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Temperature Derating Factors

Wire Temperature Rating

For ambient temperatures other than 40 C (104 F), multiply the ampacities shown above by the appropriate factor shown below.

Ambient Temp. C	200 C	250 C	450 C	Ambient Temp. F
41-50	0.97	0.98	0.99	106-122
51-60	0.94	0.95	0.99	124-140
61-70	0.90	0.93	0.96	142-158
71-80	0.87	0.90	0.95	160-176
81-90	0.83	0.87	0.93	177-194
91-100	0.79	0.85	0.92	195-212
101-120	0.71	0.79	0.89	213-248
121-140	0.61	0.72	0.86	249-284
141-160	0.50	0.65	0.84	285-320
161-180	0.35	0.58	0.81	321-356
181-200	--	0.49	0.78	357-392
201-225	--	0.35	0.74	393-437
226-250	--	--	0.69	439-482
251-275	--	--	0.65	483-527
276-300	--	--	0.60	528-572
301-325	--	--	0.55	573-617
326-350	--	--	0.49	618-662
351-375	--	--	0.42	663-707
376-400	--	--	0.34	708-752

The use of this table for establishing ampacity ratings is an inexact procedure. This table should only be used as a starting point when establishing ratings for any given situation. Values may be lesser or greater than those given in the table because of the influence of installation method, environment, number of conductors, conductor composition and size, ambient temperatures, insulation types, etc.

It is recommended that design engineers desiring accurate ampacity data closely study the 1996 National Electrical Code Handbook , Article 310-15-310-84. Additional information can be derived from AIEE, Paper Number 57-660. "The calculation of the temperature rise and load capability of cable systems" by J. H. Neher and M. H. McGrath.

This paper was presented to the AIEE general meeting in Montreal, Quebec, Canada on June 24-28, 1957, and was published in the "AIEE Transactions," Part 3 (power apparatus and systems), Volume 76, October, 1957, pp. 752-772.

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