[hvyhln]

I am buying a home with a 100amp entrance. I am putting in an electric hot water heater, but already have an electric range and dryer. Do I need a subpanel for the water heater or do I have to change the whole entrance??

 

[NolaTigaBait]

I am buying a home with a 100amp entrance. I am putting in an electric hot water heater, but already have an electric range and dryer. Do I need a subpanel for the water heater or do I have to change the whole entrance??

you need to do a load calc....how may square feet, other appliances, ...if i had to guess, yes, you will need a bigger service

 

[handyman78]

Just to clarify- in a typical residence your MAIN breaker is what determines the amount of amperage into your house. You are saying subpanel but if it comes off of the main entrance, you haven't gained any amperage. A subpanel just makes more divisions (and more control) from the main entrance.

An upgrade in your MAIN service may be necessary (to 200a possibly) but as already mentioned- a load calc needs to be done to determine if it is really necessary.

 

[J. V.]

How many spare breaker slots do you have. If you have 2 you can install a 2 pole 30 amp breaker and run #10/3 cable for the WH. If you have the spare slots don't concern yourself with a load calculation. Your panel is designed to hold as many breakers that it will accept.
If you do not have any spare breakers you can use tandem breakers to free up some space. You must be sure your panel will accept tandems and how many! 100 amp services are not always to small. In fact it may be just right for your house.