7.4 KiB
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Wiring Method Selection |
Wiring Method Selection
Determine where each wiring method can be used.
Important
This note covers general, system-agnostic selection. For certain specific applications, It may be necessary to deviate from the general case.
%% TODO Scripts by others tend to beg questions of edge cases. It's my intent that if a rule holds in all cases, even the super special one you're thinking I didn't consider, that I specifically say so.
If I didn't say so, and your edge case is not covered, you should seek clarification. %%
Wire Material Selection
pdi-estimating preference is to use aluminum conductors.
If pdi-bid-estimating's proposal sells aluminum 100A and larger use aluminum for grounds, even if the feeder schedule uses copper.1
Example Wiring Method Selection
Below is an example Wiring Method Selection for a ibc-construction-types#^5-over-1 construction.
[!example] Wire Material:
- Less than 100A: CU
- 100A and larger: AL
Outdoors:
- Above-ground: EMT-Comp
- Underground: PVC w/ Rigid Elbows
Levels 1-2 (Type IA):
- Concealed: MC
- Exposed to Structure:
- Up to 2"C: PVC In-Slab
- 2-1/2"C and larger: EMT-SS OH
Levels 3-5 (Type IIIA):
- Concealed: NM/SER
- Exposed to Structure: PVC OH
%% This needs reworking. I know PVC is preferable to MC in the case of feeders, subfeeds, and possibly home runs, but I think they swap for branch. %%
Standard Indoor Wiring Methods
NM/SER < PVC in Slab < MC < EMT-SS < EMT-Comp
NM/SER Overhead
Nonmetallic-sheathed cables are combustible, so not permissible in Type ibc-construction-types#Type I Fire Resistive Non-Combustible, ibc-construction-types#Type II Non-Combustible constructions.
Concealment
Cable type wiring methods (MC, NM, SE) may only be used where they will be concealed from view.
PVC In-Slab
May require rigid stub-ups in certain cases, which could make EMT more cost-effective. Consult with your conest#ConEst Senior Estimator in this case.
Important
Be careful of ambiguous terminology in specifications and PDI proposals: "underground" and "below grade" are sometimes used inappropriately to describe PVC in-slab. It may be that rigid stub-ups are only necessary when emerging from grade (dirt), but not when emerging from the slab.
Precast Slab
Typical of garage construction.
PVC in-slab may not be utilized for precast slabs.
Slab Thickness
Conduits may only be routed through slabs of thickness at least 3 times the outer diameter of the conduit.
| Conduit Size | Minimum Slab Thickness |
|---|---|
| 1/2" | 3.0 in |
| 3/4" | 3.5 in |
| 1" | 4.0 in |
| 1-1/4" | 5.0 in |
| 1-1/2" | 6.0 in |
| 2" | 7.5 in |
| 3" | 9.0 in |
| 2-1/2" | 10.5 in |
| 3-1/2" | 12.0 in |
| 4" | 13.5 in |
| 5" | 17.0 in |
| 6" | 20.0 in |
Shear Walls
construction-methods#Shear Walls, like those around stairwells and elevator shafts, may be routed through with special permission.
Post-Tensioned Slabs
Raceway considerations for construction-methods#Post-Tensioned Slab (namely if PVC may be utilized, and if so, what sizes) are contentious.2 3 4
Pan Deck Slabs
Raceway considerations for construction-methods#Pan Deck Slab (namely if PVC may be utilized, and if so, what sizes) are contentious.5
MC Cable Overhead
Market Type = Assisted Living: MC AP cable may be required.
Concealment
Cable type wiring methods (MC, NM, SE) may only be used where they will be concealed from view.
[!important] Exception: Temporary provisions for unfinished spaces (e.g. core and shell pdi-building-types#Retail)
[!info] This is not a code requirement, though it is frequently stated to be. It is, however, near universal specification.
Multi-Circuit Homeruns
Different schools of thought exist on whether to use multi-neutral MC or pipe and wire for homeruns.6 Which is more cost-effective is apparently subject to transient market conditions. Seek clarification from your senior on a job by job basis.
EMT Overhead
Set-screw fittings are cheaper than compression, but may not be acceptable.
[!info] EMT has a third coupling type "indenter", which are crimped onto the conduit using a special tool. These are usually only mentioned in specifications to say that they are unacceptable.
Where only set-screw fittings with two screws are permitted no change to takeoff is necessary.7
Standard Outdoor Wiring Methods
PVC Underground < EMT-Comp < IMC
PVC Underground
May require rigid stub-ups.
EMT Overhead
Use compression fittings.
Nonstandard Indoor Wiring Methods
PVC Overhead
Not usually permissible, except in Florida.
Where acceptable usually utilized in lieu of EMT.
By Specification
[!info] Example Specs Relevant to Wiring Methods Selection
26XXXX - Raceway And Boxes For Electrical Systems
26XXXX - Conduits For Electrical Systems
26XXXX - Underground Ducts And Raceways For Electrical Systems
26XXXX - Low-Voltage Electrical Power Conductors And Cables
26XXXX - General Requirements For Electrical
26XXXX - Common Work Results For Electrical
Flow Chart
flowchart
A[Construction Type]
A -- I, II --> B[Can be run in slab?]
B -- No --> D[Concealed?]
B -- Yes --> PVC
D -- No --> EMT
D -- Yes --> MC
A -- III, IV, V --> C[Concealed?]
C -- No --> PVC
C -- Yes --> NM/SER
Other Considerations
Fire Rated Feeders
Where required see fire-resistive-wiring-methods-takeoff.
PDI estimating practice is to run all "emergency" feeds (life-safety or just standby) in a 2-hour rated assembly. This usually requires no change to takeoff for high-rises since electrical riser rooms are themselves fire-rated.
Liquidtight Flex
- nfpa-70_356_lfnc --- "Carflex"8
- nfpa-70_352_lfmc --- "Sealtite"9
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↩︎[!cite] joel-jansen 2025-08-15, in reference to belle-meade-plaza (pp.) No affect on takeoff. Routing in slab still possible.
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↩︎[!cite] joel-jansen 2025-08-21, in reference to belle-meade-plaza (pp.) PT slabs do have some affect on takeoff. 3in conduit can not be run in an 11in PT slab.
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↩︎[!cite] joel-jansen 2025-08-15, in reference to belle-meade-plaza (pp.) Do not route in slab.
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↩︎[!quote] Art Baldwin (pp.) Multi-circuit MC is not cost-effective when considering the difficulty of rework. Use conduit, even EMT if necessary.
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2025-10-15_09-32-00#Vanderbilt Central Neighborhood Residential College Spec Interpretation ↩︎
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From Carflex®, manufactured by Carlon. ↩︎
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From Anaconda Sealtite®, manufactured by ANAMET Electrical. ↩︎