# Why is the neutral conductor sometimes undersized in AC applications?

Why are electricians allowed to sometimes lower the size of the grounded (neutral) conductor?

If alternating current is bidirectional then wouldn't the neutral technically be an ungrounded (hot) conductor 60 times a second? (For 60hz)

I can get my head around multiwire branch circuits sharing a neutral and phases and what not, but this one stumps me.

• Seems to me it might be the other way around. Two lives returning through one larger neutral. But some sort of reference would help clear this one up. Though with GFI breakers that would be a bad idea. Jan 11, 2018 at 4:48
• It depends if it is a "return neutral", as in single phase systems, or a "star center neutral" as in 3 phases systems. Jan 11, 2018 at 11:30
• Archonosx has the code referenced below for a good answer. Another case is the multi wire branch circuit where 2 hotts are using only 1 neutral. This specific case won't overload the single neutral because L1 & L2 are 180 degrees out of phase. Jan 11, 2018 at 14:08

Why are electricians allowed to sometimes lower the size of the grounded (neutral) conductor?

As DrMoishe points out some pieces of equipment use mostly 240 volts and very little 120 volt power. It is the 120 volt power that uses the neutral. Therefore the National Electrical Code allows the downsizing of the neutral in limited application. This is NOT blanket permission to downsize all neutrals. It is only applicable in a few instances.

It is possible to downsize a feeder or service neutral according to the calculations in the National Electrical Code here:

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) deter‐ mined by the following basic calculations:

(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 system; or a 3-wire, 2-phase system; or a 5-wire, 2-phase system

Informational Note: See Examples D1(a), D1(b), D2(b), D4(a), and D5(a) in Informative Annex D.

(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

Informational Note: 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.

Regarding branch circuits, the code allows the downsizing of ranges and cooking equipment here, attention to exception 2:

210.19(A)(3) Household Ranges and Cooking Appliances. Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8 3∕4 kW or more rating, the minimum branch-circuit rating shall be 40 amperes.

Exception No. 1: Conductors tapped from a 50-ampere branch circuit supplying electric ranges, wall-mounted electric ovens, and counter-mounted electric cooking units shall have an ampacity of not less than 20 amperes and shall be sufficient for the load to be served. These tap conductors include any conductors that are a part of the leads supplied with the appliance that are smaller than the branch-circuit conductors. The taps shall not be longer than necessary for servicing the appliance.

Exception No. 2: The neutral conductor of a 3-wire branch circuit supplying a household electric range, a wall-mounted oven, or a counter-mounted cooking unit shall be permitted to be smaller than the ungrounded conductors where the maximum demand of a range of 8 3∕4-kW or more rating has been calculated according to Column C of Table 220.55, but such conductor shall have an ampacity of not less than 70 percent of the branch-circuit rating and shall not be smaller than 10 AWG.

There are no other allowances for a downsized neutral that I can find in the National Electrical Code.

Most dryers and ranges are wired with NM cable and will therefore have full sized neutrals anyway. If you are wiring with conduit, you could use this exception for the range.

The dryer circuit neutral has to be able to carry the full imbalance of the load. Since that load is unknown because you are wiring a receptacle not a piece of equipment, then the neutral must be full sized. These circuits are normally #10 wire and the range exception sets the minimum at #10 so there would be no reduction even if dryers used the same exception.

Hopefully, this will help to clarify this subject.

For some devices, most current is consumed across the 240 V L1 and L2, while the neutral gets only an amp or two. For example, an electric dryer heating element might draw 30 amps when hot (and a considerably higher surge current before it warms, since cold resistance is lower) across the 240 V lines. The electric motor, timer and other 120 V devices in the dryer need perhaps 5 A.

That said, it depends on the electric code whether a smaller neutral is allowed.

• You seem to be describing a North American centre-tapped 2-phase system where the phases at 180°. You should probably clarify this in your answer. Jan 11, 2018 at 6:49
• @Transistor Not necessarily. In a Y configuration 3-phase system load can be connected either line-neutral or across lines. A lot of three phase appliances do exactly that (even if for increased voltage). Norway uses delta three phase config where there is no neutral and all connections are more or less equivalent to line-neutral. Typical 4 (or 5) conductor wire has all conductors of the same diameter, not one 3 times the size of others Jan 11, 2018 at 11:26
• @Transistor North America uses a center-tapped single phase system. Jan 11, 2018 at 14:11
• @Tester101 that is a matter of perspective and naming conventions. It can equally well be called a two-phase 180degree system. The fact that the two phases are generated from one phase of the three phase transmission system is not overly relevant to the customer.. Jan 11, 2018 at 15:34
• @Trevor_G From Wikipedia: "A split-phase or single-phase three-wire system is a type of single-phase electric power distribution.". Which is not the same as a Two-phase system. Jan 11, 2018 at 15:53

why are electricians allowed to sometimes lower the size of the grounded (neutral) conductor?

Because in the USA, houses are typically provided with a "split phase" supply. In most of the rest of the world, houses mostly use a single-phase supply - which is what you probably have in mind as it is the simplest system. Industry and some houses are supplied with a three-phase supply which I will ignore here.

In a single phase AC supply, you are right, the polarity reverses many times a second so that current flows in the other direction. This also means that the voltage is reversed. A single phase system has two conductors. In the US these are described as hot and neutral. In other countries the hot is known as live or line.

Since the current/voltage reverses many times a second, it may seem that these labels are arbitrary. However there is a convention that one of these wires is connected to ground near the main panel. It is a convention that all the fuses, breakers and switches are in the other wire. This makes for a more safe arrangement.

In a split phase system, you have three conductors, We can call these h1, h2 and neutral. There is 240 VAC across h1 and h2. This allows high-powered appliances to be connected by thinner wires than would be needed by a 120V appliance of the same power. losses in the wires are also lower. It is important to note that none of the current for a 240V appliance runs through neutral, it all flows through h1 and h2.

So wires to a 240V receptacle can use a thinner neutral wire where the appliance uses 240V for its main load, even if it also uses 120V for a small part of its load. The same would apply to wires serving a mixture of 240V and 120V outlets.

There is more current flowing back and forth along the h1 and h2 wires than flows back and forth through the neutral wire.

If alternating current is bidirectional then wouldn't the neutral technically be an ungrounded (hot) conductor 60 times a second? (For 60hz)

No.

You should remember that voltages are relative. there are no absolute voltages. The labels hot and neutral are arbitrary so far as the electron flow is concerned.

Note that you can take a 9V PP3 battery and label the two connectors 1000000 volts and 1000009 volts. Or -509 and -500. All the usual calculations can then be applied (e.g. ohms law V=IR) and get the correct results. This is because we are always dealing with voltage differences between two points. We are used to labelling one connector 0V for simplicity and we usually choose the least positive connector to label as 0V. This is DC of course. For AC the choice is even more arbitrary. Nevertheless the convention is useful.

Neutral is always tied to ground and there should be very little difference in voltage between neutral and ground. There will be some difference because of the resistance in the wire - but this should never be more than a few volts.

Neutral is never "ungrounded hot" as that would imply a significant voltage difference between neutral and ground - which doesn't happen.

What this means is that we are arbitrarily labelling neutral as 0V and measuring the voltage on the hot side relative to the neutral or ground conductors.

What is called "netural" is not decided by directionality of current. It's decided by which conductor is pegged to be near ground.

In low-voltage DC design, you may feel obligated to put battery - to backplane/return/GND, because you are working in semiconductors, and they care, and most semiconductors are architected for Vcc+. But you get a pre-semiconductor toy, say that is motors and limit switches, and put the batteries in backwards - it works fine! So the idea of dirctionality relating to ground doesn't even apply in DC world.

The AC transformer output is always varying in voltage. Until it's grounded, it is an isolated system - none of it has a relation with ground. We establish that relationship for a number of reasons, but it's arbitrary. You could tie L1 pole to ground if you like; then L1 would be neutral, N would be 120V, and L2 would be 240V. When we peg it in the middle, we get 240V from the poles, but each pole is only 120V from earth.

Still, neutral is not ground. It's just near ground because of our best efforts to peg it at ground inside the main service. As soon as you get outside the main panel, neutral may not be at ground potential, differing by voltage drop, or more if there is a wire problem.

As such, calling it the grounded conductor is misleading and wishful thinking, and a weird glitch because of what happens when lawyers write code books. It is a conductor (it is meant to carry current under normal circumstances), but it is certainly not grounded except inside the main service.

The neutral wire only carries the imbalance current between phase a and phase b, that's the main reason that it is allowed to be smaller. Neutral wire is tied to ground and also the center tap on the secondary (output) of your transformer out on the pole (right in the center of the secondary coil, making a reference point to 0v gnd, that splits the 240v potential into two usable half, or it's full 240v potential)

In other words, imagine you only have 2 breakers on in your house, 120volts only. One breaker is on phase a and the circuit is pulling 15 amps, the other breaker is on phase b pulling only 12 amps. You will see roughly 3 amps on the neutral wire at the service, even though a total of 18 amps is seen between phases a and b.

Keep in mind that the neutral size is only usually reduced on the service entrance conductors and not on branch circuits.