stand-by generator sizing as per 702.5

[Billy Nobles]

i recently submitted drawings to coral springs Florida to install a 30kw stand-by generator in a residential application with a 200 amp automatic transfer switch.
it was rejected by plan review stating that the generator kW rating was not enough to handle the existing load of 178 amps that was taken from the original load calculations that the city had on record.
we all know that load diversity is such that it is very unlikely that a generator with an output of 125 amps would ever be overloaded in a residential application as this. the generator has a built-in 125 amp main breaker that would trip if this event ever happened.
my understanding is that code article 702.5 takes this into consideration and allows us to size a generator based upon what loads will be operated at one time and allows the user to select the loads connected to the system and do load shedding if necessary. please comment!!

 

[charlie b]

Re: stand-by generator sizing as per 702.5

I am inclined to agree with the city. That opinion is based more on design considerations, and not so much as an attempt to strictly interpret the code. But to address your specific question, I see two key words in 702.5: "intended " and "connected."

You could do your own load calculation, and probably come up with the same 178 amps that the city has on record. But how much of that 178 amps is "intended" to be operated on the generator? The word "intended" carries with it the notion of deliberate choice. The problem is that no such choice has been made. Every single load in the house will be "connected" to the generator, so the clear intent is to operate any and all loads at the same time. There are no provisions for load shedding; you are relying on the owner to notice that the generator is close to being overloaded, and to shed some load before starting some other load. Not likely to happen, especially if the generator's load meter is outside, not right next to the electric stove or dryer or whatever load the owner is about to start.

You are also relying on the generator breaker to serve as the one any only method of preventing overload. That is not what I would call a "good engineering practice." We should never rely on a protective action to terminate an event, if we can take steps to prevent the event from occurring in the first place.

There are several ways of resolving this issue. The best, and regrettably most expensive, is to create a sub-panel that will carry all of the loads that you want to have running on the generator. You feed that panel, and not the main panel, from the generator via the ATS. Another solution is to install an automatic load shedding scheme. Take an aux contact from the generator breaker, and trip off the HVAC unit (or whatever else is needed to bring the total load below 125 amps). You might also be able to sell the AHJ on manual load shedding if you provided an easy way for the owner to check the load before turning something else on. A remote display of the generator's load might be enough.

 

[kevinware]

Re: stand-by generator sizing as per 702.5

I am assuming the intended use of this Gen. set will be a whole house unit?? If that is the case then looking at 702.5 "Capacity and Rating"
An optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. Optional standby equipment shall be suitable for the maximum available fault current at its terminals. The user of the optional standby system shall be permitted to select the load connected to the system." Just an apprentice's opinion but if the intended use is a whole house unit then the standby gen. set should be sized to handle the service that is calculated for the whole house.....But lets see what others with more experience have to say....

 

[davedottcom]

Re: stand-by generator sizing as per 702.5



I HATE Grey Areas!

Dave

 

[iwire]

Re: stand-by generator sizing as per 702.5

I strongly disagree with the city.

I am of the opinion the NEC is not a design specification and that an optional generator can be connected to any amount of load.

I think this is one of the clearest code sections we talk about.

702.5 Capacity and Rating.
An optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. Optional standby system equipment shall be suitable for the maximum available fault current at its terminals. The user of the optional standby system shall be permitted to select the load connected to the system.

Charlie

You are also relying on the generator breaker to serve as the one any only method of preventing overload. That is not what I would call a "good engineering practice."

The same can be said of any multi outlet branch circuit.

 

[davedottcom]

Re: stand-by generator sizing as per 702.5

Billy, I understand and agree 100% with the city's opinion but unless they have a local ordinance they are incorrectly siting a code violation. I wish it was a violation, for many reasons I've discussed here before, but it's not. Now that I think of it, Bob (Iwire) was one of the ones who made me realize this before! It wasn't easy for me to swallow then and I still don't like it, but the code clearly says "Intended" which leaves the door wide open for the owner to "intend" on using any combination of loads not exceeding the 125 amps available.
The fact that different "combinations" of loads are possible is where I think the problem arrises from. The owner may want to run the HVAC system for a while, then decide to shut it off and use the range to cook thanksgiving dinner for a few hours!
Just as the home owner is "Trusted" to not max out the utility power source by maxing out every circuit in his panel at the same time, he is trusted to regulate the power consumption during back up power mode as well, be it a more limited source, but still the same in theory.

Poor design: I think so
Code Violation: Nope


Dave

 

[suemarkp]

Re: stand-by generator sizing as per 702.5

What's the safety issue in overloading a generator? Isn't it self limiting, and you'll just get a severe voltage drop if you load it up too far? Will it burn up if overloaded?

 

[davedottcom]

Re: stand-by generator sizing as per 702.5

Suemarkp, I don't know the answer to your question but my concerns are based on the main breaker tripping on the generator causing another complete power outage. These systems are automatic, and one who purchases them assumes the "back up" power will be working in a utility outage.
BUT, this is not always the case...
Let's say Joe Shmo has an auto Gen installed. Loaded up WAY over it's max output. Joe leaves for vacation thinking his pets are in a climate controlled invironment and his 3 refrigerators and freezers are well protected in the event of an outage. Well, while he's on vacation the utility power goes out. His Gen starts, it's immediately overloaded because the Heat is on & no one is there to regulate the loads, the Gen. Main Breaker trips, no more power... although the Generator continues to run...and run... and run. Turns out the power is out for days because of some ice storm downing some power lines. Joe's on the beach in FL thinking, "good thing I have the generator keeping the house heated". But when Joe gets home he finds the generator running but no power on, no heat, pipes frozen & broken, bad food in refridges...pets DEAD!

It's not an electrical safety issue it's a design issue that I feel the NEC should address.

Dave

 

[charlie b]

Re: stand-by generator sizing as per 702.5

Originally posted by davedottcom:I HATE Grey Areas!

Engineers don't. We like to analyze the grey, to understand why it is grey, and to determine the limits of the grey.

I will concede the point made by Bob and Dave: Not a code violation. But then I never said it was. Please note the equivocal tone of my first post:

Originally posted by charlie b: I am inclined to agree with the city. That opinion is based more on design considerations, and not so much as an attempt to strictly interpret the code.

It's not what I would have in my own home, and it's not what I would design for someone else's home. But the city should not be using the NEC as the method of stopping it from happening.

 

[kevinware]

Re: stand-by generator sizing as per 702.5

Charlie b,

Would you have sized the standby system based on the calculated service size? (Whole House Unit)I am just trying to be clear about 702.5

[ October 05, 2005, 12:18 PM: Message edited by: kevinware ]

 

[charlie b]

Re: stand-by generator sizing as per 702.5

It's not my house, so I would let the owner tell me what he intends to have running on the generator. Then I would pick the next higher standard size generator, and create a design that permits the generator to power those loads, and no others.

 

[Billy Nobles]

Re: stand-by generator sizing as per 702.5

one could think of a 100 possible senerios that could happen to overload an automatic system.
it is very unlikely that an overload senerio would happen while no one is home due to the fact that the majority of the loads in a residential system are not automatic (cooking equipment, laundry equipment,dishwasher,garbage disposal,lighting loads,etc.) but i see your point to some degree.
it would more than likely happen while the residence was occupied.
i did some load tests on this house before i decided to use a 30kw unit.
with all the appliances running and every light in the house on, the load never went over 120 amps.im talking every thing too! (all four cooktop burners,oven,microwave,dishwasher,garbage disposal,water heater,a/c,washer,dryer,sprinkler pump,pool pump,garage door opener,TV's,ceiling fans,every lamp,every recessed light,every thing!!!
So I'm 100% sure a 30kw unit is more than sufficient to handle the whole load of the house.
even with people in the house no one ever uses everything at the same time as i did while conducting the load test.
i think the writers of article 702.5 understands that an optional stand-by system is not designed to replace normal power, only provide the user with the option to turn on what every loads they see fit in a power outage situation.
if no one were home the generator could not be overloaded

 

[charlie b]

Re: stand-by generator sizing as per 702.5

Your original post used language along the lines of "it is unlikely that." But you have actual test data that changes the "unlikely" to "not possible." Did you give that information to the city reviewers? It would certainly convince me.

 

[davedottcom]

Re: stand-by generator sizing as per 702.5

Billy, I agree with you. It is very unlikely you would overload your gen. My extreme scenerio was actually refering to the smaller 15 KW situations I see everyday where it is VERY likely to happen.
Both situations use 702.5. I was just saying (In my opinion!)it shouldn't be allowed but it is.

You might want to run another test with the HEAT running instead of the AC! You might be surprised at the increased load.

Dave

Editted: What type of Heat does it have?

[ October 05, 2005, 02:41 PM: Message edited by: davedottcom ]

 

[iwire]

Re: stand-by generator sizing as per 702.5

Dave I can see no safety issue with the failure of an automatic generator.

We are talking optional standby.

If a homeowner choses to rely only on an automatic generator to keep their food or pets safe they have made a poor choice.

A 100 things can happen to make that generator not start at all, start and give out or perhaps start and shutdown from overload.

What happened to personal responsibility?

 

[al hildenbrand]

Re: stand-by generator sizing as per 702.5

Charlie,

create a design that permits the generator to power those loads, and no others.

Isn't that still an overstatement of 702.5?

I mean, I know you are talking for yourself, not specifically interpreting 702. Right?

Here, let me turn the scenario this way. I have a single family dwelling that has: </font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Electric range</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Microwave</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Forced Air Gas Furnace</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">One Window AC</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Refrigerator</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Feezer</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Garage Door Opener</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Garage Work Area & Gas Fired Unit Heater, Air Compressor, Security Lighting</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Washer & Gas Dryer</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Basement Workbench (grinder, drill press, table saw, other small power tools)</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Structured Media Center (ISP, Cable, Phone)</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">A Home Office (Computer, Multi Function Printer, Answering Machine)</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">A Family Entertainment Center</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">The General Lighting Load</font>

<font size="2" face="Verdana, Helvetica, sans-serif">A 5.5 Kw portable generator plugs into and exterior inlet that goes to the subpanel that has a manual transfer switch.

The Elec. Range & AC are not supplied through the optional standby subpanel. The homeowner states that the base load is the freezer & the refir & the furnace & the ISP, computer & the office answering machine. This works out to 2.2 Kw. The HO says he will only use enough of the remaining power to not overload the generator.

Its not what "can be run", but what is "intended to be operated at one time." And: "The user of the optional standby system shall be permitted to select the load connected to the system. The circuits in the subpanel connect to much more than can be possibly be supplied by the generator, but that is not the point. The User gets to choose.

 

[davedottcom]

Re: stand-by generator sizing as per 702.5

Bob, I don't see a safety issue either. I just wish there was a demand load calc for them.
That would at least limit the power failures to mechanical failures not over loads. I think a design that allows an over load situation so easily is cheesy to say the least. We've had this discussion before and we didn't agree then either!

You haven't changed my mind!

Just kidding of course.
Respectfully,

Dave

 

[davedottcom]

Re: stand-by generator sizing as per 702.5

Al, A Manual Transfer switch FORCES the user to choose loads. If they choose all the loads at once, well then it's their own dumb fault! :roll: But an Automatic system doesn't require any load shedding action... well not until it trips the main on the Gen!

Dave

 

[iwire]

Re: stand-by generator sizing as per 702.5

Originally posted by davedottcom:
I fully understand & respect your opinion... even though it is Cheesy!

Mmmmm cheese.


Cheesy is not within the scope of the NEC.

Cheesy and not cheesy separates the good ECs from the great ECs.

 

[davedottcom]

Re: stand-by generator sizing as per 702.5

Originally posted by iwire:
Cheesy and not cheesy separates the good ECs from the great ECs.

Right on brother...I'm pickin up what your puttin down!