We are researching a new market, and have been told that for single family houses built or rewired since the 1970's, that there is usually at least 100 -120 amps available for a sub panel that runs off "the back of the meter". The load would be a tankless electric water heater.I would be interested in any thoughts on this. If it is NOT the case, what would the resolution be? Would the utility run more power from the street to the house or replace the meter socket at no charge?
Thanks!

I have no idea what is means by "the back of the meter."

Meters have a definite number of lugs, and those are all spoken for. The meter directly feeds a main disconnect of some sort, and again, those have only the connection points that are essential.

How do you install a tankless water heater? Well, the FIRST step should be a "load calculation" to see what size service is required. Then, you go check the size of the main disconnect. If it's not large enough for the load, you need a service change- regardless of whether there are available breaker positions.

To be fair, it is sometimes possible that the existing service already is oversized enough to allow for the addition of such a water heater. One simply cannot say without doing a calculation for the entire house.

An electrician, not the utility, does this service change- including replacing the meter base. No, we don't work for free.

Increasing you service size usually involves a chat with the power company, so they can determine if the local supply lines have that additional power available. If they have to make changes, they GENERALLY make the changes to their equipment without charge to the homeowner.

Blu:

A little more in addition to Reno's comments.

Also, I have no clue what you call 'back of the meter'.

What's the KW of your unit?? Your profile states 'Engineer'; what discipline are you, if you don't mind.
What area of South Carolina are you in?

A SFD with a 200 amp service, on average uses 54% of the available capacity. Subpanels are usually installed for circuit breaker capacity, or convenience of a 'local' panel for branch circuits.

A 10 KW TWH, equates to 40+/- amps at 240 volts, which should be acceptable to 'add' to the 'average 200 amp' example above. Please note that an 'actual' load calc would be required, as opposed to my example based on experiences.

Don't know about US preferences but in Europe a 10 kW unit is considered "suitable for kitchen sink use only" and if you're used to the flow rates provided by a hot water tank on city water/ a good well pump I doubt you'd be happy even with an 18 kW unit. The Germans usually install 24 kW ones nowadays, with up to 27 kW available - 400 V 3-phase with three heating elements.

18 kW units usually have a nominal flow rate of 8 l (roughly 2 gallons) per minute at 40 K temperature difference (e.g. heating 10 C water to 40 C). That's barely enough for a European shower - provided no one turns on the sink while you're in the shower.

Texas, that's a very good catch.

Our rules limit shower heads to 2.5GPM- which, of course, means that they're designed to let as much as allowed through. That's a rating at 80psi; if your supply pressure is less, you'll get less water out.

Looking at water heaters that can supply 2.5-3.0 gpm, 18KW seems to be the minimum required. Remember- that's at a rated 45F increase in temperature .... with ground water commonly at about 60F, that's only a 105F output .... a temperature often seen in hot tubs. Barely hot enough, if you ask me.

So, we can conclude that a 10KW unit -a draw guessed at by another poster - would be too small for use in a shower.

The electric tankless heaters I've seen have all called for three 60-amp, 240-volt circuits- that's about 43KW. That's adding 180 "intermittent" amps on to a 200-amp service. I'd say a service change is almost a necessity.

The smaller 18KW units are advertised as being "ideal for an apartment of three." IMO, that's being generous. That would be 40 "intermittent" amps added to what is likely a 100-amp service. Let's hope the air conditioner isn't running.

It would also be a rare 70's house that had even a 150-amp service. I suspect the majority were built with 100-amp services.

I don't know what plans the OP has - but adding a tankless water heater is a serious step. That's why I advise the use of gas-fired tankless units.

Tex & Reno:

Thanks for the input and info. As you may guess the tankless water heaters around my area are NOT electric, but gas fired.

We are all here to share and learn.

Guys, Thanks for the comments!
The term "back of meter" comes to us from utility company engineers, and apparently more than one panel can be connected to the back of the meter in a SFR.if the meter socket is less than 20-40 years old (depends on the service area). When I bought my house 20 years ago, there is a 200 amp panel, and a 100 amp panel coming of the back of the meter to supply pool pump, HVAC, etc. Seems to work fine. The key point here is that the sub panel is NOT connected to the existing 150 or 200 amp panel but from the back of the meter. Have a look at this form Duke Energy https://www.duke-energy.com/pdfs/Parallel_Main_Panels_from_a_Single_Meter_Base.pdf
24kW seems to be the right size for the southern part of the US, which is also primarily electric, not gas.

With that link, things begin to make sense.

The power company is assuming a meter base with a 400 amp intermittent / 320 amp continuous rating. That's common for utility gear - but not to be assumed for residential services.

Behind this stands the load calculation .... the total load calculation for everything served by that meter needs to be less than 320 amps.

By adding a second set of wires to that meter base, you are creating a second 'main' service. The NEC generally frowns on this. I've encountered this sort of thing only rarely, usually in some sort of duplex or apartment conversion, or in a farm setting (where one meter serves several buildings).

The use of electricity of traditional water heating is not an issue; most any service can support a 30-A, 240-V circuit. That's a LOT less than what is called for in the operation of a tankless water heater.

Even here in Arkansas, the use of a "400-amp" meter base is quite rare in homes. Most often, the meter base is part of the main disconnect assembly.

In any case, working on that meter base requires the participation of the power company. Without disconnecting the power lines themselves, that meter base is always "hot-" and simply pulling the meter is no guarantee that you've killed the power to the load side. (Many meter bases have a 'bypass').

Definitely NOT a job for the homeowner.

I see some 400a services here on big, all electric houses but it is far from the norm.

The 400/320s are popular here in the McMansions, either with two (2) 200s or 2x150s, or a 200 and a 150.

Can't say I remember any from back in the 70s though.

I'd strongly suspect in practice much smaller services would happily supply such a water heater without tripping the main though. In Germany and Austria load calcs are only required for new supplies, nothing keeps you from adding loads to an existing supply even exceeding its rating. Example: one 24 kW tankless water heaters, another 21 kW and a full-sized electric cooker (probably somewhere between 7 and 8 kW) - connected to a 25 A 230/400 V 3-ph supply! Never a blown main fuse! That rare case came up in a discussion on mains supply sizes and how much power modern households really need.

Interesting. That is a somewhat common installation here, using a feed thru Meter base with dual load lugs. It was more popular before the all in ones took over though, now we just use a 400 all in one with a second 200 amp sub feed breaker. I have had a few service replacement situations where that wouldn't work and we went with the dual feed meter base and fed into two main breaker panels.