: Generator Neutrals

Originally posted by iwire:
Ed we may be getting into a bonding / grounding confusion.

Bonding Jumper. A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.

For a separately derived system neutral to ground conection this is sized in relation to the derived conductors (12.5%)

What you described

Here, (Canada) equipment bonding conductor size is determined by the amp rating of the overcurrent device that will be operated by the equipment bonding conductor, but in no case does it ever have to be larger than the largest ungrounded conductor of the circuit.

We still call a grounding conductor and we size it exactly the same way as you.

Actually you are talking about the Main Bonding Jumper... That is what the NEC calls it whether bonding the neutral on a seperately derived system or not! And the sizes only uses the 12.5% rule when voltage is wire is over 1000 KCMIL !! see 2002 NEC 250.28 (D)Otherwise you size it according to Table 250.66 ... that table says the smallest conductor will be # 8 copper!! and on the generator, the bonding jumper is a #14 or #12 but the largest circuit conductor is # 10 supplying the 240V 30A outlet... I realize that the code does not govern the Generator manufacturers, but the point is the safety factor (the code is supposed to be minimum standards)

[ January 19, 2005, 02:15 AM: Message edited by: ddderek ]

 

[iwire]

Re: Generator Neutrals

Originally posted by ddderek:
Actually you are talking about the Main Bonding Jumper... That is what the NEC calls it whether bonding the neutral on a separately derived system or not! And the sizes only uses the 12.5% rule when voltage is wire is over 1000 KCMIL !! see 2002 NEC 250.28 (D)Otherwise you size it according to Table 250.66

I disagree, first look at the article 100 definition of 'Main Bonding Jumper'

Bonding Jumper, Main. The connection between the grounded circuit conductor and the equipment grounding conductor at the service.

No mention of SDS (Separately Derived System)

When we look at the definition of service

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

it is apparent you can not call any SDS a 'service'

Now look at 250.30 Grounding Separately Derived Alternating-Current Systems.

250.30(A)(1) Bonding Jumper. A bonding jumper in compliance with 250.28(A) through (D) that is sized for the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. Except as permitted by 250.24(A)(3), this connection shall be made at any 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. The point of connection shall be the same as the grounding electrode conductor as required in 250.30(A)(2).

Notice they do not use the term main bonding jumper only bonding jumper.

As far as the 12.5% I am aware we use the table, however if you do some math you will find the figures in the table work out to 12.5% of the supply conductors.

Originally posted by ddderek:
I realize that the code does not govern the Generator manufacturers, but the point is the safety factor (the code is supposed to be minimum standards)

OK but I think you are forgetting that the code is the minimum standard for installations that will not be tested in the field until a fault happens.

In the case of UL listed equipment the combination is tested many different ways to verify that it works like it is supposed to.

You say 'I realize that the code does not govern the Generator manufacturers' stay with that thought, do not go on to say 'the code is supposed to be minimum standards' without adding 'for field installations.'

Bob

[ January 19, 2005, 06:17 AM: Message edited by: iwire ]

 

[crossman]

Re: Generator Neutrals

The 2005 Code has put to rest the confusion with main bonding jumpers and separately derived systems.

There is a new definition and additions to 250 concerning the "system bonding jumper" which is in essence the "main bonding jumper" for a seperately derived system

MBJ is at the service

SBJ is at a separately derived system

I still say that an exact reading of 250 allows only a minimum of a #8 copper SBJ for any separately derived system. I may certainly have missed someting somewhere. There is an exception for separately derived systems of Class I, II, and III systems (power limited)

[ January 19, 2005, 11:36 AM: Message edited by: crossman ]

 

[ddderek]

Re: Generator Neutrals

Originally posted by iwire:

Now look at 250.30 Grounding Separately Derived Alternating-Current Systems.

250.30(A)(1) Bonding Jumper. A bonding jumper in compliance with 250.28(A) through (D) that is sized for the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor.

You spelled it out right there... SDS bonding jumper sized according to 250.28 (main Bonding jumper sizing) which refers you to table 250.66 like I said. Now you may be very well correct in that a 12.5% jumper would be safe based on the calculations in the table... but the code does NOT officially allow for a bonding jumper for a seperately derived system to be any smaller than # 8... and crossman clarifies it by definition above...

[ January 19, 2005, 02:38 PM: Message edited by: ddderek ]

 

[iwire]

Re: Generator Neutrals

Originally posted by ddderek:
You spelled it out right there... SDS bonding jumper sized according to 250.28 (main Bonding jumper sizing) which refers you to table 250.66 like I said.

Yes it is sized to the same table as a main bonding jumper is sized but that does not in anyway mean that it is called a main bonding jumper which is what you said.

Originally posted by ddderek:
Actually you are talking about the Main Bonding Jumper... That is what the NEC calls it whether bonding the neutral on a seperately derived system or not!

The table is actually called Grounding Electrode Conductor for Alternating-Current Systems

And none of what jumpers we are talking about are GECs

Originally posted by ddderek:
Now you may be very well correct in that a 12.5% jumper would be safe based on the calculations in the table...

That is not what I said, what I was pointing out to Ed was that our (Ed is from Canada) bonding jumpers for a service or SDS are roughly 12.5% of the supply conductors.

Originally posted by ddderek:
but the code does NOT officially allow for a bonding jumper for a separately derived system to be any smaller than # 8...

I did not say that either, what I did say is the NEC has nothing to do with the size of the bonding jumper on a UL listed assembly.

In my opinion you are getting to caught up with the words in the code and forgetting the intent.

Lets look at this another way, what size conductors come off of the generator to the outlets on it?

Probably 12 or 10 AWG?

Now jumping them together with an 8 AWG makes little sense electrically.

You have got to forget the NEC when it comes to UL listed equipment.

How about an electric stove, if I run NM to feed a stove I will need to use 6 AWG copper if I use a 50 amp breaker.

Now inside the stove what size wires will you find protected by the 50 amp breaker?

12 AWG, could I do this myself in the field, no.

But it is perfectly acceptable inside the UL listed stove and it is not dangerous.

Bob

 

[tstern]

Re: Generator Neutrals

What will cause the stray voltage to take the shortest path to ground? Have you not ever used or studied parrell circuits? Is the voltage really trying to get to earth? I don't think so but it is seeking the other side of the source.

 

[ddderek]

Re: Generator Neutrals

WAIT a second!!!! What is the code article and section that you are saying refers to only one bonding point for the grounded conductor??? I just took a code update course today and not only is it not spelled out, it actually had a picture of a high voltage situation with dual bonding, electrodes, and paths and all legal... I could not find anything disallowing the double bonding parallel paths or not. HELP if you know?

 

[don_resqcapt19]

Re: Generator Neutrals

ddderek,
250.24(A)(5).

 

[ddderek]

Re: Generator Neutrals

Don, If you use this section as your arguement,
250.24 A 5, then in the case where your gen feed comes into the main panel and terminates on a two pole breaker that is mechanically linked to the main and allows you to run any circuit with gen backup, THAT two pole breaker could be referred to as yourservice disconnecting means and in fact the bond at the service would be acceptable?

Just as with a home service, the grounded conductor is bonded at the utility transformer AND your home service disconnecting means... TWO bonds! but bonded at the service disconnecting means!

-Derek