Can anyone help clear up the intent of words used in 702.5? Does the article mean that the optional system supply (gen set) must be large enough to supply EVERYTHING in the panel which it feeds or does this mean that the owner(homeowner, storeowner,etc.) of the installed system can turn loads on and off in order to prevent an overload to the generator feed. I've seen several system panels being fed by a generator not capable of powering all of the loads simultaneously in the panel which it feeds but will do an excellent job of powering select loads, as needed. The confusion is over the word "select". Selected when?... at the installation or during the operation of? Optional systems are being installed everywhere nowdays and I'm a little confused about what that article actually means. Thank you.
Article 702.5 Optional Standby
- Thread starter patc
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ron
Senior Member
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- New York, 40.7514,-73.9925
Re: Article 702.5 Optional Standby
Is this a manual start system or autostart auto transfer?
There is some logic involved.
Is this a manual start system or autostart auto transfer?
There is some logic involved.
Re: Article 702.5 Optional Standby
I'm not sure what version of the NEC you are using, but the 2002 states the "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" This sounds to me like you can size it to run only some of the loads on a panel.
Of course, if the transfer switch is automatic, there must be some way to keep it from automatically connecting the entire load on the panel. I think that's probably the "logic" Ron refered to.
Steve
I'm not sure what version of the NEC you are using, but the 2002 states the "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" This sounds to me like you can size it to run only some of the loads on a panel.
Of course, if the transfer switch is automatic, there must be some way to keep it from automatically connecting the entire load on the panel. I think that's probably the "logic" Ron refered to.
Steve
sceepe
Senior Member
Re: Article 702.5 Optional Standby
Would it really matter what type of xfer switch you have? You still need to have protection for the conductors between the generator and the ATS such that they will not overheat if 100% of the load is transferred by the ATS or if the owner keeps turning on breakers in the emergency panel on by one.
I suppose as a matter of convenience, it would be nice not to trip the gen set's main line breaker as soon as the ATS transfers to the generator. But if that is your concern the best solution is to go full size on the gen set.
Also, expecting an owner to carefully monitor his load is asking the impossible. I imagine a guy wanting to add just one more breaker. Everything is fine until his wife fires up the hair dryer and then pop, its all dark and quite again. Sounds like a major hassle to me. The guy will probably be looking for a bigger gen set in no time (or a new wife).
Would it really matter what type of xfer switch you have? You still need to have protection for the conductors between the generator and the ATS such that they will not overheat if 100% of the load is transferred by the ATS or if the owner keeps turning on breakers in the emergency panel on by one.
I suppose as a matter of convenience, it would be nice not to trip the gen set's main line breaker as soon as the ATS transfers to the generator. But if that is your concern the best solution is to go full size on the gen set.
Also, expecting an owner to carefully monitor his load is asking the impossible. I imagine a guy wanting to add just one more breaker. Everything is fine until his wife fires up the hair dryer and then pop, its all dark and quite again. Sounds like a major hassle to me. The guy will probably be looking for a bigger gen set in no time (or a new wife).
Re: Article 702.5 Optional Standby
Sounds to me that everyone else may be as uncertain of the actual intent of the language as I am. I understand that with an automatic xfer the possiblity would be greater to overload a genset which may be too small for the entire load of the panel but more importantly,what is the code actually saying? There may be situations which you may have an auto xfer and there may be times when you have a manual xfer, but the bottom line is does the code allow for a situation(because its an optional system) for the owner to turn loads off and on in order to not overload the genset. I'm not making an argument either way, it just appears that this truely could be interpreted either way. Common sense would tell you not to allow a xfer to occur which could O/L the circuit and render the sytem useless but can we select what is on and what is off after the fact? As a follow up to this, does anyone know what the process is to get a formal interpretation of this, just in case we can't get a clean answer? Thanks again.
Sounds to me that everyone else may be as uncertain of the actual intent of the language as I am. I understand that with an automatic xfer the possiblity would be greater to overload a genset which may be too small for the entire load of the panel but more importantly,what is the code actually saying? There may be situations which you may have an auto xfer and there may be times when you have a manual xfer, but the bottom line is does the code allow for a situation(because its an optional system) for the owner to turn loads off and on in order to not overload the genset. I'm not making an argument either way, it just appears that this truely could be interpreted either way. Common sense would tell you not to allow a xfer to occur which could O/L the circuit and render the sytem useless but can we select what is on and what is off after the fact? As a follow up to this, does anyone know what the process is to get a formal interpretation of this, just in case we can't get a clean answer? Thanks again.
Re: Article 702.5 Optional Standby
A generator has the inherent ability to be current limiting. The voltage will start sagging when the current exceeds the maximum rating.
Lights will dim, motor starters will drop out. Motors with no starters will smoke or hopefully the overheat switch shuts it down.
A generator can withstand a 20% voltage drop before the undervoltage relay dumps the exciter.
A generator has the inherent ability to be current limiting. The voltage will start sagging when the current exceeds the maximum rating.
Lights will dim, motor starters will drop out. Motors with no starters will smoke or hopefully the overheat switch shuts it down.
A generator can withstand a 20% voltage drop before the undervoltage relay dumps the exciter.
Re: Article 702.5 Optional Standby
Bennie, I agree that this could be a design issue but the design must be based on the intent of the article. If we can't define the intent, can we logically make a decision as to whether a design is adequate or not? There are way too many of these systems out there for this to be as vaguely worded as it is. Thank you.
Bennie, I agree that this could be a design issue but the design must be based on the intent of the article. If we can't define the intent, can we logically make a decision as to whether a design is adequate or not? There are way too many of these systems out there for this to be as vaguely worded as it is. Thank you.
Re: Article 702.5 Optional Standby
patc:
The language in the code seems clear enough to me. Size the generator for the loads you want to run. Ignore the rest. Do you have a different version (99 perhaps) that reads different than my qoute above?
Steve
patc:
The language in the code seems clear enough to me. Size the generator for the loads you want to run. Ignore the rest. Do you have a different version (99 perhaps) that reads different than my qoute above?
Steve
hforney500
Member
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- New Jersey
Re: Article 702.5 Optional Standby
700.5 Capacity.
(A) Capacity and Rating. An emergency system shall have adequate capacity and rating for all loads to be operated simultaneously. The emergency system equipment shall be suitable for the maximum available fault current at its terminals.
Maybe I am missing something here. There is no mention of an electrical panel in this code section. Loads to be connected to the Emergency Power Supply System are selected and the genset is sized based on the load. The loads can come from different electrical panels. A transfer switch would be used for each selected load, or the most common practice would be to install an emergency panel and rerout all circuits to the new emergengy panel thus only having to use one transfer switch.
700.5 Capacity.
(A) Capacity and Rating. An emergency system shall have adequate capacity and rating for all loads to be operated simultaneously. The emergency system equipment shall be suitable for the maximum available fault current at its terminals.
Maybe I am missing something here. There is no mention of an electrical panel in this code section. Loads to be connected to the Emergency Power Supply System are selected and the genset is sized based on the load. The loads can come from different electrical panels. A transfer switch would be used for each selected load, or the most common practice would be to install an emergency panel and rerout all circuits to the new emergengy panel thus only having to use one transfer switch.
Re: Article 702.5 Optional Standby
Remember that my post is for Art. 702, not 700. Art. 700 states under its "scope" that Emergency Systems are for Legally Required systems. I am not asking about a legally required system. My post is about Optional systems. 702 says that optional stanby system shall have adequate capacity for all of the equipment intended to be used at one time and that the user shall be able to select the load connected to the system. If the owner does not intend to use all if the load at the same time can he "select" to turn some off? If not, many of the installations for OPTIONAL standby systems(generators) installed at homes are too small for the load in the panel that they supply. Once again I'm not making an argument either way but it still sounds like the verbage used in 702.5 could be a little more clear. Is there any other reference that could clear this up for me.Thanks
Remember that my post is for Art. 702, not 700. Art. 700 states under its "scope" that Emergency Systems are for Legally Required systems. I am not asking about a legally required system. My post is about Optional systems. 702 says that optional stanby system shall have adequate capacity for all of the equipment intended to be used at one time and that the user shall be able to select the load connected to the system. If the owner does not intend to use all if the load at the same time can he "select" to turn some off? If not, many of the installations for OPTIONAL standby systems(generators) installed at homes are too small for the load in the panel that they supply. Once again I'm not making an argument either way but it still sounds like the verbage used in 702.5 could be a little more clear. Is there any other reference that could clear this up for me.Thanks
Re: Article 702.5 Optional Standby
patc: The NEC is not intended to be a design manual.
Your answer to your dilemma is straightforward engineering fundamentals.
Count the beans and size the jar to fit.
Reverse engineering; Size the jar, then determine how many beans will fit.
patc: The NEC is not intended to be a design manual.
Your answer to your dilemma is straightforward engineering fundamentals.
Count the beans and size the jar to fit.
Reverse engineering; Size the jar, then determine how many beans will fit.
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Re: Article 702.5 Optional Standby
Patc:
this language is 702 was added a few code cycles ago. It allows the user to control the loads on the generator to prevent overloading. The intent was to allow load shedding to prevent overloading of the generator. You can have a 10 kw genset with 50 kw of load, the generator is protected from overload by its output circuit breaker. Since its an optional system, its up to the user to calculate and control the connected load, which is what bennie and the others are saying.
Patc:
this language is 702 was added a few code cycles ago. It allows the user to control the loads on the generator to prevent overloading. The intent was to allow load shedding to prevent overloading of the generator. You can have a 10 kw genset with 50 kw of load, the generator is protected from overload by its output circuit breaker. Since its an optional system, its up to the user to calculate and control the connected load, which is what bennie and the others are saying.
Re: Article 702.5 Optional Standby
IMO since this question is refering to something that is not required but optional then the loads to be connected are also optional and selectable. Now as far a homeowner being able select these loads this is where you must play teacher, salesman and electrician all at once. Whether you feel this is part of it or not, think again . Because in the scenario given a couple of posts ago about the hair dryer, it won't be the wife or the generator at fault it will be you.(as far as they are concerned)
Now to answer your question the generator can supply any load as far as code is concerned, as long as it does not become overloaded under normal conditions. If this is a home owner set up when you install the generator panel limit what is connected to only what the gen set can supply. If this is a comercial or industrial application consult someone in your area with lots of experiance in generators and transfer switches.
IMO since this question is refering to something that is not required but optional then the loads to be connected are also optional and selectable. Now as far a homeowner being able select these loads this is where you must play teacher, salesman and electrician all at once. Whether you feel this is part of it or not, think again . Because in the scenario given a couple of posts ago about the hair dryer, it won't be the wife or the generator at fault it will be you.(as far as they are concerned)
Now to answer your question the generator can supply any load as far as code is concerned, as long as it does not become overloaded under normal conditions. If this is a home owner set up when you install the generator panel limit what is connected to only what the gen set can supply. If this is a comercial or industrial application consult someone in your area with lots of experiance in generators and transfer switches.
Re: Article 702.5 Optional Standby
Thank you Tom, while I really wasn't making an argument either way, I feel that you best understand the question and have provided an answer that I agree with. I provded the initial post because I have had some discussion with others who feel that if there is a total load of 10k in the optional panel, the generator must be able to produce enough power to supply that load. But then we looked at the words a little more closely and found that it may be the end user's resposibility to assure that the system load was applied at an amount ("select"..turn stuff off or on)which would not overload the system...the optional system. Maybe we can put a few more beans in the jar than it will hold...just not at the same time. Thanks all
[ March 12, 2004, 01:17 PM: Message edited by: patc ]
Thank you Tom, while I really wasn't making an argument either way, I feel that you best understand the question and have provided an answer that I agree with. I provded the initial post because I have had some discussion with others who feel that if there is a total load of 10k in the optional panel, the generator must be able to produce enough power to supply that load. But then we looked at the words a little more closely and found that it may be the end user's resposibility to assure that the system load was applied at an amount ("select"..turn stuff off or on)which would not overload the system...the optional system. Maybe we can put a few more beans in the jar than it will hold...just not at the same time. Thanks all
[ March 12, 2004, 01:17 PM: Message edited by: patc ]
Re: Article 702.5 Optional Standby
Patc, Bennie is right on target, cynical, but on target. There are a few design approaches you can take and it all revolves around budget verses convenience.
The most economical approach is to install a small generator and manual transfer switch to only carry critical loads. You would have to instruct the owner, that when power is lost to turn everything off , then start and transfer generator. After Transfer then only turn on critical loads. If the owner goes over the limit, then the generator will trip off line, not burn up wiring.
Other approaches can be taken at much greater expense like a full sized generator and ATS, or dividing up the critical loads from non-critical to separate panels and use an ATS with proper size generator on the critical panel.
It?s all design issues, not a code issue.
Patc, Bennie is right on target, cynical, but on target. There are a few design approaches you can take and it all revolves around budget verses convenience.
The most economical approach is to install a small generator and manual transfer switch to only carry critical loads. You would have to instruct the owner, that when power is lost to turn everything off , then start and transfer generator. After Transfer then only turn on critical loads. If the owner goes over the limit, then the generator will trip off line, not burn up wiring.
Other approaches can be taken at much greater expense like a full sized generator and ATS, or dividing up the critical loads from non-critical to separate panels and use an ATS with proper size generator on the critical panel.
It?s all design issues, not a code issue.
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