200amp vs 300amp

aj33March 18, 2013

Hi All,
We are starting Electrical soon on our new house. Couple of electricians want to do 200 amp. Another one I talked to suggested 300 amp.He calculated total load from all electrical appliances. I just talked to one of the electricians and he says that a 200 Amp service lets you draw up to 400 amps. 200 amps from each wire.

Does it make sense? If my highest possible load adds up to say 215 amps, will I be able to draw it with a 200 amp service without causing any problems or inviting trouble from the electrical company?

As an aside - I got 4 quotes - range 11750 to 22000 - all are working off of the same plans.

Thanks in advance for your help.


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This is a single-family house?

We have 100A in a 7-person household with lights, Tv's and computers left on all the time and way too few circuits for what they service. We never trip a breaker and have never tripped my main at any 100A house.

Also things are moving to be more efficient today, not less. 20 bulbs left on at 100W would have been 2000W back in the day (16A). Today that would be 4A in LEDs.

IOW, I think 200A is already overkill, and uless this is a multi-family dwelling 300A would be a waste.

    Bookmark   March 18, 2013 at 3:32PM
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It is a single family home. Everything is electric in the house. Heating system will be geothermal.

I agree with you - just wanted make sure.


    Bookmark   March 18, 2013 at 3:51PM
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I have 400A in my house. Overkill? Perhaps, but with two 200A panels in the house, I have plenty of slots for surge protection, etc. Plus my two geothermal units use a total of 10 slots (5 different 220V circuits).

    Bookmark   March 18, 2013 at 4:13PM
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Water heater too? Tankless electric? That would make a difference. Or traditional water heater?

    Bookmark   March 18, 2013 at 4:16PM
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It is possible that the post misquotes one or both of the electricians- but the statements as written concern me.

    Bookmark   March 18, 2013 at 6:11PM
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I have a desuperheater and two traditional hot water tanks. The first one is turned off and is used as a buffer tank. The second one has power (30A) but since the DSH gets the incoming water to 120F, it only has to heat a little. Of course, in the spring/fall when the weather is nice, the unit isn't running so no "free hot water". I have power running to both tanks for a total of 4 slots. However, I've never turned on my buffer tank's breaker. Probably a waste of a breaker and 10awg cable.

My 4ton unit has a 50A circuit for the compressor, etc, a 60A/30A circuits for the 15KW emergency heat for a total of 6 slots. I also have a 2ton unit in the attic that handles the upper floor. It has a smaller emergency heat so just 2 220V circuits for another 4 slots.

That totals 14 slots just for my HVAC & hot water system.

    Bookmark   March 18, 2013 at 11:45PM
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You do not size the supply based on the highest possible load. There is never a situation where your water tank is drawing full power, oven is on full blast, heat is on full, a/c is on full, all lights are on, vacuum is running, hair dryer is blowing, etc. etc.

And (as bus_driver said) I hope you misunderstood and misquoted what the electricians said because it is not just all about the amps. Voltage is considered too. A 200 amp service is supplying 200 amps at 240 volts. Which can be considered 2x200A at 120v. But your actual usage will have some devices that require 240v, and some at 120v. So it isn't a cut and dry calculation (although it should be fairly simple for an actual licensed electrician).

    Bookmark   March 19, 2013 at 10:25AM
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Yes, I do have a tankless electric. I was just looking at an ecosmart unit on Amazon and spec says - 112 amps for one heater.

Thanks everybody - Geothermal information is also very helpful. Good to know how amps are calculated. I have to list all 120 and 240 appliances, add it all up and apply some sort of discount factor to account for non-simulateous usage. There must be some heuristic or more firm guidance to do that...


    Bookmark   March 19, 2013 at 10:32AM
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The easiest way is to just ask the electricians if they did a load calculation or how they came up with what they are recommending.

If you want to get a general idea if they are in the right ballpark, you could google a website that will do the calculation for you where you just have to punch in the values. I linked to an example below, but I haven't actually used it or even read the instructions or anything, so I'm not actually recommending this one as a good one.

Maybe try a few different calculators and see if they agree.

But be aware that there may be something specific about your situation that isn't taken into account by these general calculators. Which is why a good electrician's opinion is probably the better option.

Here is a link that might be useful: Calculator

    Bookmark   March 19, 2013 at 12:14PM
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The electric tankless will also pull even more than 112 A when it kicks on to heat water.

Resistance heating elements pull their rated power AFTER they have heated up, but just like induction motors (like in your heat-pump) pull significantly motors current for a very short time.

Incandescent bulbs do the seating, but the filament is so small it is not usually many cycles of the power line (though many an engineer has blown out his drive transistor by using the running wattage to determine the starting current of a bulb).

Larger heaters prolong the starting draw (more mass meas longer heat up time) and large induction motors can really stretch it out till they come up to full speed ('hard start kits' and more start capacitance to shorten the start up on time).

The heat pump should have a LRA (Locked Rotor Amperes) number on the name plate.
This provides a decent estimate of the starting current the motor will pull, but you still need to round up (often you end up going to the next larger available breaker size anyway and that covers the problem unless the feed run is long).

There are a couple methods for doing load estimates in the NEC.

Ask to see at least one of them on paper.

For habitable rooms that require receptacles every 12 feet, put them in every 6 feet.

Not using extension cords on every lamp you need to plug in is nice (and safer).

    Bookmark   March 19, 2013 at 2:22PM
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That is lots of good information. Thank you all.

I will use the calculator and report back, right not aI am away on a business trip.

Thanks again!!


    Bookmark   March 20, 2013 at 6:11PM
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Hi All,

regarding load discount factor:

Loads are not just summed up to compute the service size needed. There are discounts and multipliers, such as:

Lighting load beyond 3000VA is discounted to 35%.
Water heater rating has a multiplier of 125%.

Appliance (not including dryer, water heater etc.) load beyond 4 is discounted to 75%.

In short, it is best left to the professional but make sure that he is taking everything into account AND doing the calculation.

Mine arrived at 200 amp because 99% of the people get 200 amp service AND the fact that materials for 300 amp service add another $1k or more to the cost.

    Bookmark   March 25, 2013 at 1:22PM
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If your new 200A service is almost maxed out with the geothermal loads and normal house loads, I would strongly advise to upsize the service now, while it's still relatively cost effective. If you wait and need it later, the $1k in materials plus the cost of labor will look like a bargain. Appliances are not accounted for in a load calc unless they are fixed in place- dishwashers, garbage disposal, etc. The online calculator linked to in this thread doesn't look like it takes the largest motor load into consideration, which would be a major oversight. Leave the calculations to professional electricians. Guess what happens when the service is undersized? It doesn't work correctly (trips), gets ripped out, and redone, all at significant expense. Don't get a 200A service because that's what 99% of other services are; have several load calcs done by qualified electricians, decide how much room you want for future expansion, and go from there. FYI- there's a standard and an optional calc for dwellings. Cheers, Mike

    Bookmark   March 26, 2013 at 8:11AM
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At this stage the labor is pretty much the same for 200 vs. 300 vs. 400 service.

The cost of the material will add up, but like mm11 says - it's cheap now and big $$$, hassle and mess if you need it later.

Also, people don't really have a grasp on how much power electronics can use. A good home theater can can draw 2000-3000 watts and 25 + amps while running. Have a workshop ? Saws, Air compressors and dust collectors, welders can draw huge loads at startup and consume breaker slots fast. Most kitchens can consume 20 breaker slots by themselves.

Sure you could split the 200 into a couple of sub panels, but why not get the xtra juice too ?

    Bookmark   March 26, 2013 at 2:24PM
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Thanks for your suggestions.

I agree with you that extra cost at this stage could prove to be money well spent. I do have a table saw and I hook up a shop vac to it when I use the saw.

Thanks again.


    Bookmark   March 26, 2013 at 3:43PM
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