There is no such thing as "permissible current and voltage"
in circuit board design, (except by safety regulations, and
those requirements differ).

There is a relationship between current, trace size and temperature
(which is described by the curves in IPC-2221 Figure 6-4)
so by increasing the current or decreasing the trace cross section, 
you increase the temperature.

The curves will show the "temperature rise" above ambient for
any given trace size and current. The key is in knowing what your 
"ambient temperature" is. 
...and that's why we designers make the big bucks (just kidding!)

The caution is that you don't want the temperature to affect
the integrity of the board material, so for example if the Tg of
a particular material is 170 degrees C, we may limit the maximum 
operating temperature to something like 135 degrees C 
(by adjusting the trace width or copper thickness, or by moving 
the trace from an internal to an external layer, or by pouring copper 
planes near it to dissipate the heat, etc.)
 
also, clearances define maximum voltage, not trace size.

You can get a better description of these concepts by downloading
a draft of the new IPC-2152 (Current Carrying Capacity) at ipc.org 
under "Status of Standardization" in the Knowledge menu.

hope that helps,
Jack

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