This chart will assist you in making a rapid evaluation of the exposed surface area required to cool the average electronics, if the approiximate wattage to be dissipated and the acceptable temperature rise are known.
Select here for Painted Steel Enclosures
Select here for 304 Stainless Steel Enclosures
Select here for 347 Stainless Steel Enclosures
Select here for 410 Stainless Steel Enclosures
This chart is based on an altitude of 3300 feet or less. Consult the equipment manufacturer for higher altitudes.

NEMA 4 Enclosure Sizing Chart For Painted Steel Enclsoure.

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Power in Enclosure(WATTS)	Surf. Area (sq.ft.) for 10C rise	Surf. Area (sq.ft.) for 15C rise	Surf. Area (sq.ft.) for 20C rise	Surf. Area (sq.ft.) for 25C rise
25	5	3	2	2
50	10	7	5	4
75	15	10	7	6
100	20	13	10	8
125	25	16	12	10
150	29	20	15	12
175	34	23	17	14
200	39	26	20	16
225	44	29	22	18
250	49	33	25	20
275	54	36	27	22
300	59	39	29	24
325	64	42	32	25
350	69	46	34	27
375	74	49	37	29
400	78	52	39	31
425	83	56	42	33
450	88	59	44	35
475	93	62	47	37
500	98	65	49	39
525	103	69	51	41
550	108	72	54	43
575	113	75	56	45
600	118	78	59	47
625	123	82	61	49
650	127	85	64	51
675	132	88	66	53
700	137	92	69	55
725	142	95	71	57
750	147	98	74	59
775	152	101	76	61
800	157	105	78	63
825	162	108	81	65
850	167	111	83	67
875	172	114	86	69
900	176	118	88	71
925	181	121	91	73
950	186	124	93	75
975	191	127	96	76
1000	196	131	98	78
1025	201	134	100	80
1050	206	137	103	82
1075	211	141	105	84
1100	216	144	108	86
1125	221	147	110	88
1150	225	150	113	90
1175	230	154	115	92
1200	235	157	118	94
1225	240	160	120	96
1250	245	163	123	98
1275	250	167	125	100
1300	255	170	127	102
1325	260	173	130	104
1350	265	176	132	106
1375	270	180	135	108
1400	275	183	137	110
1425	279	186	140	112
1450	284	190	142	114
1475	289	193	145	116
1500	294	196	147	118
1525	299	199	150	120
1550	304	203	152	122
1575	309	206	154	124
1600	314	209	157	125
1625	319	212	159	127
1650	324	216	162	129
1675	328	219	164	131
1700	333	222	167	133
1725	338	225	169	135
1750	343	229	172	137
1775	348	232	174	139
1800	353	235	176	141
1825	358	239	179	143
1850	363	242	181	145
1875	368	245	184	147
1900	373	248	186	149
1925	377	252	189	151
1950	382	255	191	153
1975	387	258	194	155
2000	392	261	196	157
2025	397	265	199	159
2050	402	268	201	161
2075	407	271	203	163
2100	412	275	206	165
2125	417	278	208	167
2150	422	281	211	169
2175	426	284	213	171
2200	431	288	216	173
2225	436	291	218	175
2250	441	294	221	176
2275	446	297	223	178
2300	451	301	225	180
2325	456	304	228	182
2350	461	307	230	184
2375	466	310	233	186
2400	471	314	235	188
2425	475	317	238	190
2450	480	320	240	192
2475	485	324	243	194
2500	490	327	245	196
2525	495	330	248	198
2550	500	333	250	200
2575	505	337	252	202
2600	510	340	255	204
2625	515	343	257	206
2650	520	346	260	208
2675	525	350	262	210
2700	529	353	265	212
2725	534	356	267	214
2750	539	359	270	216
2775	544	363	272	218
2800	549	366	275	220
2825	554	369	277	222
2850	559	373	279	224
2875	564	376	282	225
2900	569	379	284	227
2925	574	382	287	229
2950	578	386	289	231
2975	583	389	292	233
3000	588	392	294	235
3025	593	395	297	237
3050	598	399	299	239
3075	603	402	301	241
3100	608	405	304	243
3125	613	408	306	245
3150	618	412	309	247
3175	623	415	311	249
3200	627	418	314	251
3225	632	422	316	253
3250	637	425	319	255
3275	642	428	321	257
3300	647	431	324	259
3325	652	435	326	261
3350	657	438	328	263
3375	662	441	331	265
3400	667	444	333	267
3425	672	448	336	269
3450	676	451	338	271
3475	681	454	341	273
3500	686	458	343	275
3525	691	461	346	276
3550	696	464	348	278
3575	701	467	350	280
3600	706	471	353	282
3625	711	474	355	284
3650	716	477	358	286
3675	721	480	360	288
3700	725	484	363	290
3725	730	487	365	292
3750	735	490	368	294
3775	740	493	370	296
3800	745	497	373	298
3825	750	500	375	300
3850	755	503	377	302
3875	760	507	380	304
3900	765	510	382	306
3925	770	513	385	308
3950	775	516	387	310
3975	779	520	390	312
4000	784	523	392	314
4025	789	526	395	316
4050	794	529	397	318
4075	799	533	400	320
4100	804	536	402	322
4125	809	539	404	324
4150	814	542	407	325
4175	819	546	409	327
4200	824	549	412	329
4225	828	552	414	331
4250	833	556	417	333
4275	838	559	419	335
4300	843	562	422	337
4325	848	565	424	339
4350	853	569	426	341
4375	858	572	429	343
4400	863	575	431	345
4425	868	578	434	347
4450	873	582	436	349
4475	877	585	439	351
4500	882	588	441	353
4525	887	592	444	355
4550	892	595	446	357
4575	897	598	449	359
4600	902	601	451	361
4625	907	605	453	363
4650	912	608	456	365
4675	917	611	458	367
4700	922	614	461	369
4725	926	618	463	371
4750	931	621	466	373
4775	936	624	468	375
4800	941	627	471	376
4825	946	631	473	378
4850	951	634	475	380
4875	956	637	478	382
4900	961	641	480	384
4925	966	644	483	386
4950	971	647	485	388
4975	975	650	488	390
5000	980	654	490	392

NEMA 4 Enclosure Sizing Chart For 304 Stainless Steel Enclsoure.

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Power in Enclosure(WATTS)	Surf. Area (sq.ft.) for 10C rise	Surf. Area (sq.ft.) for 15C rise	Surf. Area (sq.ft.) for 20C rise	Surf. Area (sq.ft.) for 25C rise

NEMA 4 Enclosure Sizing Chart For 347 Stainless Steel Enclsoure.

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Power in Enclosure(WATTS)	Surf. Area (sq.ft.) for 10C rise	Surf. Area (sq.ft.) for 15C rise	Surf. Area (sq.ft.) for 20C rise	Surf. Area (sq.ft.) for 25C rise

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Notes:
#1. When the above data disagrees with actual manufacturer data, always default to actual manufacturer data.
#2. These values are for educational purposes only.
Due to the wide range of circuit applications, the author assumes no responsibility for the application of these values.

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NEMA 4 Enclosure Sizing Chart For 410 Stainless Steel Enclsoure.

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Power in Enclosure(WATTS)	Surf. Area (sq.ft.) for 10C rise	Surf. Area (sq.ft.) for 15C rise	Surf. Area (sq.ft.) for 20C rise	Surf. Area (sq.ft.) for 25C rise