I have a question for you ,I know you are sharp with this type of stuff. I have a vac. system 2HP (15 amp. pull )on a 20 amp.breaker and the 3 1/4 HP router (15amp) on the same breaker, on 10 ga. copper wire running from the panel to the outlet boxes, I just about always I fire up the Vac.system 1st.but not always and than the router and never pop the breaker, how can that be ?
It all comes down to what the numbers mean. If one throws an ammeter on the line to each piece of equipment, one is likely to see that neither the vacuum nor the router is actually drawing the amperage indicated on the label. Why?
Let's look at that 3-1/4 HP router. Routers tend to be rated in peak power or current. Peak is the operative word here. One would not normally push the router hard enough to sustain the peak current. Peak means one can push the router to this value without damage to the router. However, in most work, one wouldn't achieve peak.
Similarly, the vacuum will draw the 2 HP only under certain conditions. Most of the time, these conditions don't exist and the vacuum draws less power and current.
So, let's say you have a 1/4" double-flute spiral bit in the router, and you're proceeding with making a groove in oak, at normal feed speed. What power is being taken? Definitely not the 3-1/4 HP. Probably below 1 HP.
Being the diligent worker, you don't allow the vacuum to struggle against a clogged bag and you empty the vacuum before it fills more than half way. Probably less than 1 HP draw.
Together you have less than 2 HP, which is less than the 20 amp breaker rating.
Another issue is that your router may have a soft-start, which limits the current on start-up. So, it's smart for one to start the vacuum before the router. The vacuum probably has the greater starting current.