EMT: Crush and Impact Testing Results
The National Electrical Code® (NEC®) allows EMT (Article 358) to be used in areas subject to physical damage, but not severe physical damage. In contrast, MC cable (Article 330) can’t be used in areas subject to any level of physical damage. To see the real difference in performance for these two wiring methods, the Steel Tube Institute Conduit Technical Committee contracted with Intertek (ETL SEMKO) to conduct comparative crush and impact testing — and EMT clearly outperformed MC cable in both tests.
Crush tests were performed in accordance with UL 1569, Section 25, Crushing Test – All Cable, but with slight deviations in order to test the EMT and take both products to failure. This test shows each raceway’s ability to endure field hazards such as construction variables, structure shifts and physical abuse (such as forklift damage or the pressure of construction workers stepping on the raceway). Each test sample was placed in an Instron® universal testing machine, laid over a steel rod, and topped with a compression plate. Each sample was tested at a compression rate of 0.50 ± 0.05 in. / min. until one or more of the indicators signaled that contact occurred between the circuit conductors or between one or more of the circuit conductors and any grounding conductor, the conduit body or both; the maximum load obtained at the moment of contact was then recorded. The crush test results proved that EMT has a higher point of failure than MC cable when subjected to field hazards.
Similarly, impact tests were conducted in accordance with UL 1569 Section 24, Impact Test, but with a slight deviation. This test is an important indicator of the raceway’s ability to handle mechanical abuse, such as the impact of an item dropped on the raceway during the construction process. Each test sample was placed in an impact test apparatus, laid over a steel rod, and impacted at the center. Upon impact, connected neon lamps were observed to see if they lit, indicating contact between the circuit conductors or between circuit conductors and the grounding conductor, conduit body or both; the maximum impact height was then recorded. EMT exhausted the limits of the testing apparatus without failure, while MC cable failed when subjected to much less impact force.
The results are clear: Whether subjected to field hazards (crushing) or mechanical abuse (impact), EMT outperforms MC cable — making it the better wiring method for physical protection.