Second year Lesson 10 of 13 About 16 min

Services and panels

What you’ll learn

• The path from the utility transformer through the meter and the main disconnect to the panel.
• Why a 120/240 V service gives you 120 V and 240 V, and what the neutral carries.
• The one place neutral and ground are joined, and why subpanels keep them apart.
• Which parts of a panel stay live with the main off.

A service is everything between the utility’s wires and the first overcurrent device in the building: the service conductors, the meter, the main disconnect, and the panel it feeds. This lesson follows the power from the pole (or the pad) to a branch circuit, names each part, and explains the two ideas that trip people up: what the neutral actually carries, and the one place it is joined to ground. How big any of it must be is a code-book question; how it works is here.

From the utility to the panel

The utility transformer (on a pole, or a green box on a pad) steps the distribution voltage down to what the building uses. The service conductors come from there, overhead (a service drop to a mast) or underground (a service lateral), to the meter, which only measures energy. Next is the main disconnect — usually the main breaker at the top of the panel, sometimes a separate switch — the one place you can switch off everything the service feeds. Then the panel: busbars that the branch-circuit breakers clip onto, a neutral bar, and a bar for the bonding conductors.

Who owns what varies: in most places the utility owns everything up to the meter, and you own from the meter base on. Your utility’s service guide and your code book both have a say in the service; find out which rules your area follows before you plan one.

120/240 V split-phase

A house service in Canada and the US is usually 120/240 V single-phase. The transformer’s secondary is one winding with a tap in the middle. The two ends are L1 and L2; the middle is the neutral. From either end to the middle is 120 V; from end to end is 240 V. Each row of breaker spaces down the panel is on the opposite line from the row above it (the left and right spaces in one row share a line), so a single-pole breaker gives you 120 V and a double-pole breaker, straddling both, gives you 240 V.

Commercial buildings often get a three-phase service instead — 120/208 V, 277/480 V in the US, or 347/600 V in Canada. Same parts, three lines instead of two; the three-phase lesson covers the voltages.

Neutral and ground: joined once

The neutral is a current-carrying conductor. The bonding (equipment grounding) conductors are not — they only carry current during a fault, long enough to trip the breaker. The two systems are tied together at one point: the main bonding jumper at the service, which is also where the grounding electrode connects. Everywhere downstream they are kept apart. (A separately derived system — the secondary of a dry-type transformer, for one — gets its own bond at its source; your code book gives the details.) If they were joined again in a subpanel or a box, normal neutral current would split and flow on the bonding conductors, conduit and enclosures — metal that people touch.

Worked examples

1. Neutral current on a split-phase panel

1. The 120 V loads on L1 add up to 20 A; on L2, 12 A.
2. The neutral carries the difference: 20 − 12 = 8 A.
3. Move a 4 A load from L1 to L2 and both sides carry 16 A: the neutral current drops to 0 A. That is what balancing a panel does.

2. A 240 V load

1. A 4,800 W water heater on 240 V: I = P ÷ V = 4,800 ÷ 240 = 20 A.
2. That 20 A goes out on L1 and back on L2. The neutral carries nothing from it — a straight 240 V load does not need one.
3. Were it a 120 V heater of the same wattage, it would draw 4,800 ÷ 120 = 40 A. Higher voltage, half the current: why big loads run at 240 V.

Try one: 15 A on L1, 15 A on L2, all 120 V loads

The neutral carries 15 − 15 = 0 A. Perfectly balanced.

Sur le chantier

Check yourself

7 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.

Not started yet.

1. On a 120/240 V single-phase service, what voltage do you read from L1 to L2?

   A 120 V B 208 V C 240 V D 0 V
   Answer and why

   C. L1 and L2 are the two ends of the transformer’s secondary. The neutral is its centre tap, so each line is 120 V to neutral and the two together are 240 V.

2. Where are the neutral and the equipment ground (bonding) system joined together?

   A In every panel and junction box B At the service (and at the source of a separately derived system, such as a transformer secondary) — never again downstream C At each receptacle D Nowhere, ever
   Answer and why

   B. The main bonding jumper joins them at the service. Downstream — in a subpanel, a box, a device — they stay separate, or normal neutral current would flow on the bonding conductors and metal parts.

3. L1 is carrying 20 A of 120 V loads and L2 is carrying 12 A of 120 V loads. About how much current is on the neutral?

   A 32 A B 20 A C 12 A D 8 A
   Answer and why

   D. On a split-phase service the two lines are opposite each other, so the neutral carries only the difference: 20 − 12 = 8 A. Balance the two sides and the neutral current falls toward zero.

4. A 4,800 W, 240 V water heater is on a double-pole breaker. What current is on each line, and on the neutral?

   A 20 A on each line, none on a neutral B 40 A on each line C 10 A on each line, 20 A on the neutral D 20 A on one line only
   Answer and why

   A. I = 4,800 ÷ 240 = 20 A, flowing out on L1 and back on L2. A straight 240 V load does not use the neutral at all.

5. What is the main disconnect for?

   A Measuring energy B Letting you switch off everything the service feeds, in one place C Bonding the neutral D Reducing voltage
   Answer and why

   B. The main breaker or switch is the one place to shut off the whole building’s supply. The meter measures; the main disconnects.

6. In a subpanel fed from the main panel, the neutral bar and the ground bar are…

   A bonded together with a jumper B kept separate C the same bar D connected only to the grounding electrode
   Answer and why

   B. The neutral-to-ground connection is made once, at the service. In a subpanel the neutral bar is isolated from the enclosure and the bonding conductors land on their own bar.

7. Why can you not just tighten a service-entrance lug with the main breaker off?

   A You can — the main breaker makes it dead B The line side of the main is still live from the utility; only the utility can make it dead C The breaker would trip D Lugs are never tightened
   Answer and why

   B. Switching off the main kills the load side. The conductors from the meter to the main’s line terminals are still energised, with the utility’s fault current behind them. Treat them as live until the utility has isolated the service.

Start the quiz over

Questions fréquentes

Voir aussi

• Lire le répertoire des circuits d’un panneau
• Continuité des masses ou mise à la terre
• Sous tension, neutre, mise à la terre
• 240 V receptacle wiring — a load that uses L1 and L2 and no neutral.
• Cadenassage, pas à pas
• Grounding and bonding — the first-year lesson this builds on.
• All second year lessons