Is MCM the same as kcmil?

Yes. Both mean one thousand circular mils, so 350 MCM and 350 kcmil are the same conductor. MCM is the older notation using the Roman numeral M for a thousand; kcmil is the modern SI-style spelling and is what current standards and labels use. Nothing about the conductor changes with the name.

What AWG is 250 kcmil?

There is no exact AWG equivalent. 250 kcmil is 250,000 circular mils, which sits between 4/0 AWG at about 211,600 cmil and the next step up the ladder. AWG stops being practical past 4/0, which is exactly why large conductors are specified directly in kcmil instead.

Where does AWG stop and kcmil start?

4/0 AWG, about 211,600 circular mils, is the largest common AWG size. Above it the trade switches to kcmil, starting at 250. The AWG scale is geometric and its numbers get unintuitive at the large end, while kcmil states the cross-sectional area directly.

How many circular mils is 500 kcmil?

500,000 circular mils. One kcmil is 1,000 circular mils, and a circular mil is the area of a circle one mil (0.001 in) in diameter, so kcmil times 1,000 always gives circular mils exactly.

Is one 500 kcmil conductor the same as two 4/0 in parallel?

Close but not equal. Two 4/0 conductors give about 423,200 circular mils against 500,000 for a single 500 kcmil, so the parallel pair has roughly 15% less copper. Parallel sets are common anyway because they are far easier to bend, pull and terminate than one very large conductor.

What size conduit does 500 kcmil need?

A single 500 kcmil THHN conductor needs at least 1-1/4 inch EMT, and a three-conductor set with a ground needs 3 inch. Large conductors are usually the reason a raceway jumps two trade sizes.

kcmil Wire Size Chart — MCM to AWG

Conversion, geometry and estimated ampacity for large conductors, from 250 kcmil to 750 kcmil, with the 4/0 AWG crossover shown for reference.

Quick answer

kcmil and MCM are the same unit — one thousand circular mils — so 350 MCM is 350 kcmil, or 350,000 circular mils. AWG runs out at 4/0 (about 211,600 cmil) and the trade switches to kcmil above it, starting at 250 kcmil. There is no exact AWG number for any kcmil size; the two scales simply meet at 4/0.

kcmil size chart — area, diameter, resistance and ampacity

Every value is computed from the stated area, so the columns stay internally consistent.

Size	Circular mils	Bare dia (in)	THHN OD (in)	Ω/1000 ft	75 °C (A)	90 °C (A)
250 kcmil conduit fill	250,000	0.500	0.711	0.0415	257	289
300 kcmil conduit fill	300,000	0.548	0.766	0.0346	289	326
350 kcmil conduit fill	350,000	0.592	0.817	0.0296	319	360
400 kcmil conduit fill	400,000	0.632	0.864	0.0259	348	393
500 kcmil conduit fill	500,000	0.707	0.949	0.0207	403	454
600 kcmil conduit fill	600,000	0.775	1.051	0.0173	454	511
750 kcmil conduit fill	750,000	0.866	1.158	0.0138	524	591

Copper conductors. Ampacity is a modelled estimate at 75 °C and 90 °C, not a code table value.

Formula estimate

Source & method: Ampacity figures are modelled, not transcribed. Read the methodology page for the fit and its deviation before using a value for design.

MCM to AWG — where the two scales meet

AWG sizes large enough to be compared with kcmil, listed in circular mils so the crossover is exact.

AWG size	Circular mils	Equivalent kcmil	Nearest listed kcmil size
4 AWG conduit fill	41,741	41.7 kcmil	250 kcmil (smaller by 499%)
3 AWG conduit fill	52,635	52.6 kcmil	250 kcmil (smaller by 375%)
2 AWG conduit fill	66,371	66.4 kcmil	250 kcmil (smaller by 277%)
1 AWG conduit fill	83,693	83.7 kcmil	250 kcmil (smaller by 199%)
1/0 AWG conduit fill	105,535	105.5 kcmil	250 kcmil (smaller by 137%)
2/0 AWG conduit fill	133,077	133.1 kcmil	250 kcmil (smaller by 88%)
3/0 AWG conduit fill	167,806	167.8 kcmil	250 kcmil (smaller by 49%)
4/0 AWG conduit fill	211,600	211.6 kcmil	250 kcmil (smaller by 18%)

Large AWG conductors expressed in circular mils and kcmil, with the nearest kcmil step.

Source & method: Conversion is arithmetic, not a lookup: kcmil = circular mils ÷ 1000, and circular mils for an AWG size come from the ASTM B258 diameter formula squared. Nothing here rounds to a "standard equivalent" — the percentage difference is shown instead, because on large feeders that gap decides whether a substitution is acceptable.

Parallel conductor equivalents

Large feeders are frequently pulled as parallel sets. This is what each set adds up to.

Set	Total circular mils	Equivalent kcmil	Est. 75 °C ampacity (A)
2 × 4/0 AWG	423,200	423 kcmil	460
2 × 250 kcmil	500,000	500 kcmil	514
2 × 350 kcmil	700,000	700 kcmil	638
2 × 500 kcmil	1,000,000	1000 kcmil	806
3 × 500 kcmil	1,500,000	1500 kcmil	1209
4 × 500 kcmil	2,000,000	2000 kcmil	1612

Total copper area and estimated 75 °C ampacity for parallel sets, ignoring adjustment factors.

Source & method: Area adds linearly across a parallel set; ampacity does not, in practice. The last column multiplies single-conductor ampacity by the number of conductors, which is the ceiling before adjustment. Parallel sets in one raceway are more than three current-carrying conductors, so bundling adjustment applies — derate before using these numbers.

Raceway area consumed by large conductors

Size	Area per conductor (sq in)	1 conductor	3 conductors	4 conductors
250 kcmil	0.397	1"	2"	2-1/2"
300 kcmil	0.461	1-1/4"	2-1/2"	2-1/2"
350 kcmil	0.524	1-1/4"	2-1/2"	2-1/2"
400 kcmil	0.586	1-1/4"	2-1/2"	3"
500 kcmil	0.707	1-1/4"	2-1/2"	3"
600 kcmil	0.868	1-1/2"	3"	3"
750 kcmil	1.053	1-1/2"	3"	3-1/2"

Area per conductor and the smallest EMT trade size for one, three and four conductors of that size.

Source & method: A dash means no listed EMT trade size holds that set; that pull moves to a larger raceway type or a different method. Full tables for every conduit type are on the conduit fill chart.

Reading kcmil sizes correctly

A circular mil is the area of a circle one mil across — one thousandth of an inch. It exists so conductor area can be written without carrying π around: area in circular mils is simply the diameter in mils squared. A conductor 0.5 in across is 500 mils, so 250,000 circular mils, or 250 kcmil. That is the whole conversion, and it is why kcmil numbers scale intuitively while AWG numbers run backwards.

MCM and kcmil are interchangeable on drawings, cut sheets and cable jackets. Older US documentation writes 500 MCM; current standards write 500 kcmil. If a submittal mixes both, they are not two different conductors and there is nothing to reconcile.

Stranding changes the physical conductor without changing the kcmil figure. A 500 kcmil conductor has 500,000 circular mils of metal regardless of whether it is Class B 37 strand or a flexible Class I rope lay, but the overall diameter, bend radius and lug requirements differ between them. The outside diameters on this page are nominal THHN/THWN-2 over standard Class B stranding; verify against the actual cable when the raceway or the termination is tight.

Two practical points on large feeders. First, aluminium dominates above about 4/0 because copper at those sizes is heavy and expensive; a common substitution is to move up roughly one and a half to two sizes in kcmil for the same current, and the ampacity chart shows the gap for each size. Second, voltage drop rarely governs large feeders on short runs, but service and site-distribution runs are often long enough that it does — check the drop before settling on a size, because upsizing after the pull is not an option.

Frequently asked

Is MCM the same as kcmil?: Yes. Both mean one thousand circular mils, so 350 MCM and 350 kcmil are the same conductor. MCM is the older notation using the Roman numeral M for a thousand; kcmil is the modern SI-style spelling and is what current standards and labels use. Nothing about the conductor changes with the name.
What AWG is 250 kcmil?: There is no exact AWG equivalent. 250 kcmil is 250,000 circular mils, which sits between 4/0 AWG at about 211,600 cmil and the next step up the ladder. AWG stops being practical past 4/0, which is exactly why large conductors are specified directly in kcmil instead.
Where does AWG stop and kcmil start?: 4/0 AWG, about 211,600 circular mils, is the largest common AWG size. Above it the trade switches to kcmil, starting at 250. The AWG scale is geometric and its numbers get unintuitive at the large end, while kcmil states the cross-sectional area directly.
How many circular mils is 500 kcmil?: 500,000 circular mils. One kcmil is 1,000 circular mils, and a circular mil is the area of a circle one mil (0.001 in) in diameter, so kcmil times 1,000 always gives circular mils exactly.
Is one 500 kcmil conductor the same as two 4/0 in parallel?: Close but not equal. Two 4/0 conductors give about 423,200 circular mils against 500,000 for a single 500 kcmil, so the parallel pair has roughly 15% less copper. Parallel sets are common anyway because they are far easier to bend, pull and terminate than one very large conductor.
What size conduit does 500 kcmil need?: A single 500 kcmil THHN conductor needs at least 1-1/4 inch EMT, and a three-conductor set with a ground needs 3 inch. Large conductors are usually the reason a raceway jumps two trade sizes.