Respectfully, that doesn't make sense. If you have an electric hub wheel the size of the rim will dictate the possible tires that can fit on it. My gut feeling is that an electric hub wheel will be on the wide side and putting a narrow tire woudn't be recommended. If the rim could take a 28mm tire I wouldn't do it unless you were very light as the added weight and wear would benefit from a wider tire and most likely one that is fairly puncture resistant, ie. heavy. So whatever benefit you're seeking from a narrower tire will be complicated by the greater wear and harsher ride. Are you sure your electric wheel can take a 28mm tire and why are you going for that size of tire?