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#92775 - 04/07/05 07:11 AM Service Calcs
electcontrctr Offline
Member

Registered: 04/07/05
Posts: 15
Loc: Fairbanks, AK USA
Hello,
I am new to the forum and was looking for a little help.

I need to calculate a residential service and the inspector says it should be calculated at 125%..(is that correct?)

The total sqf is 1308 living and 1792 total (including garage)..

it is total electric. here are the loads:

HW-4500
Dryer-5000
Range-10500
A/C-2.5 Ton
Heat (Heat Pump)
Dishwasher-1600
Disposal-690
Attic Fan-150
Microwave (950VA)(does this need to be counted if it is cord and plug connected?)

-----------------------

If anyone could help it would be appreciated.

Thanks,

Brad




[This message has been edited by electcontrctr (edited 04-07-2005).]

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#92776 - 04/07/05 09:25 AM Re: Service Calcs
BobH Offline
Member

Registered: 10/16/04
Posts: 134
Loc: Newfane, N.Y USA
Are you using a demand factor or just adding everything together?

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#92777 - 04/07/05 09:27 AM Re: Service Calcs
electcontrctr Offline
Member

Registered: 04/07/05
Posts: 15
Loc: Fairbanks, AK USA
Hi,
No, I used an online calculator..

i havent done it by hand yet but I may because i dont think the calculator is correct..

i get lower amps when i double the sqf!

thanks

brad

[This message has been edited by electcontrctr (edited 04-07-2005).]

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#92778 - 04/07/05 12:23 PM Re: Service Calcs
markp Offline
Member

Registered: 04/12/04
Posts: 60
Loc: Kent, WA, USA
You added most of the nameplate values in an edit, so that will help with people getting you answers. But you need to clarify the heat pump and airconditioner situation. A heat pump usually provides both the heat and AC, so is the AC a separate one or is this just the compressor part of the heat pump? You also need to know how the air handler is setup -- can all the resistance heat in it be used while the compressor is running, or is part of it locked out? If you don't know, assume that all the strip heat and the outside compressor can run together. Is there a nameplate on the airhandler indicating MCA, or FLA and heat KW? We also need the nameplate value for the compressor portion (which should be listed as MCA or RLA+FLA). You could guess 18 amps for that, but you shouldn't guess as the values can be all over the place.

I don't understand your inspector's 125% comment. Residential services rarely have anything that is continuous, and its those items you multiply by 125%. All the standard calculation mandates is a 25% increase for the largest motor you have and that's it. But having a 25% growth margin is probably a good design.

My rough answer, based on what you've provided, is: 164 amps

I assumed the heat pump was 46 + 18 amps at 240V, that the disposal was the largest motor, and that the water heater, disposal, dishwasher, attic fan, and microwave could all have a .75 value used because of 4 or more fastened in place equipment items.

A 200A service should be perfect for this situation unless they plan on adding a hot tub. For most installs, the next size up is a 320A meterbase and a second main panel (a 300A or 400A service), which causes a substantial price increase.
_________________________
Mark
Kent, WA

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#92779 - 04/07/05 12:32 PM Re: Service Calcs
electcontrctr Offline
Member

Registered: 04/07/05
Posts: 15
Loc: Fairbanks, AK USA
Hello,
Thank you for replying. The heat pump is two seperate units..one installed over the garage and the compressor outside..the compressor will not run at the same time as i understand it..it is a trane unit...i am trying to get the exact nameplate rating..

the inspector said the calculation for the service had to be done at 125% and i do not understand where that is coming from..i tried to call back but got no answer..i will try again tomorrow..

thanks

brad

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#92780 - 04/07/05 01:18 PM Re: Service Calcs
electcontrctr Offline
Member

Registered: 04/07/05
Posts: 15
Loc: Fairbanks, AK USA
Hi,
Here is what I came up with using the following:

65% of the heating load..230.(C)4
----------------------------------

General Light and Receptacle Load
1308sqft x 3VA = 3924VA
--------------------------------------------------------------------------------
Small Appliance Circuits Load
2 x 1500VA = 3000VA
--------------------------------------------------------------------------------
Laundry Branch Circuit(s) Load
1 x 1500VA = 1500VA
--------------------------------------------------------------------------------
Total General Load Less Demand
sum of above less T220.11 demand
sum of above = 8424VA
first 3000 at 100 % = 3000
next 117,000 at 35% = 1898
remainder at 25% = 0
sum less demand = 4898VA
--------------------------------------------------------------------------------
Fastened In Place Appliances Load
water heater = 2400VA
dishwasher = 1600VA
disposal = 690VA
attic fan = 150VA
microwave = 950VA
sum less 75% demand = 4342VA
--------------------------------------------------------------------------------
Clothes Dryer Circuit(s)Load
Total dryer circuit(s) load = 5000VA
--------------------------------------------------------------------------------
Cooking Appliance(s) Load
range 10.5kVA
T220.19 Column A = 8kVA
T220.19 Column B = 0kVA
T220.19 Column C = 0kVA
Total calc.d range load = 8kVA
--------------------------------------------------------------------------------
Heating or Air-Conditioning Load
Heater = 9360VA
--------------------------------------------------------------------------------
Largest Motor Load
25% of largest motor = 172.5VA
--------------------------------------------------------------------------------
Total Computed Load
31772.5 Volt-Amps
--------------------------------------------------------------------------------
Computed Amperage
31772.5 divided by 240 = 132Amps

-------------------------------

Neutral Loads
General Neutral Load = 4898VA
--------------------------------------------------------------------------------
Fastened In Place Appliances Neutral Load
dishwasher = 1600VA
disposal = 690VA
attic fan = 150VA
microwave = 950VA
Total before 75% demand(4 or more) = 3390VA
Total FIP Neutral Load = 2542VA
--------------------------------------------------------------------------------
Clothes Dryer Neutral Load
220.22 allows 70% of 5000VA
Clothes Dryer Neutral Load = 3500VA
--------------------------------------------------------------------------------
Cooking Appliances
range = 10500VA
220.22 allows 70% of 10500VA
Cooking Appliances Neutral Load = 7350VA
--------------------------------------------------------------------------------
Heating or Air Nuetral Load = 0VA
--------------------------------------------------------------------------------
Largest Motor Neutral Load = 172.5VA
--------------------------------------------------------------------------------
Total Neutral Load = 18462.5VA
--------------------------------------------------------------------------------
Total Neutral Amperage = 77Amps

TOTAL SERVICE CONNECTED LOAD: 132A

Now if I multiply that times the 125% the inspector wants then I get 165A...

I wont do that unless he can show me where it is in the NEC or local ordinance..

thanks

brad

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#92781 - 04/07/05 09:17 PM Re: Service Calcs
DougW Offline
Member

Registered: 06/08/03
Posts: 1083
Loc: North Chicago, IL
I got 151A myself (using my Excel spreadsheet calculator program)

The closest thing to a "125% rule" we have around here is many AHJ's require 25% of the breaker spaces in a new construction panel to be empty, to allow room for expansion.

[This message has been edited by DougW (edited 04-08-2005).]

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#92782 - 04/08/05 12:18 PM Re: Service Calcs
markp Offline
Member

Registered: 04/12/04
Posts: 60
Loc: Kent, WA, USA
Brad, you've done one thing I don't agree with and I think you made one mistake. In your calculation, you have 2400VA for the water heater, yet you said it was 4500VA. Perhaps you're thinking that only one element is used at a time (which is true), but usually a 4500W heater will have two 4500W elements, of which either the upper or lower one is used (but not both).

What I don't agree with is the 220.30(C)(4) usage. This only applies to the optional calculation, not the standard method that you used. You can recalculate using the optional method, but that still brings up whether you should be using 65% or 100% of the heat pump rating. Try to talk to the HVAC guy, but most heat pumps run the electric heat when the heat pump is in defrost mode or when the outside temperature is below the "balance point". If this is a cold area (like noth of Georgia), I would hope the electric heat and compressor run at the same time. There is also emergency heat, which runs all the electric heat but without the compressor. It is possible to have X KW for defrost, Y KW supplemental heat (when its real cold), and Z KW for emergency. So its difficult to say how much of the installed strip heat is being used in each mode, unless there is only 1 strip. If you can't get answers to this, I think you need to assume that all the strip heat could be used with the heat pump compressor.

[This message has been edited by markp (edited 04-08-2005).]
_________________________
Mark
Kent, WA

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#92783 - 04/09/05 08:57 PM Re: Service Calcs
Bob Offline
Member

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
All heat pumps I have seen have the compressor and strip heat running at the same time.
An A/C or heat pump should be considered continous load. 220.14 references 430.24,.25 and .26 and 440.6 and requires 125% of the largest motor. In a residence the A/C unit is usually the largest motor.
220.30C2 requires the heat and compressor be added at 100%. The 65% rule is for fixed electrical space heat.
Based on the caculations, the service should include 125% of the FLA of the compressor load not 125% of the entire load as suggested by the inspector

electcontrctr
Your post shows an A/C unit and a heat pump. Is it both?


[This message has been edited by Bob (edited 04-10-2005).]

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#92784 - 04/10/05 07:48 AM Re: Service Calcs
electcontrctr Offline
Member

Registered: 04/07/05
Posts: 15
Loc: Fairbanks, AK USA
hi,
i am no hvac expert but as i understand it the hvac system includes the following: a compressor unit located outside and a blower/heat combo located above the garage cieling...the compressor gets a 40A disco and the unit above the garage gets a 60 breaker. this is a heat pump made by trane..

the compressor runs until the heat strips come on then the compressor is shut off...

once i get the job done and pass inspection i will request a written letter from the jurisdiction explaining the 125% requirement he asked for...i am just going to install a 200A main panel and move on..

i appreciate the help

brad

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