Engineering services
Power System Studies
Short-circuit, coordination, arc-flash and load-flow for commercial and industrial facilities across Maricopa County.
A power system study is not a closeout deliverable. The short-circuit number sets the bus bracing on the switchgear, and APS will not approve a commercial meter enclosure for manufacture without seeing that rating, so the modeling starts before the gear is released. We collect the field data, build and maintain the one-line, and do the installation work the study calls for.
Four studies, one model
Short-circuit, coordination, arc-flash and load-flow are four outputs of one model. You build the one-line once, populate it with real impedance and length data once, then run it four ways. Anyone selling them as four separate engagements is charging four times for a single round of data collection.
| Study | What it answers | What it changes |
|---|---|---|
| Short-circuit | Maximum and minimum fault current at every bus | Interrupting ratings, bus bracing, conductor withstand |
| Protective device coordination | Which device opens first for a given fault | Trip unit settings, fuse class and size, device selection |
| Arc-flash | Incident energy in cal/cm2 at a stated working distance | Labels, PPE, work permits, whether a task can be done energized at all |
| Load-flow | Steady-state voltage and loading at every bus under a given configuration | Transformer taps, feeder sizes, where capacity actually runs out |
The relationship between coordination and arc-flash is the one that surprises people. Speeding an upstream breaker up to cut incident energy can break selectivity; slowing it down to gain selectivity raises incident energy at the bus below it. You cannot optimize both everywhere. The engineer has to be told which buses a maintenance electrician actually opens, and the rest gets the compromise.
Why the arc-flash study anchors a safe work practice program
OSHA does not publish an incident energy table. What it requires, at 29 CFR 1910.132(d)(1), is that the employer assess the workplace for hazards that make personal protective equipment necessary and select PPE that matches what was found. For electrical work the consensus method for that assessment is NFPA 70E, and 70E's arc flash risk assessment is what produces a number in calories per square centimeter that a PPE selection can be defended against.
Without a study you are working off the PPE category tables, which carry hard limits on the fault current and clearing time they cover. On a Phoenix service fed from a large utility transformer, plenty of gear sits outside those limits. The point of the study is that the guessing stops.
The code side is separate and narrower, and it gets conflated constantly. NEC 110.16(A) requires a field-applied arc-flash warning label on switchboards, panelboards, industrial control panels, meter socket enclosures and motor control centers in other than dwelling units that are likely to require examination while energized. NEC 110.16(B) adds a specific label for service equipment rated 1,200 amperes or more: nominal system voltage, available fault current, clearing time and the date the label was applied. NEC 110.24 requires service equipment in other than dwelling units to be field marked with the maximum available fault current and the date of that calculation. None of the three requires an incident energy study. They are marking rules on the installation. The study obligation sits on the employer, through OSHA and 70E, and it does not go away because the building passed inspection.
We do not write your electrical safety program and we do not train your people to it. We produce the study, apply the labels, and make the installation changes the study calls for. The written program, the training records and the energized work permits stay in the facility's own document set.
What has to be collected before anyone can model anything
The model is only as good as the data behind it, and studies go wrong in the same three places every time.
The utility contribution
The available fault current at the service comes from the serving utility, not from a rule of thumb and not from the last set of drawings. In the Valley that means APS or SRP, and the city name will not tell you which. SRP's own list of the fifteen cities it serves reads "served all or in part" — Phoenix, Scottsdale, Glendale, Peoria and Avondale are all on it, and APS also serves parts of each. The boundary cuts through neighborhoods and in places through individual streets. Read the utility name off a bill for the site, check the APS Phoenix Metro service territory map, or call APS at 602-371-7171 or SRP at 602-236-8888.
Ask for the maximum available fault current at the point of delivery, and treat the answer as a snapshot with a date on it. Utilities change out service transformers and re-source feeders without writing to the customer, which is one of the main reasons a study has a shelf life.
The equipment data
- Transformer kVA, primary and secondary voltage, percent impedance and X/R, taken from the nameplate rather than the schedule.
- Conductor size, material, quantity per phase and run length for every feeder in the model. Length moves the answer as much as size does.
- Overcurrent device manufacturer, frame, type, trip unit model, and the settings as found: long time, short time, instantaneous, ground fault. Photograph the dials.
- Fuse class as well as ampere rating. A 400 amp Class J and a 400 amp Class L are not interchangeable in the model.
- Motor horsepower, voltage and starting method for anything large enough to contribute to a fault.
- Generators, PV inverters and battery systems, with their fault contribution characteristics and their transfer scheme.
The gear you cannot open safely
Some equipment will not come apart safely without an outage, and we do not open energized gear to collect data. Anything that cannot be read dead goes onto an exception list with the assumption we used recorded against it, and it is picked up at the next planned shutdown. A study built on undocumented assumptions is worse than none, because the label on the door looks equally authoritative either way.
There is a procurement reason to front-load all of this. APS ESRM 302.9 requires every commercial meter enclosure to be submitted for APS review and approval before the equipment is manufactured, and the submittal must show bus bracing, meaning the AIC rating, alongside the one-line, ampacity, voltage, phase and service disconnect count. SRP's Electric Service Specifications carry the parallel rule on the supply side: the short-circuit rating of any disconnect or overcurrent device must be at least equal to the service equipment. If the study lands after the switchboard is released, an undersized bracing rating is a re-order, not a field fix — which is why we run it alongside the service and distribution package rather than after it.
When a study has to be refreshed
NFPA 70E requires the arc flash risk assessment to be updated when a major modification or renovation takes place, and reviewed at intervals not exceeding five years. Everyone quotes the five years. The modification trigger is the one that bites, because in an occupied building it fires far more often.
NEC 110.24 carries the parallel rule on the marking side: where modifications affect the maximum available fault current, the calculation is redone and the field marking is updated.
In rough order of how often we see them, these are the events that invalidate a study.
- The utility changes the service transformer or re-sources the feeder. Your fault duty moves and no letter arrives.
- A generator, PV array or battery system is added. New sources, new contributions, and on the generator side a fault characteristic that looks nothing like the utility's.
- A breaker is replaced with a different trip unit, or settings are changed by someone chasing a nuisance trip and never recorded.
- A series-rated combination is broken because the downstream device was swapped for one that was never part of the tested combination.
- Significant motor load is added or removed.
- A panel is added, a feeder is extended, or a section is added to an existing switchboard lineup.
If you run a campus, hospital or plant under the City of Phoenix Annual Facilities Program, keeping the model current is not housekeeping. Electrical plans submitted under the AFP require a one-line diagram, load calculations and fault current calculations under the Phoenix Building Construction Code. The program is worth having, since the same staff review the plans and inspect the work, but it assumes you can produce current fault current calculations on demand. That is a good reason to fold the model into a maintenance program rather than re-buying it every five years.
Code edition is a local question in Maricopa County
Arizona has no statewide building or electrical code. Every municipality adopts and amends independently, and the Valley is currently split across three NEC cycles. That is not trivia. It decides what the plan reviewer holds you to.
| Edition | Jurisdictions | Notes |
|---|---|---|
| 2023 NEC | Phoenix, Mesa, Chandler, Glendale, Surprise, Avondale | Phoenix effective 1 Aug 2025 by Ordinance G-7397; Chandler 1 Jul 2025 by Ordinance 5108; Mesa 8 Jan 2026; Glendale 9 Jan 2026 |
| 2020 NEC | Scottsdale | Effective 1 Jan 2023 alongside the 2021 I-Codes |
| 2017 NEC | Unincorporated Maricopa County | Adopted 17 Aug 2022 with the 2018 I-Codes |
| Confirm first | Gilbert, Peoria, Tempe | We could not verify the current adopted edition from a primary source. Call the city building department before design; Gilbert Development Services is 480-503-6700 |
For studies it matters because the relevant articles have not been static. The arc-energy-reduction requirements at NEC 240.87 for circuit breakers and 240.67 for fuses, and the labeling language at 110.16, have all moved between the 2017, 2020 and 2023 editions. The same 1,200 amp service can face different requirements two miles apart because it crossed a city line. Confirm the adopted edition before design, not at the plan review desk.
Build plan review into the schedule as well. The City of Phoenix July 2026 turnaround report shows commercial plan review averaging 30 to 33 calendar days on initial review, with minor commercial running 8 days over goal and slower than major projects. That is before the first correction cycle.
Scope, and where a study is the wrong purchase
We collect the field data, build and maintain the one-line, reconcile the model against the approved gear submittals, apply the labels, and do the installation work the study calls for: trip unit settings, zone-selective interlock wiring, energy-reducing maintenance switching, breaker and fuse changes, and the grounding corrections that surface during the survey. The modeling and the report are stamped by a licensed professional engineer through our engineering side. We do not present a contractor's spreadsheet as an engineered study.
Where this is the wrong purchase: a single 200 amp panel in a strip retail suite does not need a modeled study. The utility's available fault current, equipment with an interrupting rating at or above it, and the NEC 110.24 field marking are the right answer, and we will say so rather than sell a model. A tenant improvement that adds a subpanel to a building with a current study is a revision to that model, not a new engagement, and it should be priced that way.
Data collection in an occupied building usually means a planned shutdown. Our posted hours are Monday to Thursday 6:00 AM to 3:00 PM and Friday 6:00 AM to 2:00 PM, so an evening or weekend outage is scheduled work with a date agreed in advance. On new construction, and on any design-build job, start early: the study drives the gear order, and the gear order drives the rest of the job.
Frequently asked questions
Is an arc-flash study required by law in Arizona?
There is no single Arizona rule to point at, because Arizona has no statewide building or electrical code. Every city and the county adopt and amend independently. The requirement reaches you two different ways. OSHA requires an employer to assess the workplace for hazards that make personal protective equipment necessary and to select PPE accordingly, and NFPA 70E is the consensus standard used to do that for electrical work, which is what produces an incident energy number. Separately, the NEC articles your jurisdiction has adopted require arc-flash warning labels and available fault current marking on certain equipment. The labeling is a code requirement on the installation. The study is a safety requirement on the employer, and passing a building inspection does not satisfy it.
How often does an arc-flash study have to be redone?
NFPA 70E requires the arc flash risk assessment to be updated when a major modification or renovation takes place and reviewed at intervals not exceeding five years. In practice the modification trigger fires more often than the five-year clock in an occupied building. A new generator or battery system, a utility transformer change, a breaker replaced with a different trip unit, added motor load, or a new switchboard section all change the answer. NEC 110.24 carries the parallel rule for the available fault current marking on service equipment: recalculate and update the marking when modifications affect the maximum available fault current.
Where does the available fault current number come from?
The serving utility. In Maricopa County that is APS or SRP for most sites, and you cannot tell which one from the city name or the ZIP code, because the two territories cut through neighborhoods and in places through individual streets. Read the utility name off an existing bill for the site, check the APS Phoenix Metro service territory map, or call APS at 602-371-7171 or SRP at 602-236-8888. Ask for the maximum available fault current at the point of delivery, and treat what comes back as a snapshot with a date on it, because utilities change transformers and re-source feeders without notifying the customer.
Can the study be done before the switchgear is ordered?
It should be. APS requires every commercial meter enclosure to be submitted for its review and approval before the equipment is manufactured, and that submittal has to show the bus bracing AIC rating along with the one-line, ampacity, voltage, phase and the number of service disconnects. If the short-circuit numbers arrive after the gear is released for manufacture, an undersized bracing rating becomes a re-order rather than a field correction. We run a preliminary study on design data, then reconcile it against the approved submittals once the actual gear is selected.
What is coordination, and why should an owner care about it?
Coordination is the study of which protective device opens first for a given fault. Without it, a fault on one branch circuit can open the main and take the whole building down, which is a business interruption rather than a tripped breaker. It also works the other way. Coordinating everything for selectivity tends to slow upstream devices down, and a slower upstream device means higher incident energy at that bus. The two goals conflict, so the engineer has to be told which buses get worked on most often and where the compromise should land.
Do you have to shut the building down to collect the data?
For some equipment, yes. We do not open energized gear to collect data. Most of a survey can be done from nameplates, submittals and as-builts, but breaker trip settings and internal connections usually require the section to be dead. Anything that cannot be safely read is listed as an exception with the assumption we used recorded against it, and it is picked up at the next planned outage. Our posted hours are Monday to Thursday 6:00 AM to 3:00 PM and Friday 6:00 AM to 2:00 PM, so an after-hours or weekend shutdown is scheduled in advance rather than called out.
Does a small tenant improvement need a power system study?
Usually not. A single 200 amp panel in a retail suite does not justify a modeled study. What it does need is the available fault current from the utility, equipment with an interrupting rating at or above that number, and the field marking the NEC requires on service equipment. If the building already has a current study, adding a subpanel is a revision to the existing model rather than a new engagement, and it should be priced that way. We will tell you which of those two situations you are in before quoting.
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