Service Calculation Needs to be Changed

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Dennis Alwon

Dennis Alwon

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Retired Electrical Contractor
I just did a calculation for a friend who is thinking about installing an electric on demand water heater at 27kw

If you do an optional calculation the service load comes in at 125 amps however the water heater load is 112 amps. Imo, the water heater and car chargers should not be using the demand factors because this is crazy.

The standard calculation calls for 225 amp service. This is a small condo 1200 sq. ft with no heat strips but just a heat pump (2 ton) at 2000 watts (approximate)

What are your opinions..... The car charger, not in this example, is on for many hours but the water heater may only be on for as long as a shower may take. This could still be an issue if you shower in the evening when the car is being charged.


1652188043167.png
 
on the surface it seems too much, but the real answer might be to monitor total load for say a month and see what it looks like and take into consideration that any short time load levels above actual conductor ampacity aren't necessarily going to cause significant heating that will damage the conductor insulation.

That said the load patterns in your friend's house might not be the same as with other people, so a true study of what should be NEC requirements needs to be more than just one household.
 
I agree it's time for a comprehensive study and perhaps changes. The CMPs recognized the changes in lighting loads and added exceptions to 220.12. Instant water heater and EV charging introduce new factors not taken into account on the existing formulas.
Luckily, the ECs often recognioze this as you do and adjust accordingly on single dwellings but it would be interesting to see the exact effect on multi-family,
 
Dennis Alwon said:
I just did a calculation for a friend who is thinking about installing an electric on demand water heater at 27kw

If you do an optional calculation the service load comes in at 125 amps however the water heater load is 112 amps. Imo, the water heater and car chargers should not be using the demand factors because this is crazy.
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Those electric on demands are popular here as our electric rates are low. I was real skeptical for a long time but haven't seen any issue with them. I have never installed nor seen one on a service smaller than 200 Amps though.
For me I always get a chuckle when I do these new homes install a new 200A service (send the loads to the POCO) then the POCO comes and sets a 25 kVA pad-mount for two+ homes.
 
kwired said:
on the surface it seems too much, but the real answer might be to monitor total load for say a month and see what it looks like and take into consideration that any short time load levels above actual conductor ampacity aren't necessarily going to cause significant heating that will damage the conductor insulation.

That said the load patterns in your friend's house might not be the same as with other people, so a true study of what should be NEC requirements needs to be more than just one household.
Click to expand...
The problem is, how long is "short"? And how many times can you exceed the rating for a "short time" before the cumulative insulation damage becomes significant? Whatever significant means. If you can't measure it, you can't control it.
 
kwired said:
on the surface it seems too much, but the real answer might be to monitor total load for say a month and see what it looks like and take into consideration that any short time load levels above actual conductor ampacity aren't necessarily going to cause significant heating that will damage the conductor insulation.

That said the load patterns in your friend's house might not be the same as with other people, so a true study of what should be NEC requirements needs to be more than just one household.
Click to expand...
You can't really do a load calculation based on that particular person's use of the home. At some point someone else will live there so the calculation really has to encumber all possible situations
 
tortuga said:
Those electric on demands are popular here as our electric rates are low. I was real skeptical for a long time but haven't seen any issue with them. I have never installed nor seen one on a service smaller than 200 Amps though.
For me I always get a chuckle when I do these new homes install a new 200A service (send the loads to the POCO) then the POCO comes and sets a 25 kVA pad-mount for two+ homes.
Click to expand...
I've seen 200 amp panels that you would think were overloaded, maybe have 25 or even 30 kW of electric heat, water heater or even two (tank style) range, dryer, plus lighting and etc, (before LED's) yet they never had any problems. Also seen a few 100 amp panels that probably pushing limits and likely would be over 220 load calculations yet never trip the main. There is often more load diversity than people realize, but still not every user will be the same.

JMO but I think particularly in places where the supply water is rather cold, a storage tank ahead of the on demand type heater would reduce the rating needed for the heater, no added heat to storage tank or if so set it to be not much over room temperature, then tank losses would be negligible.
 
Look at the irony of the situation. The on demand electric water heater draws draw a 112 lamps the service is only a 125 amps so if the air conditioner comes on and you're taking a shower you're over a 125 amps with nothing else going
 
Dennis Alwon said:
Look at the irony of the situation. The on demand electric water heater draws draw a 112 lamps the service is only a 125 amps so if the air conditioner comes on and you're taking a shower you're over a 125 amps with nothing else going
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If you had 112 amps of cooking appliances on the surface it looks about the same, but actual load diversity would likely be very different than for the on demand water heater.

At same time actual heating of the supply conductor may not be too extreme but you have a OCPD that will respond if you let it be loaded that way for too long. But the heating effects will probably show up over time, particularly at termination points.
 
Dennis Alwon said:
Look at the irony of the situation. The on demand electric water heater draws draw a 112 lamps the service is only a 125 amps so if the air conditioner comes on and you're taking a shower you're over a 125 amps with nothing else going
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The Code tells you right up front that it's provisions are not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

In this example, you know that a 125A service, though the minimum size required for compliance with Code provisions, will not be adequate for good service...so install a larger service.
 
david luchini said:
The Code tells you right up front that it's provisions are not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

In this example, you know that a 125A service, though the minimum size required for compliance with Code provisions, will not be adequate for good service...so install a larger service.
Click to expand...

Yes, of course, that is the thing to do but I brought this up because other people may think it is fine without better direction from the NEC. I think it is time this is addressed but I am sure it is too late for the 2023.
 
Your calc came out right at the limit (125A). Are you sure you have the correct value for HVAC? A 2 ton heat pump is typically about 15A MCA at 240V (more like 3500 watts, not 2000). This could bump the service up a step and provide more margin.
 
I’m with you on this.
Seems to me that iirc that any tank heater under so many gallons (my memory eludes me for the moment what size that is for load calculation reasons). For circuit sizing needs to be figured as a continuous load @125% that being said one would think that it should be applied after your demand factor portion of calculation.
 
Dennis Alwon said:
You can't really do a load calculation based on that particular person's use of the home. At some point someone else will live there so the calculation really has to encumber all possible situations
Click to expand...
Sure you can..
220.87
 
IMHO the issue is that the water heater itself needs to have a load calculation done. The heater is rated 27kW maximum, but its actual consumption will depend upon how much hot water is actually being used at what temperature lift. Think of the water heater like a 27kVA transformer. The actual consumption will depend on the 'downstream' loads.

If the incoming water is on the cold side (40F) and the customer likes hot, high flow showers, then a single shower might use all 27kW.

If the water heater feeds many loads (2 showers, 4 lav sinks, 1 kitchen sink, 1 dishwasher, 1 clothes washer), servicing a family with several kids, then it is likely that all 27kW will be used for extended periods of time. (And is probably too small)

If the water heater feeds a small house for a couple, then it will be a much smaller load.

-Jon
 
winnie said:
IMHO the issue is that the water heater itself needs to have a load calculation done. The heater is rated 27kW maximum, but its actual consumption will depend upon how much hot water is actually being used at what temperature lift. Think of the water heater like a 27kVA transformer. The actual consumption will depend on the 'downstream' loads.

If the incoming water is on the cold side (40F) and the customer likes hot, high flow showers, then a single shower might use all 27kW.

If the water heater feeds many loads (2 showers, 4 lav sinks, 1 kitchen sink, 1 dishwasher, 1 clothes washer), servicing a family with several kids, then it is likely that all 27kW will be used for extended periods of time. (And is probably too small)

If the water heater feeds a small house for a couple, then it will be a much smaller load.

-Jon
Click to expand...
40F is some pretty cold incoming water, if raising from that level 27kW might not get it warm enough, does depend on flow rate.
 
Dennis Alwon said:
1652188043167.png
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I came up with this but that's putting the Heat pump in with the total added load.

calc.jpg
 
Dennis Alwon said:
I just did a calculation for a friend who is thinking about installing an electric on demand water heater at 27kw

If you do an optional calculation the service load comes in at 125 amps however the water heater load is 112 amps. Imo, the water heater and car chargers should not be using the demand factors because this is crazy.

The standard calculation calls for 225 amp service. This is a small condo 1200 sq. ft with no heat strips but just a heat pump (2 ton) at 2000 watts (approximate)

What are your opinions..... The car charger, not in this example, is on for many hours but the water heater may only be on for as long as a shower may take. This could still be an issue if you shower in the evening when the car is being charged.


View attachment 2560530
Click to expand...
If he is your friend, you should talk him out of the electric on-demand and into getting gas instead!
 
Little Bill said:
If he is your friend, you should talk him out of the electric on-demand and into getting gas instead!
Click to expand...

He is my friends son who lives in DC in a condo. There is no gas there but I am going to try and talk him out of it. He has a 150 amp service and has 15kw heat that he was going to have disconnected and just use the heat pump since he is surround by apts. top, bottom and sides. Only 2 sides exposed to outside.

I also just learned that there is communal laundry so no washer/dryer in the calculation. It may actually work with just the 2 of them. I worry about the next owners
 
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