I put an addition on and have installed electric baseboard heat and electric on-demand whole house water heater. Due to the increased demand my electric panel was upgraded from 100 amp to a 200 amp panel. But I am unsure if the electric company upgraded the transformer when this was done. My electric bill has increased drastically(doubled), it don’t matter if it’s winter or summer it remains pretty consistent. But when the on-demand water heater kicks on the lights in my house pulsate or when another high demand electric appliance kicks on there is a subtle dimming. Large consumption electric appliances that were existing and still in use are electric hot water heater, electric clothes dryer, electric range heating n addition to the increased load. What’s the easiest way to confirm the transformer was upgraded?

  • Were the wires from the transformer to the house upgraded? What you added(electric heat and on demand) just wondering if the upgrade was enough. Just those two could take 200 or more amps, unless you have a tiny house. Edit in the amount of amps for the heaters and on demand.
    – crip659
    Commented Dec 21, 2023 at 21:32
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    It's not clear how whether or not the transformer was upgraded would be a DIY question. Whether the power company upgraded their transformer or not is entirely up to them.
    – Greg Hill
    Commented Dec 21, 2023 at 21:56
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    You've made a potentially unfounded assumption that the transformer is the problem. I'm no sparky, but I've never even heard that come up in hundreds of panel upgrade conversations. What makes you think it's a factor?
    – isherwood
    Commented Dec 21, 2023 at 22:05
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    With all of that electrical need, I'd be surprised if 200A would be sufficient if you did a load calculation on your electrical appliances/devices. The electric on-demand water heater is a HUGE power draw in itself, and common wisdom around here is that they usually are not usually cost effective at all, once all costs of them are accounted for.
    – Milwrdfan
    Commented Dec 21, 2023 at 22:18
  • You should measure the voltage at your service panel and see how it varies from no-load to full-load. The fact that you can see fluctuations in your doesn’t mean much.
    – nobody
    Commented Dec 21, 2023 at 23:13

3 Answers 3


Frame Challenge - Load Calculation needed - a.k.a., maybe 200A isn't enough

Any time you have a significant increase in your electricity usage, such as:

  • Adding on-demand electric water heating
  • Increasing electricity requirements for HVAC (e.g., adding baseboard electric heat)
  • Adding EV Charging
  • Turning a garage or basement (lights, opener, misc. small loads) into a workshop (big tools) or accessory dwelling unit (ADU) (cooking equipment, water heating, etc.)

or a number of other large and/or significant continuous use loads, you need to do a Load Calculation. This takes into account the size of your house, required circuits (kitchen countertop circuits, bathroom, laundry, etc.), water heating (if electric), HVAC (which can range from almost nothing for gas heat to moderate amounts for air conditioner/heat pump to huge amounts for electric resistance heat), EV charging, major fixed appliances, cooking equipment (oven, cooktop, etc.) and comes up with a single number that represents the expected electric load. In certain situations you need multiple Load Calculations - one for the utility service and another for each main panel or subpanel (depending on configuration).

Once you have these numbers, you can actually do stuff if the numbers are OK. Or increase your service or change your plans if the numbers are not OK.

A typical house without electric resistance heat, without a large EV charging circuit and without on-demand electric water heating might have a Load Calculation of 50A or 100A or 150A or anything in between. However:

  • Electric resistance heat - each section might be 500W to 2,500W, which is no big deal. But if you have a few of those sections in each of several rooms, you could easily end up with 10,000W (41A) or more. Obviously that will only be a factor in winter, but it is an important piece of the load calculation.
  • EV charging is usually configurable based on your needs (for most people 30A or even 20A will be enough) and your available supply (total utility feed minus Load Calculation of everything else). Some people insist on 50A or more, but unless you have automatic load shedding (which is becoming more common), that has a huge impact on your Load Calculation.
  • On-demand water heating is the worst when it comes to load problems. Why? A typical whole-house on-demand water heater is rated at 27 kW to 36 kW. That translates into 112A to a whopping 150A!

If you have a house with a moderate 80A Load Calculation (and usually far less in actual use) and want to add 150A for on-demand water heating, and let's presume you got rid of a typical 23A large electric tank water heater, so a net gain of 127A, what will happen if you don't do a Load Calculation?

  • Your electrician or contractor will, correctly, tell you that 100A service just isn't enough.

  • The standard upgrade is from "whatever old thing you had less than 200A up to a standard modern 200A". So that's what the utility does and your electrician does:

    • Transformer if needed - probably not needed, but the utility will take care of it if needed
    • Wires from street to your house
    • Meter if needed (most utilities have put in smart meters over the last 25+ years, so it was likely already replaced with a standard 200A-capable meter)
    • Main panel replacement to handle the larger total load and install the new circuits

But without that Load Calculation, you've gone from 80A to 207A, more than your 200A official capacity when everything is going at once. And even more when the new electric resistance heat is added in! But the reality is that most houses, most of the time don't actually get anywhere near their Load Calculation 95% of time. In addition, most breakers don't trip exactly at their official limit unless they are run at that limit for a long time.

But since you didn't do that Load Calculation, that would have said (guessing) 240A, you have plenty of times hitting on the order of 200A. No breaker trip (because main is near limit and each individual circuit is running within normal parameters). But you are pushing your overall supply of 200A to the limit and causing problems.

In fact, to realistically use whole house on-demand electric water heating for anything but the smallest house, you might well need a 320A (continuous)/400A (total max.) upgrade. Really.

Don't blame the transformer. Blame on-demand water heating.

Yes, tank water heaters do waste a little electricity in the form of heat lost through the tank when no water is being used. But very little because of good insulation - the outside of a modern water heater is cool to the touch. Yes, there are limits to how much hot water you can get from a tank water heater, but there are some clever tricks, an electric water heater is more clever than it seems. And if you find a standard tank water heater just isn't enough, add a second one - two 5500W 50 Gallon electric water heaters will have lower peak demand than 1/2 of a 27 kW tankless and less than 1/3 of a 36 kW tankless. That peak demand is what really matters.

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    The EV and possibly other loads could be managed with EMS / Load Management. For instance having the tankless interrupt electric space heating or A/C, which are storage loads that can afford to stop working for a simple 30 minute shower without undue discomfort. Commented Dec 22, 2023 at 0:18
  • It seems fairly unlikely that a whole service upgrade from 100A to 200A was done un-permitted and without a new load calculation.
    – nobody
    Commented Dec 22, 2023 at 0:55
  • My hunch is that the service upgrade was done permitted but that the utility and/or AHJ either doesn't require a Load Calculation to be included or lets you get away with just plugging in one number and trusts that the electrician didn't pull the number of the air. Commented Dec 22, 2023 at 1:11
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    I wouldn't count on their not having been a load calc, @nobody. I'd agree 100% that it was permitted - I doubt there's a PoCo in the US that will turn power back on without an approved inspection - I know mine didn't
    – FreeMan
    Commented Dec 22, 2023 at 16:29

If you suspect your incoming voltage is insufficient or poorly regulated, a call to your utility should bring them out to measure it and take corrective action. They may need to change a tap on the transformer or replace it (their call).
In most jurisdictions, the size of the line feeding your house is determined by the size of the panel. When I upgraded from 100 to 200 amp service, the utility company replaced the line (no charge). I had to replace the line from their connection point on my house down to the meter, the meter base, and the line into my new panel.

If you want to check the voltage yourself, have a voltmeter, and know how to do it safely, you can measure the line voltage coming in from the meter under as many conditions as you wish (e.g., baseboard heaters on/off, dryer, etc.).
The safest way is to measure at a 120 receptacle (rather than removing the cover on the breaker panel). Measure at a receptacle while there is no load on its branch because the load will reduce the voltage you read vs at the meter. Then repeat using a receptacle on the other side of the 240 volt feed.
After my upgrade, I measured the voltage at 126-128 VAC, using a calibrated RMS reading meter. I asked my utility company to reduce it to avoid stressing devices meant for 110-120 VAC. They came out, confirmed my readings, and changed the tap on the pole transformer. Now my meter reads 118-120, which is great. As for the utility bill, I would not attribute the increase to insufficient voltage, but rather greater energy usage by your additional heating devices. (Resistive heating is much less efficient than say, a heat pump).


The easiest way is to locate and check the transformer and see if it looks new. Older ones will be weathered and pssible rusting. Your contractor or electrician should have contacted the power company concerning your panel and meter change out so they would have checked things on their end. You should call the power company and tell them you've added load and that there's excessive voltage drop. They should come out and set a monitoring device to check the voltage for a 24 to 36 hour period. If the transformer is overloaded, then they will change it out and possibly the cables from the pole to your weather head.

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    Is this a common thing? I'm completely unaware of transformer upgrades being needed in most cases.
    – isherwood
    Commented Dec 21, 2023 at 22:06
  • In my case the only thing the electric company needed to upgrade was the feeder wires... and they procrastinated on that because in air you can get away with the wires running a bit hotter than the spec calls for. (Not that I'm actually drawing 200A at any time, yet.)
    – keshlam
    Commented Dec 21, 2023 at 22:52
  • Yes, it's common for the utility to take a "wait and see" attitude regarding service drop and transformer upgrades. They know from experience that customers demanding big service upgrades usually don't increase their actual loads all that much. They can surveil smart meter data. Commented Dec 22, 2023 at 0:19
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    @isherwood It's common for them, at least in Florida where I worked, to check the loading on the transformers when significant residential load is being added. adding AC units, electric heating and on demand water heaters would send up flags. I think now most utilities have some sort of transformer load management systems in place where they can check the loading on them.
    – JACK
    Commented Dec 22, 2023 at 1:15
  • Not really sure of the DV on this... I've done this work for years.
    – JACK
    Commented Dec 22, 2023 at 1:18

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