10.1 Residential Service Upgrade Integrated Lab

Lab scenario

A homeowner is upgrading a 1970s single-family dwelling from a 100 ampere overhead service to a new 200 ampere meter-main with an exterior service disconnect and a feeder to an interior panelboard. The dwelling has 2,400 square feet of living area, two 20 ampere small-appliance branch circuits, one laundry circuit, an electric range rated 12 kW, a 5 kW clothes dryer, a 4.5 kW water heater, a 1.2 kW dishwasher, a 0.9 kW disposal, a 7.5 kW electric heat strip, and a 4 ton air-conditioning condenser. The owner may add an electric vehicle charger later, but it is not on the permit drawings.

Treat this as a master-level plan check, not a race to a single ampere number. First decide which loads are actually part of the submitted scope. Future equipment is not sized by guessing, but a supervisor may require spare capacity, raceway planning, or a written exclusion depending on the contract and local utility coordination. On an ICC master exam, do not invent future load unless the problem states it. On a job, document what is excluded so the customer does not assume the service was engineered for every future appliance.

Calculation order

Start with the dwelling load method allowed by the applicable NEC cycle listed for the exam or jurisdiction. R16 candidates work from the 2023 NEC, T16 from the 2020 NEC, and G16 from the 2017 NEC if the licensing agency still uses it. The article structure and table numbers may shift by cycle, so your book tabs should lead you to dwelling unit load calculations, service conductor sizing, grounding electrode conductor sizing, and equipment bonding.

A disciplined order is: general lighting load by floor area, required small-appliance and laundry circuits, fixed appliances, cooking equipment, clothes dryer, heating and cooling comparison, and then final service or feeder load. Keep volt-amperes and watts consistent. For single-phase dwelling calculations, many exam problems expect you to convert the final volt-ampere result to amperes at 120/240 volts only after demand factors are applied. Converting each load too early can create rounding drift and wrong answer choices.

Use a worksheet. Mark each load as continuous, noncontinuous, demand-adjusted, nameplate, or largest of heat and air-conditioning. For this scenario, electric heat and cooling are not simply added together if the code rule lets you use the larger noncoincident load. The 7.5 kW heat strip may control over the condenser depending on the stated condenser load. If the condenser question gives minimum circuit ampacity and maximum overcurrent protection, those values are for branch-circuit conductors and short-circuit protection, not necessarily the dwelling service demand load.

Code-navigation plan

Open book does not mean slow book. Use index entries for dwellings, services, service conductors, grounding electrodes, bonding, and equipment short-circuit current rating. In the dwelling calculation article, put a finger on the standard and optional methods and ask which one the question author is using. If a problem says use the optional method, do not mix standard method demand factors into the answer. If it says calculate the minimum service rating, answer the minimum required by the code method, not the round number a contractor prefers to sell.

After the load result, move to conductor and equipment checks. The service disconnect, meter-main, panelboard, service entrance conductors, grounding electrode conductor, main bonding jumper, and intersystem bonding termination all have separate rule paths. A common exam trap is to size the grounding electrode conductor from the overcurrent device instead of from the service-entrance conductor material and size, where that rule applies. Another trap is using equipment grounding conductor tables for grounding electrode conductors. They are not the same table or concept.

Field supervision decisions

Before approving the job, verify utility service drop requirements, point of attachment, working clearance, service disconnect location, bonding of metal water piping, and the grounding electrode system. Older homes may have a metal underground water pipe electrode, a concrete-encased electrode that was never exposed, ground rods added by a prior installer, or a disconnected bonding jumper at a water heater. The master electrician should direct the crew to inspect before demolition, because a missing electrode path discovered after the service is off creates schedule pressure and unsafe improvisation.

Fault current matters. The available fault current from the utility must be compared to the service equipment short-circuit current rating and interrupting ratings. Do not assume a 200 ampere residential service has harmless fault current. A larger transformer, short service lateral, or multi-dwelling utility arrangement can create available fault current above the rating of common equipment. If the problem gives available fault current and an equipment rating, compare them directly. If it asks for field marking, navigate to the service equipment marking rule and answer what must be documented.

Red flags and exam traps

Watch for multiwire branch circuits moved into a new panel without handle ties or common trip where required. Check AFCI and GFCI requirements by location and outlet type under the governing cycle. Confirm neutral isolation in the interior panel when the service disconnect is outside. Once the service disconnect is the service equipment, the feeder to the interior panel needs separate neutral and equipment grounding conductor paths. A bonding screw left in the downstream panel is a supervisory defect, not a minor trim issue.

For exam purposes, do not let field preference overwrite code minimums. Many contractors install a 200 ampere service because it is practical and marketable, but the calculated minimum might be lower. Conversely, a 200 ampere label does not cure undersized conductors, wrong temperature column assumptions, missing bonding, or equipment not rated for available fault current. The master-level skill is separating load calculation, conductor ampacity, overcurrent protection, grounding and bonding, and listed equipment limitations into decisions that can each be defended.

Structured Decision Aid

• Confirm existing service size, new load list, dwelling demand factors, grounding electrode system, and utility requirements.
• Calculate load before choosing service conductors, service disconnects, and overcurrent protection.
• Check GFCI/AFCI, surge protection, bonding, and panel labeling as part of the upgrade review.
• Separate minimum code answer from design recommendation when the scenario includes future expansion.