Thread: Under sized Neutral feeder

Can anyone tell me where the NEC talks about under sizing the neutral feeder in a conduit sub-panel. Or if not for a sub-panel a main panel.

Thanks

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Originally Posted by Rob Colecchi

Can anyone tell me where the NEC talks about under sizing the neutral feeder in a conduit sub-panel. Or if not for a sub-panel a main panel.

Thanks

Rob:

See Section 220.61(B) in the 2008 NEC for: "Feeder or Service Neutral Load."

(B) Permitted Reductions.

Originally Posted by Rob Colecchi

Can anyone tell me where the NEC talks about under sizing the neutral feeder in a conduit sub-panel. Or if not for a sub-panel a main panel.

Joe beat me to it, and with a better answer too.

Gentlemen:

See this last sentence in the 2008 NEC in Section 310.15(B)(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.

"The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met."

Also, did you know that:

"Based upon tests which have been made involving the maximum heating
that can be produced, an uninsulated conductor employed in a service cable
assembly is considered to have the same current-carrying capacity as the
insulated conductors even though it may be smaller in size."

See the attached page for Type SE Cables from the UL White Book.

I am here to please short and sweet! Got to go now to work it is 4:11 am in Baghdad!

Where was this post yesterday? Don't have access to NEC codes. Is this small enough? Pretty sure 6 & 12 gauge.

Joe and Jerry,

Thanks as usual you guys are always helpful.

Originally Posted by Joe Tedesco

See Section 220.61(B) in the 2008 NEC for: "Feeder or Service Neutral Load."

- 220.61 Feeder or Service Neutral Load.
- - (A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.
- - - Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor multiplied by 140 percent.
- - (B) Permitted Reductions. A service or feeder supplying the following loads shall be permitted to have an additional demand factor of 70 percent applied to the amount in 220.61(B)(1) or portion of the amount in 220.61(B)(2) determined by the basic calculation:
- - - (1) A feeder or service supplying household electric ranges, wall-mounted ovens, counter-mounted cooking units, and electric dryers, where the maximum unbalanced load has been determined in accordance with Table 220.55 for ranges and Table 220.54 for dryers
- - - (2) That portion of the unbalanced load in excess of 200 amperes where the feeder or service is supplied from a 3-wire dc or single-phase ac system; or a 4-wire, 3-phase, 3-wire, 2-phase system; or a 5-wire, 2-phase system
- - (C) Prohibited Reductions. There shall be no reduction of the neutral or grounded conductor capacity applied to the amount in 220.61(C)(1), or portion of the amount in (C)(2), from that determined by the basic calculation:
- - - (1) Any portion of a 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system
- - - (2) That portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system
- - - - FPN No. 1: See Examples D1(a), D1(b), D2(b), D4(a), and D5(a) in Annex D.
- - - - FPN No. 2: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral-conductor currents.

Originally Posted by Joe Tedesco

See this last sentence in the 2008 NEC in Section 310.15(B)(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.

"The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met."

- 310.15 Ampacities for Conductors Rated 0–2000 Volts.
- - (B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.16 through Table 310.19, and Ampacity Table 310.20 and Table 310.21 as modified by (B)(1) through (B)(6).
- - - (1) General. For explanation of type letters used in tables and for recognized sizes of conductors for the various conductor insulations, see Table 310.13(A) and Table 310.13(B). For installation requirements, see 310.1 through 310.10 and the various articles of this Code. For flexible cords, see Table 400.4, Table 400.5(A), and Table 400.5(B).
- - - (2) Adjustment Factors.
- - - (3) Bare or Covered Conductors.
- - - (4) Neutral Conductor.
- - - (5) Grounding or Bonding Conductor.
- - - (6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

220.61 Feeder or Service Neutral Load.
- - (A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.

Do each of the double pole 50a. conductors apply here, even though they cancel each other on the neutral current?

(And I confess, I wrote it up as undersized without knowing for sure .)

Originally Posted by Vern Heiler

220.61 Feeder or Service Neutral Load.
- - (A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.

Do each of the double pole 50a. conductors apply here, even though they cancel each other on the neutral current?

(And I confess, I wrote it up as undersized without knowing for sure .)

Vern,

1) Let's presume that the entire panel only contains 240 volt breakers.
- In this scenario there is no unbalanced load through the neutral as anytime one side of a double pole breaker trips it shuts the load down, which stops all current flow through that load.

2) Let's presume that a panel is more realistically loaded with double pole breakers and single pole breakers, and let's presume that the panel is perfectly balanced under "normal" load conditions (although we also know that never really happens in real life).
- In this scenario there is no unbalanced load through the neutral UNTIL something is shut off or trips a breaker, in which case the other ungrounded conductor is now more heavily loaded, which loads the neutral to that same amount. This condition happens all the time, and the extent of this condition changes all the time.

Let's presume we are working with 2) above, and ALL the breakers on one ungrounded conductor phase leg are off or none of the equipment on those breakers is on (either of which causes the same thing).
- Now, ALL the current from ALL the 120 volt loads on the other ungrounded conductor phase leg are drawing current through that ungrounded conductor phase leg AND through the neutral.
- That condition would cause the greatest "between the neutral conductor and any one ungrounded conductor", i.e., between the neutral and the other ungrounded conductor (the ungrounded conductor with only 240 volt loads remaining in operation on it).

Not sure if I explained that quite right as I was not quite sure exactly if that was what you were asking about.

Jerry I understand how it works in real life, its the wording of the code section that doesn't make sense.

It says "between the neutral conductor and any one ungrounded conductor." And there are two ungrounded conductors connected to the 240v breaker.

If 120v breakers were installed on one side of the panel with current drawn from every other one, the neutral current would be additive. That could happen and makes more sense than what the code wording does to me.

Originally Posted by Vern Heiler

Jerry I understand how it works in real life, its the wording of the code section that doesn't make sense.

It says "between the neutral conductor and any one ungrounded conductor." And there are two ungrounded conductors connected to the 240v breaker.

If 120v breakers were installed on one side of the panel with current drawn from every other one, the neutral current would be additive. That could happen and makes more sense than what the code wording does to me.

Vern,

The code's wording allows for "any one" ungrounded conductor. To state it your way would require much more duplicative wording and would actually (I think) make the code wording much more cumbersome to write and work with in order to cover all potential contingencies. The existing code wording covers all contingencies by simply referring to "the neutral" and "any one" ungrounded conductors. Meaning this applies to all types of wiring systems and number of phases, with the exception of those covered in the exception.

Many more exceptions would be needed, or the code much more verbose, to try to write it differently.

I think the penny just dropped!
Jerry I just re-read your post. Is the NEC referring to the buss bar as the "ungrounded conductor" ?

Originally Posted by Vern Heiler

Jerry I just re-read your post. Is the NEC referring to the buss bar as the "ungrounded conductor" ?

The NEC is referring to the ungrounded conductor which feeds the bus bar, and (of course) the same current load will will be on that ungrounded conductor as is on that bus bar.

- 300.1 Scope.
- - (B) Integral Parts of Equipment. The provisions of this article are not intended to apply to the conductors that form an integral part of equipment, such as motors, controllers, motor control centers, or factory assembled control equipment or listed utilization equipment.

- 408.3 Support and Arrangement of Busbars and Conductors.
- - (A) Conductors and Busbars on a Switchboard or Panelboard. Conductors and busbars on a switchboard or panelboard shall comply with 408.3(A)(1), (A)(2), and (A)(3) as applicable.
- - - (1) Location. Conductors and busbars shall be located so as to be free from physical damage and shall be held firmly in place.
- - - (2) Service Switchboards. Barriers shall be placed in all service switchboards such that no uninsulated, ungrounded service busbar or service terminal is exposed to inadvertent contact by persons or maintenance equipment while servicing load terminations.
- - - (3) Same Vertical Section. Other than the required interconnections and control wiring, only those conductors that are intended for termination in a vertical section of a switchboard shall be located in that section.
- - - - Exception: Conductors shall be permitted to travel horizontally through vertical sections of switchboards where such conductors are isolated from busbars by a barrier.

New NEC Article 100 Definition in the 2008 NEC describes:

Ungrounded. Not connected to ground or to a conductive
body that extends the ground connection.

So what is the easy answer for standard residential, single/split phase (120/240).
That pic definitly looks wrong... as the neutral was the same size as most of the loads.

Worst case would be if home owner lost one leg of their service.. placing half the breakers on the neutral (which just happened to a neighbor this weekend).

So for a 200 Amp service, the neutral would have to be at least 100amp to cover half the breakers (assuming max cap.).
And I saw a 70% rating in the code, so would that be 200amp * 70% = 140amp minimum size for neutral.

Seems better to use the same size, not much savings compared to all the load calcs required to be legal.

So that panel with #6 power, should have had a #8 as a minimum for the neutral.
(The Panel itself should be limited to 50 amps total, from an upstream breaker)