Microsoft changed the behaviour of the ASCII 160 character used for the non-breaking space in Word 2013. It now conforms to the CSS space rules, which means it expands/contracts like other spaces when using justification.

If you need a fixed-width non-breaking space, you can use the Narrow No-Break Space (202F, Alt-X). You could create a macro with a keyboard shortcut for this (IIRC, Ctrl-Alt-Space is normally free).

One not-so-obvious difference is that the Narrow No-Break Space appears as a space without the · or º that appears for other spaces when the formatting display is turned on.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]