Emergency genrators

[1jottie]

The question I have is when we do new construction and design the electrical service the most common for us is a 400 amp service with 2- 200 amp usally main breaker loadcenters. Nowadays most of our client are putting in emergency back up generators. When sizing the size of the generator to do the entire home would you need to size the generator for the entire 400 amp service or would you just do load calculations for the home itself since it will never be drawing close to 400 amps. I not sure what the correct answer is. I know what works but I would like to know for sure the correct way.
Thanks
Jan

 

[stew]

put all your circuits you want to be run off the gen set in one panel. Then if you use a manual transfer system on the panel no load calc is required. It is then up to the user to decide which circuits to use up to the gen set capacity. If you are using an automatic transfer switch you will then have to do a load calc for the circuits to be run off the gen set and not to exceed the generators rating.

 

[iwire]

To go along with stews good suggestion, do not call this an 'emergency' generator. To the NEC it is an 'Optional Standby' generator. The rules in Article 700 Emergency Systems are much tougher than the rules in Article 702 for Optional standby systems.

As far as the loading here are some of the relevant sections from the 2011.

702.4 Capacity and Rating.

(A) Available Short-Circuit Current. Optional standby
system equipment shall be suitable for the maximum available
short-circuit current at its terminals.

(B) System Capacity. The calculations of load on the
standby source shall be made in accordance with Article
220 or by another approved method.

(1) Manual Transfer Equipment. Where manual transfer
equipment is used, an optional standby system shall have
adequate capacity and rating for the supply of all equipment
intended to be operated at one time. The user of the
optional standby system shall be permitted to select the
load connected to the system.

(2) Automatic Transfer Equipment. Where automatic
transfer equipment is used, an optional standby system
shall comply with (2)(a) or (2)(b).

(a) Full Load. The standby source shall be capable of
supplying the full load that is transferred by the automatic
transfer equipment.

(b) Load Management. Where a system is employed that
will automatically manage the connected load, the standby
source shall have a capacity sufficient to supply the maximum
load that will be connected by the load management system.

 

[stew]

And as per usual the code guru with the cool computer program comes up with all the relevant code sections !. That is why this forum is so cool.( i coulda done that too but am too lazy to look it up lol)

 

[topgone]

The question I have is when we do new construction and design the electrical service the most common for us is a 400 amp service with 2- 200 amp usally main breaker loadcenters. Nowadays most of our client are putting in emergency back up generators. When sizing the size of the generator to do the entire home would you need to size the generator for the entire 400 amp service or would you just do load calculations for the home itself since it will never be drawing close to 400 amps. I not sure what the correct answer is. I know what works but I would like to know for sure the correct way.
Thanks
Jan

If you want to know more, read the rules in NFPA 110.

IMO, it's the local AHJ who determines what class your installation is (time in hours that the emergency power supply system is to operate without being recharged or refueled.) and the level (1 -critical and essential to life safety; 2 - power to equipment that could create hazards, hamper rescue or firefighting operations and its failure would be less critical to human life and safety). Whatever equipment falling under the life safety category will have to be considered as prime power requirement. Basically, all designers would want to cover for all the electrical requirements but some owners are content with substantial compliance.

 

[iwire]

The OP said it was for a home, that pretty much rules out any emergency system.

 

[qcroanoke]

The OP said it was for a home, that pretty much rules out any emergency system.

Exactly! an optional standby system is OPTIONAL. You don't have to have it. It is not required by code for life safety.
I wonder how many homeowners have declined an optional standby system because the price quoted was too high from an EC that did not understand the difference.

 

[topgone]

The OP said it was for a home, that pretty much rules out any emergency system.

Like a "home for the aged" perhaps? Vague as it is, I just assumed seeing a 400A service, no simple home to me, or did I miss a lot?

 

[T.M.Haja Sahib]

Does the code reserve the emergency generators for specific occupancies?

 

[roger]

Does the code reserve the emergency generators for specific occupancies?

Read the scopes of articles 700, 701, and 702.


Roger