The 83% Rule: Sizing Dwelling Services and Feeders – Lesson 9.4

Sizing Services and Feeders: Breaker First, Then Wire

Lesson 9.4 from the Electrical Code Coach covers a type of question that shows up on almost every journeyman and master exam: sizing the overcurrent protective device (OCPD) and conductors for services and feeders. The lesson looks at commercial and dwelling unit installations separately, because they follow different rules.

The most important habit from this lesson is simple: always select your overcurrent protective device first, then choose your wire size.

The OCPD also sets the rating of the service. If your calculated load leads you to a 200-amp main breaker or fuse, you have a 200-amp service. As the coach explains:

“If I’m sizing services and I end up with a 200 amp overcurrent protective device and that is the main breaker on that service, that is now a 200 amp service.”

Make sure this idea is clear before you move on. The dwelling unit rules later in this lesson depend on it.

Commercial Services: The Simple Case

The lesson starts with commercial work, which may seem backwards but makes sense. Commercial service sizing uses only the basic ampacity rules, with no special exceptions.

Worked Example: 80,000 VA Three-Phase Load

Question: What size main breaker and copper conductors would you select for a business with a total calculated load of 80,000 VA on a 120/208V three-phase system with 75°C terminals?

Step 1 – Find the load in amps. Every three-phase calculation needs a little setup first. Multiply 1.732 by the system voltage:

• 1.732 × 208 = 360
• 80,000 VA ÷ 360 = 222 amps

Step 2 – Check for demand factors. There are none here, so you size the service normally.

Step 3 – Size the OCPD. Go to Table 240.6(A) and pick the next standard size above 222 amps. That is a 225-amp breaker.

Step 4 – Size the conductor. Go to the main ampacity table, Table 310.16. Use the copper side and the 75°C column, then move down to a conductor that can carry 222 amps. That is 4/0 AWG copper.

That’s all there is to it. Nothing new is needed, just the skills from earlier lessons.

Worked Example: When the Breaker Is Already Given

Question: What size copper wire would you select for a commercial service with a 125-amp main breaker and a total connected load of 118 amps on a 120/208V system with 75°C terminals?

Here the question has already sized the OCPD for you. Check for demand factors (there are none), then go straight to Table 310.16. In the 75°C copper column, #1 AWG copper is the smallest conductor that can carry 118 amps.

The commercial process is:

1. Find the total connected load in amps
2. Check for demand factors
3. Size the OCPD from Table 240.6(A)
4. Size the conductor from Table 310.16

Dwelling Units: Meet the 83% Rule

Residential services have an extra rule. It applies to services and feeders between 100 and 400 amps for:

• Single-family dwellings
• Individual dwelling units in two-family and multifamily buildings

This is known as the 83% rule. It lets you size service and main feeder conductors at 83% of the service rating. The coach warns that this part of the lesson is dense:

“The next few slides are very dense, so I want you to slow down, take your time, watch it as many times as you need — but I know that you can do it.”

Why You Can’t Just Use Table 310.16

Take a dwelling with a calculated load of 47,000 VA at 240V. That works out to about 196 amps, so you would choose a 200-amp main breaker. Normally, the next step would be to size the wire for 196 amps using Table 310.16.

That is not the method for dwelling services between 100 and 400 amps. Use the dedicated dwelling table instead:

• 2017 NEC: Table 310.15(B)(7)
• 2020 and 2023 NEC: Table 310.12

You could work out the 83% by hand, but the table has already done it. The coach calls it a cheat sheet.

How to Read the Table

As with every NEC table, work from top to bottom and left to right, using the column headings to find your way.

• Left column: The service or feeder rating in amps. It stops at 400, because the rule only applies up to 400 amps.
• Middle column: Copper conductor size
• Right column: Aluminium conductor size

Find your service rating on the left, then move across to the conductor material you need. For a 100-amp service in copper, find 100 and read the copper column.

When the 83% Rule Applies and When It Doesn’t

On the exam, the hard part is often deciding whether the rule applies at all.

Single-Family Dwellings

Scenario 1: Meter socket, riser and main panel inside. The conductors go up the riser and into the main breaker panel inside. All of this is service conductor, and the 83% rule applies to every part of it, as long as the service is between 100 and 400 amps.

Scenario 2: Meter-main combination outside, feeding a main panel inside. The rule covers the service riser conductors. It also covers the feeder from the meter-main to the first panel inside, but only if that panel carries the entire load of the dwelling.

The coach points out the catch:

“It has to be the panel that feeds the entire structure. You’re not allowed to use it on other sub panels or on any sub panel that does not feed the entire structure.”

Two-Family and Multifamily Dwellings

For these buildings, the rule applies to feeders only, and the feeder must supply the entire dwelling unit.

Picture an apartment attached to a larger building where the main service sits. The service riser from the meter centre does not use the 83% rule. Size it with the standard rules from earlier lessons. The individual feeder running to each apartment can use the 83% rule.

This saves real money. Across many single-family houses or apartment units, being able to use a smaller conductor than normal ampacity sizing would require adds up to a big material saving.

Worked Examples: Using the Dwelling Table

Copper Conductors

Question: What size copper wire and main breaker would you select for a service with a total calculated load of 47,000 VA for a single-family dwelling on a 120/240V system with 75°C terminals?

1. Find the load in amps: 47,000 VA ÷ 240V ≈ 196 amps
2. Check whether the rule applies: Is it between 100 and 400 amps? Yes. Is it a single-family dwelling? Yes. The 83% rule applies.
3. Size the OCPD: From Table 240.6(A), the next size up from 196 amps is a 200-amp breaker
4. Size the conductor: In Table 310.12, find 200 amps on the left and read across to copper: 2/0 AWG copper

Aluminium Conductors

Question: Same dwelling and same 47,000 VA load, but the question asks for aluminium conductors.

The steps are the same, and you still end up with a 200-amp main breaker. This time, read across to the aluminium column: 4/0 AWG aluminium.

Exam writers like to catch people with this. Read each question twice and check whether it asks for copper or aluminium before you commit to an answer.

The Bottom Line

The process for sizing services and feeders:

1. Always size the OCPD first. Its rating is the service rating.
2. Commercial: Size the conductor from Table 310.16 based on the load and the terminal temperature rating.
3. Dwelling, 100–400 amps: Check whether the 83% rule applies. If it does, use Table 310.12 (Table 310.15(B)(7) in the 2017 NEC).
4. Check the scope: The rule applies to service conductors and to feeders carrying the entire dwelling load. It never applies to a subpanel that serves only part of the structure, or to the main service of a multifamily building.

The coach’s advice for mastering these questions:

“When it comes to these type questions, the only way to beat it is through repetition.”

How NEC Mastery Fits Into This

Service and feeder sizing can go wrong at several points. You might miss a demand factor, choose the wrong table, read the wrong column, or apply the 83% rule where it doesn’t belong. The fix is plenty of practice, and that’s what NEC Mastery offers.

• 8,000+ exam-style questions give you as many commercial and dwelling service sizing problems as you need. You practise the “OCPD first, then wire” routine until it becomes automatic.
• Detailed explanations referencing specific NEC articles show you exactly why an answer used Table 310.16 or Table 310.12. That helps you learn where the 83% rule applies and where it doesn’t.
• Timed mock exams weighted to your exam type test how quickly you can do three-phase calculations, move between Table 240.6(A) and the ampacity tables, and spot copper-versus-aluminium traps under real exam timing.
• Practice built around your codebook means you learn these tables by finding answers in them over and over, not by memorising sizes.