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).]
Are you using a demand factor or just adding everything together?
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).]
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.
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
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
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).]
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).]
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).]
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
Try the caculator at this site.
http://www.electricalknowledge.com/SFDLoadCalc.asp You said "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."
You need to verify this. I have a Trane and the compressor runs til the temp get below 32F and then the strips come in. Both are on at the same time.
The heat load seems to be about 12kw. Take the 60 amp breaker x .80 = 48amps.
48 amps x 240 =11.5 kw. The compressor load
is 40amp x .80 = 32 amps x 240 = 6144 va.
You may have a central electric furnace and A/C. If so 220.30C4 applies. Try the caculator and give us the results.
Hello,.
Couple quick questions..
I have a 200A main panel outside with feed through lugs..
Can i feed a 100A sub panel from the feed though lugs? There is a 100A main breaker in the sub panel.
I also decided to split the load on this house..
I am feeding the 60A Heat from the main 200A panel with a 60 A. I felt that this would reduce the total load on the 100A panel and that would save me from removing the 100A panel that is already installed..I just refed the heat from the 100A to the 200A ..
I calculted the load on the sub panel just like I would for a main service right? Or would there be a different calc for the sub panell since it is no longer the main?
Here is the load I got for the 100A sub panel..
-----------------------
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
Disposal = 1000VA
Water Heater = 4800VA
Garage Door = 1000VA
Dish Washer = 1600VA
Attic Fan = 500VA
Microwave = 975VA
sum less 75% demand = 7406VA
--------------------------------------------------------------------------------
Clothes Dryer Circuit(s)Load
Total dryer circuit(s) load = 5000VA
--------------------------------------------------------------------------------
Cooking Appliance(s) Load
T220.19 Column A = 0kVA
T220.19 Column B = 0kVA
T220.19 Column C = 0kVA
Total calc.d range load = 0kVA
--------------------------------------------------------------------------------
Heating or Air-Conditioning Load
Air Conditioner = 4585VA
--------------------------------------------------------------------------------
Largest Motor Load
25% of largest motor = 1000VA
--------------------------------------------------------------------------------
Total Computed Load
22889 Volt-Amps
--------------------------------------------------------------------------------
Computed Amperage
22889 divided by 240 = 95Amps Neutral Loads
General Neutral Load = 4898VA
--------------------------------------------------------------------------------
Fastened In Place Appliances Neutral Load
Disposal = 1000VA
Garage Door = 1000VA
Dish Washer = 1600VA
Attic Fan = 500VA
Microwave = 975VA
Total before 75% demand(4 or more) = 5075VA
Total FIP Neutral Load = 3806VA
--------------------------------------------------------------------------------
Clothes Dryer Neutral Load
220.22 allows 70% of 5000VA
Clothes Dryer Neutral Load = 3500VA
--------------------------------------------------------------------------------
Cooking Appliances
220.22 allows 70% of 0VA
Cooking Appliances Neutral Load = 0VA
--------------------------------------------------------------------------------
Heating or Air Nuetral Load = 0VA
--------------------------------------------------------------------------------
Total Neutral Load = 13204VA
--------------------------------------------------------------------------------
Total Neutral Amperage = 55Amps
--------------------------------------------------------------------------------
--------------------------------------
Can anybody help?
Thanks
Bradley
Total Neutral Amperage = 50Amps
[This message has been edited by electcontrctr (edited 04-21-2005).]
I believe hanging a 100A subpanel off of a 200A feed through main panel would be a feeder tap. I think you can meet the rules of 240.21 (B)(2), so you can go 25 feet max with that feeder. Does the subpanel have a #4 copper feeder and a 100A main?
Hi,
The sub panel is 70 feet away and has a main breaker and is fed with 2-2-2-4 AL..there is a #4 cold water bond from the sub.
So I will need a 100A main from the main panel correct? I dont need a main breaker in the sub panel at all do I?
thank you for the help..
Brad
In my opinion, you need the 100-amp breaker in the main panel to protect the feeder, and the sub-panel can be a main-lug, as long as it's not in a separate building.
I would ask to see the 125% rule in writing. If your calc load is 165 A, the 125% would make the load 206 A which means a 400 amp service. The microwave needs to be included only if there is a dedicated cabinet space for the MW. Otherwise the kit small appl circuit will cover it. Use the optional calculation in the NEC. Where is the 125% of largest motor rule? I think that is industrial only.
If your calc load is over 90 A, why not put in s 200 amp service? How much more does it cost? $150, $200 ?