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I have a large formula in align:

beginalign
overliner_D(n_D) = begincases
r_D(n_D) & textrm if r_D(n_D) < b_min \
maxb_min , max b_l-f(rho_E) mid b_l leq r_D(n_D), ; 1leq l leq m& textrmotherwise 
endcases
nonumber
endalign

And it looks like this:

It overlaps with the the column on the right. How can I fix it ?

EDIT:

Here is the compilable script:

documentclass[10pt,a4paper,twocolumn]IEEEtran

usepackagegraphicx,amssymb,amsmath,amsfonts,mathtools, bm


begindocument


Postremo ad id indignitatis est ventum, ut cum peregrini ob formidatam haut ita dudum alimentorum inopiam pellerentur ab urbe praecipites, sectatoribus disciplinarum liberalium inpendio paucis sine respiratione ulla extrusis, tenerentur minimarum adseclae veri, quique id simularunt ad tempus.


beginalign
overliner_D(vecn) = begincases
r_D(vecn) & textrm if r_D(vecn) < b_min \
maxb_min , max b_l-f(rho_E) mid b_l leq r_D(vecn), ; 1leq l leq m& textrmotherwise 
endcases
nonumber
endalign

Postremo ad id indignitatis est ventum, ut cum peregrini ob formidatam haut ita dudum alimentorum inopiam pellerentur ab urbe praecipites, sectatoribus disciplinarum liberalium inpendio paucis sine respiratione ulla extrusis, tenerentur minimarum adseclae veri, quique id simularunt ad tempus, et tria milia saltatricum ne interpellata quidem cum choris totidemque remanerent magistris.


enddocument

I would suggest using a temporary variable

documentclassamsart

begindocument

Text text text text text text text text text text text text text text
text text text text text text text text text text text text text text
text text text text text text text text text text text text text text
beginalign
 r_D(n_D)
 =
 begincases
 r_D(n_D), & textrm if r_D(n_D) < b_min, \
 maxb_min,c,& textrmotherwise,
 endcases
nonumber
endalign
where
( c = max b_ell-f(rho_E) mid b_ell leq r_D(n_D), ; 1leq
ell leq m ).

enddocument

You can move “otherwise” to another line, adding some vertical space for better clarity.

documentclassarticle
usepackageamsmath

usepackagelipsum % for mock text

begindocument

lipsum*[1][1-3]
beginequation*
overliner_D(n_D) =
begincases
beginalignedat2
&r_D(n_D) && textif $r_D(n_D) < b_min$ \[2ex]
&!maxb_min, max b_l-f(rho_E) mid b_l leq r_D(n_D), 1 leq l &&leq m \
 &&& textotherwise
endalignedat
endcases
endequation*
lipsum*[2]

enddocument

For two-column output, you need some more. Something like

max b_l-f(rho_E) mid b_l leq r_D(n_D), 1 leq l leq m

should be given a name; choose what you like more, here I use B(n_D,m).

documentclass[a4paper]IEEEtran
usepackageamsmath
usepackagenewtxtext,newtxmath

usepackagelipsum % for mock text

begindocument

lipsum*[1][1-3]
beginalign*
&overliner_D(n_D) =
begincases
r_D(n_D) & textif $r_D(n_D) < b_min$, \
maxb_min, B(n_D,m)& textotherwise,
endcases
\
&B(n_D,m)=max b_l-f(rho_E) mid b_l leq r_D(n_D), 1 leq l leq m.
endalign*
lipsum*[2][1-3]
lipsum[4-9]

enddocument

Note I used newtxtext and newtxmath to avoid (horrible) formulas in Computer Modern along with Times for text.

You can write this equation smaller by help of macro medmath... from the package nccmath:

(red lines indicate text borders)

documentclassamsart
usepackagenccmath

usepackagelipsum
begindocument
lipsum[66]
beginalign*
 medmath
overliner_D(n_D) = begincases
r_D(n_D) 
 & text if r_D(n_D) < b_min \
maxbiglb_min, maxb_l-f(rho_E) mid b_l leq r_D(n_D), ; 1leq l leq mbigr
 & textotherwise
endcases
 
endalign*
enddocument

Edit:
After your editing of your question only answer of Andrew Swann (+1) can solve your problem. To his solution (and your new MWE) I can only add some off-topic remarks (see comments in code below):

documentclass[10pt,a4paper]IEEEtran % it is default twocolumn
usepackageamssymb, % load amsfonts too
 mathtools, % it is supersede of the amsmath, no need to load it (again) 
 bm
usepackagegraphicx

usepackagelipsum

begindocument
lipsum[1]
beginalign* % no numbered
overliner_D(n_D) = begincases
r_D(n_D) 
 & text if r_D(n_D) < b_min \
maxbiglb_min, c_maxbigr
 & textotherwise
endcases
endalign*
where $c_max=maxb_l-f(rho_E) mid b_l leq r_D(n_D), ; 1leq l leq m$. 

lipsum[2-7]
enddocument

which gives: