be listed.

504.10 Equipment Installation.

(A) Control Drawing. Intrinsically safe apparatus, associ ated apparatus, and other equipment shall be installed in accordance with the control drawing(s).

Exception: A simple apparatus that does not interconnect intrinsically safe circuits.

FPN No. 1: The control drawing identification is marked on the apparatus.
FPN No. 2: Associated apparatus with a marked Um of less than 250 V may require additional overvoltage protection at the inputs to limit any possible fault voltages to less than the Um marked on the product.

(B) Location. Intrinsically safe apparatus shall be permitted to be installed in any hazardous (classified) location for which it has been identified. General- purpose enclosures shall be permitted for intrinsically safe apparatus.

Associated apparatus shall be permitted to be installed in any hazardous (classified) location for which it has been identified or, if protected by other means, permitted by Articles 501 through 503 and Article 505.
Simple apparatus shall be permitted to be installed in any hazardous (classified) location in which the maximum surface temperature of the simple apparatus does not exceed the ignition temperature of the flammable gases or vapors, flammable liquids, combustible dusts, or ignitible fibers/flyings present.
For simple apparatus, the maximum surface tempera- ture can be determined from the values of the output power from the associated apparatus or apparatus to which it is connected to obtain the temperature class. The temperature class can be determined by:

(1) Reference to Table 504.10(B)

(2) Calculation using the formula:

T ~ PoRrh + Tamb

where:

T is the surface temperature

P0 is the output power marked on the associated apparatus or intrinsically safe apparatus

Rth is the thermal resistance of the simple apparatus

Tamb is the ambient temperature (normally 40°C) and reference Table 500.8(C)

In addition, components with a surface area smaller than 10 cm 2 (excluding lead wires) may be classified as T5 if their surface temperature does not exceed 150°C.
Table 504.10(B) Assessment for T4 Classification According to Component Size and Temperature
Requirement for T4
Total Surface Area Classification (Based on 40°C
Excluding Lead Wires Ambient Temperature)
<20 mm 2 Surface temperature <275°C
>20 mm 2 <10 cm 2 Surface temperature <200°C
>20 mm 2 Power not exceeding 1.3 W*
*Reduce to 1.2 W with an ambient of 60°C or 1.0 W with 80°C
ambient temperature.
FPN: The following apparatus are examples of simple ap- paratus:

(1) Passive components, for example, switches, junction boxes, resistance temperature devices, and simple semiconductor devices such as LEDs

(2) Sources of generated energy, for example, thermocouples and photocells, which do not generate more than 1.5 V, 100 mA, and 25 mW

504.20 Wiring Methods. Any of the wiring methods suitable for unclassified locations, including those covered by Chapter 7 and Chapter 8, shall be permitted
for installing intrinsically safe apparatus. Sealing shall be as provided in 504.70, and separation shall be as provided in 504.30.
504.30 Separation of Intrinsically Safe Conductors. (A) From Nonintrinsically Safe Circuit Conductors.
(1) In Raceways, Cable Trays, and Cables. Conductors of intrinsically safe circuits shall not be placed