Sharing Neutrals, Isolated Grounds etc.

[Alwayslearningelec]

This is where I get a bit confused when estimating. I was told if it is a normal 20A circuit I can us 5 wires per 3 circuits. Hot per circuit and share a ground & neutral for the three? Now If have isolated ground receptacles I need a separate ground wire for each circuit??????? So if I am running out a 6 circuit home run and 3 of those circuits are isolated grounds how many wires would I be running out to my HR box? 12? 5 for the 3 reg circuits and 7 for the iso grounds......1 hot & 1 ground per iso circuit and share the neutral makes 7 for the 3 iso circuits.


The amount wire for circuits screws me up.

 

[Smart $]

Much of how many wires you run is a design decision. Out of a 3? 4W panelboard you could run 3 "convenience" circuits and 3 "isolated" circuits in one conduit as:

Minimum wires — 6 ungrounded, 2 grounded, 1 grounding, 1 isolated grounding

or​

Maximum wires — 6 ungrounded, 6 grounded, 6 grounding, 3 isolated grounding.​

How many do you want to run?

 

[Sparky555]

Sorry I'm not following your question very well, but I'll try to explain. I rarely run isolated grounds, but by definition I suspect that means an insulated green wire per circuit. A std. circuit is 2 wire (1 grounded, 1 ungrounded). A shared neutral circuit (two circuits) is 3 wire (1 grounded, 2 ungrounded). If you don't know the proper way to terminate them on the circuit breakers, don't do shared neutrals. It creates a fire hazard. When I run multiple shared neutral circuits in the same conduit I use different color wires to keep them straight. Figure out how many circuits you need & run 2 or 3 wire circuits. In conduit you have to derate above 9 current-carrying conductors. This is all for 1 PH. It's different for 3 PH.

Dave

 

[infinity]

Sparky, could you please explain how shared neutrals are a fire hazard?

 

[iwire]

but by definition I suspect that means an insulated green wire per circuit.

Dave, as Smart pointed out the above is a up to the designer, an IG setup might require one IG conductor per outlet, one IG per circuit, or one IG for multiple circuits.

The IGs rarely run separately all the way back to the bonding point which is typically XO at the transformer.

Often the way I see it is one IG and one EGC per raceway or cable no mater how many circuits they serve. Once they reach the panel the IGs are landed on a isolated grounding bar and a single IG conductor is run from that bar all the way back to XO at the transformer supplying that panel.

 

[infinity]

Dave, as Smart pointed out the above is a up to the designer, an IG setup might require one IG conductor per outlet, one IG per circuit, or one IG for multiple circuits.

The IGs rarely run separately all the way back to the bonding point which is typically XO at the transformer.

Often the way I see it is one IG and one EGC per raceway or cable no mater how many circuits they serve. Once they reach the panel the IGs are landed on a isolated grounding bar and a single IG conductor is run from that bar all the way back to XO at the transformer supplying that panel.

I agree, one EGC and IG shared per raceway is adequate. Separate EGC's and IG's are a waste of money.

 

[iwire]

I agree, one EGC and IG shared per raceway is adequate. Separate EGC's and IG's are a waste of money.

Like IGs aren't a waste anyway?

 

[Sparky555]

Sparky, could you please explain how shared neutrals are a fire hazard?

In 1 PH terminating both circuits on the same leg makes the current additional instead of subtractive. That can put to much current through the grounded conductor unless you size it accordingly.

Dave

 

[LarryFine]

So if I am running out a 6 circuit home run and 3 of those circuits are isolated grounds how many wires would I be running out to my HR box?

Here's my calc:

3) ckts can be standard 3p MWBC, so that's 5: 3 hots, 1 neutral, one EGC (or if you use conduit, make it 4)

3) 2-wire ckts w' individual neutrals and IG's, makes 9: 3 hots, 3 neutrals, 3 IG's (use the EGC/conduit above)

That makes a total of 14, or 13 if the raceway is the EGC.

 

[LarryFine]

Sparky, could you please explain how shared neutrals are a fire hazard?

I believe the correct statement is that improperly-wired circuits using shared neutrals can be a hazard.

 

[TOOL_5150]

Like IGs aren't a waste anyway?

Not completly useless Bob. I worked as a field tech for 2 years working on nortel meridian phone systems. The systems call for an isolated ground all the way back to XO on the transformer, and it had to be a #6 copper wire. There was a problem with this one particular switch [PBX]. It would keep restarting. They take a few minutes to come completly back up, so out of a huge building with thousands of people using the phone syatem... they notice when its down. Come to find out, we had a bootleg ground that was tied to building steel. The senior tech I was with said that was the problem... it was routed back to the transformer and they no longer had the weird random restarts.

I would not have believed it unless I saw it myself. But an IG fixed that problem.

~Matt

 

[iwire]

I would not have believed it unless I saw it myself. But an IG fixed that problem.

~Matt

I am willing to bet other ways would have fixed it as well. The real problem in my mind with IGs is that in most all installations it will be compromised by some incidental contact with a non isolated ground.

Better in my mind to design the system to work properly without IGs so that it will continue to work. :smile:

 

[mattsilkwood]

Like IGs aren't a waste anyway?

i couldnt argee more a ground is a ground is a ground.
back to the op i argee with the others that its up to the designer as to how the ig is to be installed. just install to the specs and cya

 

[TOOL_5150]

I am willing to bet other ways would have fixed it as well. The real problem in my mind with IGs is that in most all installations it will be compromised by some incidental contact with a non isolated ground.

Better in my mind to design the system to work properly without IGs so that it will continue to work. :smile:

THis is very true. I still dont know why a multi-thousand dollar PBX couldnt handle a little bit of interference. Last time I talked to someone about an IG circuit, they tied the box to the EGC and the IG terminal on the recep to a ground rod. :-? :roll:

~Matt

 

[DLTravis]

Sparky, could you please explain how shared neutrals are a fire hazard?

I know you asked Sparky but I would like to chime in :roll: . As a note, I love MWBC's. BUT.....

If they are not installed with care they can be hazardous.

I recently had a problem where a table lamp and a computer let out their beloved smoke (fire hazard).

In this case the original installer used questionable connection points (i.e. terminal blocks with four wires terminated on one side). The Neutral opened from the panel but the other three neutral connections stayed connected (three phase 120/208 panel). If I was smarter I'd post a drawing of how this looks but it puts 208 VAC across a series of 120 VAC loads. This might not cause problems if you have exactly equal loads but a table lamp and a desktop PC do not have equal resistances (impedance). The light did not last long - once it opened all was good in the electron world but it was too late for our keyboarded friend - he gave his life in the fight :grin: The third phase was not being used at the time.

Granted the installer was in violation with his termination choice. At least I think he was, I never was able to find the listing for the type of terminal strips he used. But this could happen with a wire nut or any other type of device intended to terminate (4) #12 wires.

While I love and use MWBC's and will continue to use them. They do present a unique hazard when things go wrong.

Thanks for listening, er, I mean reading
Travis

 

[iwire]

So what your saying is poor electrical installation practices are hazardous .... that is not limited to MWBCs.

 

[DLTravis]

So what your saying is poor electrical installation practices are hazardous .... that is not limited to MWBCs.

True, but there are things other than poor wiring practices which cause wires to open. To deny that MWBC's pose unique hazards is illogical. To propose they not be used because of these hazards is foolish.

 

[LarryFine]

True, but there are things other than poor wiring practices which cause wires to open. To deny that MWBC's pose unique hazards is illogical. To propose they not be used because of these hazards is foolish.

DL, keep in mind that services themselves are basically MWBC's. Many times has an open neutral caused destructive voltage imbalances. They are not limited to branch circuits.

 

[iwire]

To deny that MWBC's pose unique hazards is illogical.

Installed per NEC rules I see no additional hazard, as Larry mentions essentially all circuits are supplied via a shared neutral.

 

[DLTravis]

Installed per NEC rules I see no additional hazard, as Larry mentions essentially all circuits are supplied via a shared neutral.

DL, keep in mind that services themselves are basically MWBC's. Many times has an open neutral caused destructive voltage imbalances. They are not limited to branch circuits.

Here's where I disagree:
What would have happened in this same situation (open neutral) had this not been a MWBC?

Answer:
Devices would have lost power - smoke stays where it belongs.

Simple as that.