ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Increasing demand factors in residential
by gfretwell - 03/28/24 12:43 AM
Portable generator question
by Steve Miller - 03/19/24 08:50 PM
Do we need grounding?
by NORCAL - 03/19/24 05:11 PM
240V only in a home and NEC?
by dsk - 03/19/24 06:33 AM
Cordless Tools: The Obvious Question
by renosteinke - 03/14/24 08:05 PM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
1 members (Scott35), 266 guests, and 19 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#56371 09/20/05 07:12 PM
Joined: Aug 2005
Posts: 28
C
Member
could anybody give me some help how do i calculate the size of an generator?
let s say i have a big residence with an existing 400 a service?
How do i calculate the needed wattage and how do i get my amps calculation in regards of several 240v appliances and several 110v circuits.
Any idea somebody

#56372 09/20/05 07:25 PM
Joined: Apr 2002
Posts: 7,381
Likes: 7
Member
First step is are you installing a panel with only the circuits you want the gen to power?
If yes, determine the connected load of the circuits within the new panel.
If no, determine the actual 'load' on the 400 amp service.
You can discount some items, ie: the lesser of elec heat or hvac (if installed)
Beside the gen, don't forget the transfer switch.
The above is a 'rule of thumb'.
John


John
#56373 09/20/05 08:30 PM
Joined: Apr 2002
Posts: 914
E
Member
We sell and install a bunch of generators. The best way is to have the customer turn on every item they want to use (make sure the microwave is in use) and turn off everything else. Use an amp clamp to see the actual current on these circuits. Then increase this by 25% for inrush currents and items not drawing full normal loads. This is the minimum you want to use. It's been my experience that a 40KW-50KW unit will run most houses without disruption using 2 - 200 amp transfer switches. A 15KW model will run the basics using a sub panel and a 100 amp ATS. You can email me if you need more info or to purchase a Guardian Generator from us.

#56374 09/20/05 08:51 PM
Joined: Aug 2005
Posts: 28
C
Member
thnx so far
anybody has a math formula to figure out the genny size etc?



[This message has been edited by copperseller (edited 09-20-2005).]

#56375 09/20/05 08:59 PM
Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
Personal opinion... 100% of LRA on motors of any size, and 100% of starting amps of other loads, then connected load. Assume worst case scenario unless loads are managed. If doing the whole house, assume power will go out in peak usage times. Holidays, and heat waves....


Mark Heller
"Well - I oughta....." -Jackie Gleason
#56376 09/22/05 12:16 AM
Joined: Feb 2005
Posts: 693
L
Member
EE said: "A 15KW model will run the basics using a sub panel and a 100 amp ATS."

Keep in mind that the transfer switch must be rated for the full rating of the utility, regardless of the generator output.

If you have a 200-amp service, you must use a 200-amp transfer switch, even if the generator is only 5KW.


Larry Fine
Fine Electric Co.
fineelectricco.com
#56377 09/22/05 09:18 PM
Joined: Apr 2002
Posts: 914
E
Member
"If you have a 200-amp service, you must use a 200-amp transfer switch, even if the generator is only 5KW"

Larry, you would be right if you tried to use the ATS to serve the entire load of the house, but if you select about 8-12 circuits and the ATS transfers a 60 amp sub panel the ATS only needs to be rated at 60 amps or higher. You have 60 amps going to the ATS from the utility side, gen current going to the ATS, and the ATS decides which of these serves the sub panel.

#56378 09/22/05 10:16 PM
Joined: Apr 2002
Posts: 7,381
Likes: 7
Member
OK, now we're back to the original questions.....
Ya have to know what you want to do....before ya start doin' it! Right??

John


John
#56379 09/22/05 11:29 PM
Joined: Feb 2005
Posts: 693
L
Member
EE, of course that's correct. A generator panel was not mentioned above, though.

While I've installed plenty of them, both pre-made and home-brewed, my personal preference is a whole-house transfer. Plus, when installed as the service is being constructed, it's a lot easier to do.

In a large house (the basement alone is over 3Ksq.ft.) I'm finishing up, with two 200a panels, I simply fed one of the panels (and its sub-panel) through a 200a ATS, and fed the other one straight from the 320a meter.

I divided the loads between panels according to the desire to have generator power (and within reasonable balance).


Larry Fine
Fine Electric Co.
fineelectricco.com

Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5