Sharing neutral for different circuits?

[a10jp]

Can you share neutral between 3-phase and 1-phase ckts? Is it safe for such connection? Since the neutral is used to balance a 3-phase ckts in a different manner than 1-phase circuits, can you really share them?

 

[waross]

Good question;
Canadian Electrical Code 1998
Rule 4-022 Size of Neutral Conductor.
(1) The neutral conductor shall have sufficient ampacity to carry the unbalanced load.

Rule 4-024 Common Neutral Conductor.
Providing that when in metal enclosures all conductors of feeder circuits employing a common neutral are contained within the same enclosure a common neutral shall be permitted to be employed for:
(a) Two or three sets of 3-wire single-phase feeders; or (b) Two sets of 4-wire, 3-phase feeders.

Is there an NEC equivalent?

Is this an academic question or do you have an actual installation? Do you need help sizing a common neutral?
yours

 

[a10jp]

Hi. I am reviewing a set of drawings, where the designer borrow the neutral from the 1-phase system to the 3-phase. It makes me wonder. I would assume, if the neutral are of different sizes, then it shall use the larger of the 2 sizes. I agree with Rule 4-022 you have stated above.

 

[waross]

Is this a a 4 wire delta system or two seperate transformer banks?
Is it actually a seperate single phase system or is it a 120 Volt circuit from the 120/208 volt supply?
Do both circuits originate in the same panel?

 

[a10jp]

My apology, I did not provide enough info. Yes, the 2 systems are separate. One came from a 1-ph, 3W, 75kVA xmfr, and the other from 150kVA 3-ph, 4W. So the designer creates the neutral from the 1-ph system bonded with the ground and carry the ground to the ground temrinals of the 3-phase service panel, which he will derive his neutral, although the drawings lost its clarity after that point.

 

[davidbeach]

Two entirely different neutrals. The two need to be kept entirely separate, or you will have neutral current flowing on some portion on the ground system and that is not permitted nor desired.

 

[waross]

Is this two seperate systems each with it's own neutral but using a common ground electrode?
If they are fed from the same utility there will probably be an indirect connection between them through the supply utility neutral, and the distributed grounding system of the utility neutral.
In a large building such as a shopping mall where individual tenant services are fed from dry type transformers, it was common to use the building ground grid for grounding all of the individual tenant services. The arrangement for a tenant service may be;
1> A large transformer from the supply primary to 480 Volt or 600 Volt switch gear.
2> From a breaker in the switch gear to the revenue meter.
3> From the revenue meter to a dry type transformer. The secondary of the dry type transformer may be 120/240 V or 120/208 V.
4> From the dry type transformer to the tenants service panel.
5> The panel ground, which would serve as both system ground and equipment ground would extend back to the 600 V switchgear and be connected to the main building ground bus.

This is allowed under Canadian Electric code rule #;
10-204 and other rules
10-204 Grounding Connections for Alternating-Current Systems
(4) Notwithstanding Subrule (1), for circuits that are supplied from two sources in a common enclosure or grouped together in seperate enclosures and employing a tie, a single grounding electrode connection to the tie point of the grounded circuit conductors from each power source shall be permitted.

I think that the poster has two seperate systems served by a common ground, rather than a common neutral.
My understanding of the code is that the system ground conductor must be in the the same raceway as the supply conductors. If the supply conductors for both transformers are in the same raceway, it may be acceptable to use a common grounding conductor sized for the largest system.

See also
NEC 250-26 Grounding Seperately Derived Alternating-Current Systems.
NEC 250-54 Common Grounding Electrode.
This rule would seem to require that both systems be grounded to the same ground electrode.
250-81. Grounding Electrode system.
Excep[tion No. 1 will probably also apply.

 

[jghrist]

If you ground the 1Ø neutral at the 3Ø service, you will have ground system current as mentioned by davidbeach. Grounding at two points is not permitted by the NEC.

If the neutral is not grounded at the 3Ø service, then you still have to run the neutral from the 1Ø source to the loads served by the 1Ø source. Otherwise, the return current would have to go through the 3Ø system neutral to the 3Ø service, then through the ground system to the 1Ø service. Current flowing from the 1Ø transformer has to return to the 1Ø transformer. What would you do with the 1Ø system neutral if you didn't run it with circuits served by the 1Ø source?

Tying the two neutrals together at the 3Ø service would ground the 1Ø neutral in two places because the 3Ø system neutral is grounded there.

 

[a10jp]

For the single phase transformer service, 210/105V, 1PH, 3W, first the designer derived the neutral bar from the ground electrodes connected externally to the bldg, and from this neutral bas he bonded to the ground bar, which is connected to the panel casing. Then within the same bldg, for the 3-phase 4W transformer service, which is feeding a new service disconnect, 208Y/120V, 3 PH, 4W, the designer borrowed a neutral from the signle phase service mentioned above to the new service disconnect neutral bar, which is 3 ph, and from which he made a ground bar, and bonded to the casing.

I do not know why he did not use the neutral from the 3-phase transformer to form his disconnect neutral. One more thing, this service disocnnect is feeding another bldg. Right there, it is already not in compliance with NEC art 250. But when I saw the the borrowed neutral from the single phase service to the 3-phase service, it makes me wonder if ther might be any situation where this might be allowed. But not here, one can tell clearly.