Faulty load calculation by engineer????

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bubby

Member
I'm in a pickle. We are working a 165 unit apartment project and the HVAC and plumbing was changed from gas to electric. Not on the drawings mind you, but at the field level. My concern is that even though 80A of load per unit has been added to this project, the builder is NOT willing to upgrade the services - to the buildings or the units.

Based on the previous HVAC loads being gas, the main breakers for the units are 100 amp at 240V.

Per the engineer, the electrical loads are as follows:

3,840 = Square footage average = 1280 square feet x 3
3,000 = Appliance
8,000 = Range (seems low and need to verify)
4,500 = Electric Hot water tank
1,500 = Washer
5,000 = Dryer
800 = Disposal
740 = Dishwasher
300 = Range hood

27,680

10,000 = First 10KVA at 100%
7,072 = Remainder at 40%
6,500 = Electric furnace greater than heating (10KW x .65)

23,572/240 = 98.21 amps

So, the engineer is saying the 100 amps is adequate.

But, there is also a heat pump. Couldn't the 30A, 240V heat pump and the aux. furnace also cycle together at times (althought the same engineer added a not stating that they are to be set up not to)? Also, there is a main breaker IN the furnaces, which is 60 amp. If the furnace has a 60A main, and we are feeding it with a 60 amp circuit, wouldn't we have to calculate its load at 14,400 in lieu of 10kw?

In short, I have to warranty this work. I have to ensure this work and I have to stand by this work.

If the furnace was running while someone was doing the laundry (hot water tank would be cycling also), it would trip the main.

If someone was cooking and the furnace was running and a few lights were on, it could trip the main.

In short, there are SOOO many variations that are real that could trip the 100 amp main that it doesn't pass the gut check for me. I have told the builder this but they can't be bothered.

Also, I thought electric furances were considered constant load and had to be calculated at 125%? The engineer is taking actual load based on the size of the elements and multiplying by 65%.

Any feedback for peace of mind would be appreciated.

Thank you.
 

bubby

Member
There downside for me is that the GC is not willing to pay to upgrade the services because the engineer says the 100A services are fine.

I will then be expected to provide a warranty for this engineering.

Thanks for your reply.
 

shamsdebout

Senior Member
Location
Macon,GA
I'm in a pickle. We are working a 165 unit apartment project and the HVAC and plumbing was changed from gas to electric. Not on the drawings mind you, but at the field level. My concern is that even though 80A of load per unit has been added to this project, the builder is NOT willing to upgrade the services - to the buildings or the units.

Based on the previous HVAC loads being gas, the main breakers for the units are 100 amp at 240V.

Per the engineer, the electrical loads are as follows:

3,840 = Square footage average = 1280 square feet x 3
3,000 = Appliance
8,000 = Range (seems low and need to verify)
4,500 = Electric Hot water tank
1,500 = Washer
5,000 = Dryer
800 = Disposal
740 = Dishwasher
300 = Range hood

27,680

10,000 = First 10KVA at 100%
7,072 = Remainder at 40%
6,500 = Electric furnace greater than heating (10KW x .65)

23,572/240 = 98.21 amps

So, the engineer is saying the 100 amps is adequate.

But, there is also a heat pump. Couldn't the 30A, 240V heat pump and the aux. furnace also cycle together at times (althought the same engineer added a not stating that they are to be set up not to)? Also, there is a main breaker IN the furnaces, which is 60 amp. If the furnace has a 60A main, and we are feeding it with a 60 amp circuit, wouldn't we have to calculate its load at 14,400 in lieu of 10kw?

In short, I have to warranty this work. I have to ensure this work and I have to stand by this work.

If the furnace was running while someone was doing the laundry (hot water tank would be cycling also), it would trip the main.

If someone was cooking and the furnace was running and a few lights were on, it could trip the main.

In short, there are SOOO many variations that are real that could trip the 100 amp main that it doesn't pass the gut check for me. I have told the builder this but they can't be bothered.

Also, I thought electric furances were considered constant load and had to be calculated at 125%? The engineer is taking actual load based on the size of the elements and multiplying by 65%.

Any feedback for peace of mind would be appreciated.

Thank you.


Can you issue an RFI to clear yourself? You described the problem well. I think it would be good to get it in writing to the GC and then distribute to the engineer. That way you know you tried your best and hopefully you will be covered and maybe an issue can be prevented.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
There downside for me is that the GC is not willing to pay to upgrade the services because the engineer says the 100A services are fine.

I will then be expected to provide a warranty for this engineering.

Thanks for your reply.

I don't see how you can be required to warranty someone else's work.

The only issue I see is that probably in your contract you are required to produce a product in compliance with "all applicable codes...".

If it truely does not meet code, there could be some kind of an issue that might come back to bite you.

I think you need to document your misgivings in writing to your customer (presumably the GC). in that document, you should include a statement regarding his instructions to proceed regardless of your misgivings.

IMO, unless there is some kind of contractual arrangement between you and the engineer on this project, you have no business directing this kind of communication to the engineer. It needs to go to the guy that is paying your bill, and it is up to him to deal with it.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
I'd suggest repeating the calculation with the Optional method. You'll probably get a lower result, but it would be best if you knew the nameplate values. A 10KW electric heater most likely will have a 12.5KW MCA. You use the MCA value, not the breaker rating. Many heat pump air handlers come with a 60A breaker regardless of what size heat strip is in it. But there will be fan and control loads too which is why you need the true nameplate and note one you're guessing about.

The optional calculation allows a demand factor to be applied to heat pump or fixed electric heaters. Since there's a heat pump, you probably need to use that factor, and can ignore the compressor if they wire it such that the aux heat is locked out while the compressor is on and vice versa.
 

templdl

Senior Member
Location
Wisconsin
Have you had the opportunity to put your warranty in writing?
You would want to be very specific that your installation is per the NEC and installed per that which has been designed by the engineering frim. That you are not liable for load calculation that are the sole responsibility of the EE who designed the system.
Remember that you have installed the wire as directed by the EE and protected the wire with a suitably sized breaker. As such the only thing that could occur is nuisance tripping should the EE provided you with circuit sizes that are to small.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
On closer reading it looks like he did do the optional calculation (the 8KVA for the range made me think he's using the standard). I think the 100A feeders may be a problem because of the omissions (range too small, HVAC not including fan/controls/MCA value). Not sure what YOU do....
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
You do not calculate the load of the furnace based on the 60 amp breaker. 10kw is what you use or actually the nameplate. If the heat pump is not set to cycle independently from the heat strips then they should be added together. I would get an engineer signed affidavit saying that the 100 amps is okay. If the calculated load is 98 amps then 100 amp service is adequate.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
There downside for me is that the GC is not willing to pay to upgrade the services because the engineer says the 100A services are fine.

I will then be expected to provide a warranty for this engineering.

Thanks for your reply.
You would only warranty your installation and materials you furnished, the engineers seal takes you out of any responsibilty for the design.

Roger
 

Smart $

Esteemed Member
Location
Ohio
What do the 'approved' plans say? Can you just willy nilly change things where you are?
I think not having approved revised plans is the major issue. The other issues are just consequences of that inaction. The dwelling unit panels may be a non-issue, given the load calculation. But what about the feeders' ampacity and service rating(s)?

bubby said:
I'm in a pickle. We are working a 165 unit apartment project and the HVAC and plumbing was changed from gas to electric. Not on the drawings mind you, but at the field level. My concern is that even though 80A of load per unit has been added to this project, the builder is NOT willing to upgrade the services - to the buildings or the units.



Based on the previous HVAC loads being gas, the main breakers for the units are 100 amp at 240V.

...
 

kwired

Electron manager
Location
NE Nebraska
You mentioned 30 amp heat pump, I'm guessing this is size of breaker of that unit, and not actual RLC, which is probably only 16-19 amps if it wants a 30 amp breaker, but on coldest days may only draw 4-6 amps as there is not much heat in the air available to capture, less heat = less load on the compressor, but will run almost continuously at a low level.

10 kW of heat - probably is actually 9.6 kW. This is pretty common in these type of units. 100 amps feeder to each unit is probably pushing it a little, but should be acceptable.

You have different demand factors applicable for individual dwellings than you have for the service with multiple dwellings. So you need to consider the service loading separately from each feeder.
 

kenaslan

Senior Member
Location
Billings MT
Because engineers are never wrong, have him sign that he will pay for all service calls for triped breakers. Use the calcuator shown, and ask him where the mistake is.

I know as a former contractor, that once the job is said and done, it is the contractor almost always holding the bag.
 
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