Multi-Family Dwelling Calculation

[Npstewart]

I need some opinions on the attached PDF load calculation. We were hired to take over a project where they halted construction back in 2006.
The load calculation has a few minor math errors, but most importantly the service is under-designed by it's own accounts and I am trying to see if it is possible to get it to work.

So two questions about the "Optional Dwelling unit Calc".

1) The original designer took the Range VA at 12,000. I typically take them at 8,000. Would I be able to make this change?

2)The original designer took the entire heating load (including the fan) at 65%. I have always taken ONLY the heat at 65%, and the blower fan at 100%. Which way is correct?

I really appreciate any opinions, thanks guys.

 

[Dennis Alwon]

I need some opinions on the attached PDF load calculation. We were hired to take over a project where they halted construction back in 2006.
The load calculation has a few minor math errors, but most importantly the service is under-designed by it's own accounts and I am trying to see if it is possible to get it to work.

So two questions about the "Optional Dwelling unit Calc".

1) The original designer took the Range VA at 12,000. I typically take them at 8,000. Would I be able to make this change?

For optional calculation you must take the ranges at nameplate- probably 12Kw, Art. 220.84(C)

2)The original designer took the entire heating load (including the fan) at 65%. I have always taken ONLY the heat at 65%, and the blower fan at 100%. Which way is correct?

Again I think the designer is correct-- 220.84(C)(5).

You did say MFD and Optional calculation so 220.84 is what should be used.

 

[Smart $]

I'm wondering how the designer planned to "squeeze" 824 calculated service amps through an 800A breaker???

 

[Smart $]

...

2)The original designer took the entire heating load (including the fan) at 65%. I have always taken ONLY the heat at 65%, and the blower fan at 100%. Which way is correct?

Again I think the designer is correct-- 220.84(C)(5).

You did say MFD and Optional calculation so 220.84 is what should be used.

Ummm... HVAC load for each dwelling unit is determined under 220.82(C)

Answer to the OP question is taking 65% of both is correct because the fan (aka blower) is part of the heating load. I'm assuming a forced air system where the blower is intergral with each space heater unit.

 

[chris kennedy]

I'm wondering how the designer planned to "squeeze" 824 calculated service amps through an 800A breaker???

Compact electrons.

 

[Dennis Alwon]

Ummm... HVAC load for each dwelling unit is determined under 220.82(C)

Answer to the OP question is taking 65% of both is correct because the fan (aka blower) is part of the heating load. I'm assuming a forced air system where the blower is intergral with each space heater unit.

That is not what 220.84 states. It should reference the other section if that is the intent-- You are probably right as I never did a mfd calc but it says to take the larger of the loads and does not say based on 220.82

 

[iMuse97]

Compact electrons.

But not compact conductors.

 

[Smart $]

That is not what 220.84 states. It should reference the other section if that is the intent-- You are probably right as I never did a mfd calc but it says to take the larger of the loads and does not say based on 220.82

You are not incorrect in a sense...

Table 220.84, subject to the inclusions of 220.84(C) of which you reference, applies only to service and feeders supplying three or more units of an MFDU.

The OP question was, as it seemed to me, regarding individual unit calc's, for which 220.84(C) does not apply due to the criterion of Table 220.84.

 

[Smart $]

But not compact conductors.

Wouldn't suprise me if he/she gave it a go :lol:

 

[Npstewart]

I didn't get any replies to this yesterday, im glad to see a bunch this morning

My MAIN issue here is that the original Engineer clearly put 824 amps (actually closer to 835A), and directly below called out for a 800A service. My first thought was that this was a typo on the load calc, and it was supposed to be 1000A service. But, on the riser, he is using (3) sets of #400 AL w/ a reduced neutral, which is good for 810 amps max....

My issue with the heater @65% and the range isn't really biggest concern here, I was attempting to see if 800A would work, and I think im realizing that given that we are taking a 41% diversity on the entire service as it is, I don't think I would be comfortable even if it was 799A. There is several of these buildings with 800A services that all need to be 1000A that haven't had their final inspections yet, and now is subject to plan review again. Im glad it isn't my companies name on the original drawings.

 

[charlie b]

I see a number of errors in the calculation, the last of which I consider major. :thumbsdown::thumbsdown:

1. The math is wrong. The value of “subtotal” for the loads on unit A is shown as 31142. The actual subtotal is 31787.
2. The value of “subtotal” for the loads on unit B is shown as 31637. The actual subtotal is 31775.
3. The air conditioning loads are shown as being a negative number that will be subtracted from the other items. They are not being subtracted, but the manner of displaying these values is confusing.
4. The total loads for unit A is shown as 24675. This value is correct, because it is based on the correct value of 31787 as discussed in comment 1 above. So the calculation table is inconsistent with itself.
5. The totals load value shown for unit B is also wrong, for reasons that are harder to explain. But basically the table is, here again, inconsistent with itself.
6. Calculating the individual apartment loads, using the demand factors given in 220.82, is one thing. Calculating the total service load is quite another. We are required to start all over, and count all the loads to which the service will be connected. Then, and only then, do we get to apply the demand factor from table 220.84. The calculation given to us takes the individual unit loads, which were based on use of the 10k/40% demand factor, multiplies by the number of units, and then multiplies that total by the 41% DF. That is a major error. 220.84(C) tells us to include 3VA per square foot, as an example, plus 1500 VA for all the required circuits, but it does not let us take 40% of the amount above 10K before multiplying the total by 41%. Also, the heating load has to be counted at 100%, not 65%, before applying the 41% DF.

This calculation definitely has the service undersized, and there is no way to use the 800 amp result. I would throw away the earlier calculation, and start it all over from scratch. Nothing in the existing calculation is trustworthy. :thumbsdown::thumbsdown:

 

[david luchini]

I didn't get any replies to this yesterday, im glad to see a bunch this morning

My MAIN issue here is that the original Engineer clearly put 824 amps (actually closer to 835A), and directly below called out for a 800A service. My first thought was that this was a typo on the load calc, and it was supposed to be 1000A service. But, on the riser, he is using (3) sets of #400 AL w/ a reduced neutral, which is good for 810 amps max....

My issue with the heater @65% and the range isn't really biggest concern here, I was attempting to see if 800A would work, and I think im realizing that given that we are taking a 41% diversity on the entire service as it is, I don't think I would be comfortable even if it was 799A. There is several of these buildings with 800A services that all need to be 1000A that haven't had their final inspections yet, and now is subject to plan review again. Im glad it isn't my companies name on the original drawings.

I think to know if 800A is adequate, you need to know what the "house load" is. It appears to me that the original engineer was calling out for a 50A house panel, rather than having an actual 50A house load. He may have known his actual house load was only 20A (for instance), making the 800A service OK.

 

[Npstewart]

I only posted the load calc. Basically, the EOR for this showed 64 amps of load on the panel itself, and then put a 60A MCB with a 60A feeder, and to make matters WORSE counted it as a 50A load on the load calc (as you can see from the PDF).

 

[david luchini]

I see a number of errors in the calculation, the last of which I consider major...

Calculating the individual apartment loads, using the demand factors given in 220.82, is one thing. Calculating the total service load is quite another. We are required to start all over, and count all the loads to which the service will be connected. Then, and only then, do we get to apply the demand factor from table 220.84. The calculation given to us takes the individual unit loads, which were based on use of the 10k/40% demand factor, multiplies by the number of units, and then multiplies that total by the 41% DF. That is a major error. 220.84(C) tells us to include 3VA per square foot, as an example, plus 1500 VA for all the required circuits, but it does not let us take 40% of the amount above 10K before multiplying the total by 41%. Also, the heating load has to be counted at 100%, not 65%, before applying the 41% DF.

Charlie, I agree with your other points, but I think you are off with this one. The calculation does take 100% of the loads calculated for each apartment type, including the larger of the heating or A/C, per 220.84.

The total connected load for Type A, per 220.84(C)(1)-(4) is 31787va. Per 220.84(C)(5), the larger of the heating or A/C load is 5960. This gives a total of 37747 VA for type A. For type B, the total is 37735 VA.

This is what the original engineer used for the multiple dwelling optional calc. Four unit A @ 37747 = 150988VA. Eight Unit B @ 37735 = 301880VA. Total for all 12 units = 452868VA. 41% demand per T220.84 = 185676 VA. I think the oringinal engineer is correct with this calculation.

(Also, I don't think the original was listing the A/C as a negative number, but had it in parantheses to show it was the smaller of the heating or A/C loads, and wouldn't be included.)

 

[charlie b]

If came across a signature and seal of a licensed professional engineer on a calculation that (even disregarding the math errors and the process error) makes a selection that is clearly unsafe, then I would report that person to the applicable licensing board. Our fundamental duty is to protect the health and safety of the public. If a person shows that degree of incompetence, it is a serious matter, and warrants investigation by the licensing board.

 

[Npstewart]

Charlie, you are definitely correct. The NEC goes out of its way to re-list everything in 220.85 (C) to ensure the 41% deduction isn't taken in addition to the 40% to size the feeder.

 

[charlie b]

For starters, David, we don’t know the heating load. We were told that the original engineer used a value of 65%, and we see a value of 5960 in the calculation. From that I infer, rightly or wrongly, that the heating load is about 9170. If that is right, then the total service load is off by about 65 amps for that reason alone.

But I see that I did not check enough of the math. You are right; the engineer did not make what I called the major error of double dipping the demand factors. So I withdraw that part of my comment, with apologies to the original engineer.

That said, selecting a panel that is not rated for the load to which it will be subjected is a clear violation of NEC 408.30. More to the point, it violates the basic principle of protecting the safety of the public.

 

[charlie b]

Charlie, you are definitely correct. The NEC goes out of its way to re-list everything in 220.85 (C) to ensure the 41% deduction isn't taken in addition to the 40% to size the feeder.

Yes, but as David correctly pointed out, I was wrong in thinking that the engineer had made that error.

 

[Npstewart]

My company knows this EOR of this project professionally. He is a really nice guy, but extremely old. He has been a engineer since the early 1940's! Im pretty sure he didn't renew his license last time around.

 

[charlie b]

Im pretty sure he didn't renew his license last time around.

Was this before or after he signed and sealed the calculation? You are starting to concern me.