美国墙壁插座的频率有多准确？

I have a submersible pump for which I'm trying to determine the variability in flow rate, to high precision. Since this is just an impeller driven by an AC load, what is the % error in frequency of a US wall outlet? Is there a commonly accepted 'ballpark' value, such as 60 hz +/-0.5 hz? I have a submersible pump for which I'm trying