Load Shedding: 1 Gen, 2 Loads

[bcm]

Folks,

Ever have a project that kept you awake at night? I could use some advice with a recent insomnia issue.

I have a client with 2 sets of already placed & active cell tower equipment - one set in a shelter, another set on a slab. They are each served individually with a 120/240V, 200A, single phase service ? I assume from the same transformer. The client wants to use their existing 50KW generator that keeps the slab equipment up to also run the shelter in case of power failure. Since the 50KW gen is only going to provide about 208 amps, load shedding is the issue. Usually cell clients put in a generator rated to handle their entire cell equipment load (200A, typically), but splitting the power up like this is new to me.

Once I get past the 2 new ATSes I?m sure I need, I can?t figure out what I need to do to make sure the generator only serves the client?s critical loads and doesn?t waste power on non-critical loads like shelter lighting, receptacles, etc. I?m familiar with load shedding as a concept, but not how it actually is applied. I think I need to add a panelboard somewhere for the generator?s output, but I?m not seeing how that would help.

Has anybody done something like this that can point me in the right direction on how to handle it?

Thanks so much and may you have a happy Monday!

 

[JoeStillman]

You may not be able to shed enough load.

Lights and receptacles in that type of installation will be, at most, 10% of the total. In my experience, the other 90% is equal parts telco equipment and air conditioning.

But if you want to try, then there are a couple of things you can do.

1) Re-route each service into two mains - one critical and one non critical. Feed an ATS with the critical main and a non-critical panel with the other. Move your non-critical circuits over to the non-critical panel.

2) Use two service-rated ATS's. After each ATS, you could split off to a critical panel and a non-critical panel. Put a contactor ahead of the non-critical panel to hold it off on emergency.

2) If there aren't many non-critical loads, you could just backfeed the panels from new SR ATS's and run those branch circuits through a set of contactors that switch them off when on emergency power.

3) Interlock the air conditioning controls so the systems take turns running on emergency. This could be tricky because the equipment is temperature-sensitive and the space can heat up in a few minutes.

 

[bcm]

Load Shedding: 1 Gen, 2 Loads

Thanks for those ideas!

It seems common sense that I can't just show a 50KW (208A) generator feeding (2) 200A loads without doing one of the things listed above, but I'm not finding a section in the Code that prohibits it. So far I've only found information on separately derived systems in relation to this. Can anybody point me at the section that says I can't show something that stupid on the drawings? The client doesn't know the load of either set of equipment and is too cheap at this point to pay for a load study. They just want me to show connecting the 50KW generator to both 200A loads. Article 445 doesn't seem to actually require that the generator be able to supply power for the total load. Maybe I'm not understanding it.

Thanks!

 

[GoldDigger]

Thanks for those ideas!

It seems common sense that I can't just show a 50KW (208A) generator feeding (2) 200A loads without doing one of the things listed above, but I'm not finding a section in the Code that prohibits it. So far I've only found information on separately derived systems in relation to this. Can anybody point me at the section that says I can't show something that stupid on the drawings? The client doesn't know the load of either set of equipment and is too cheap at this point to pay for a load study. They just want me to show connecting the 50KW generator to both 200A loads. Article 445 doesn't seem to actually require that the generator be able to supply power for the total load. Maybe I'm not understanding it.

Thanks!

The limitation is logical, in that the ATS cannot be allowed to connect the generator automatically to a load which exceeds its rating. The limit is attached to the rules for the ATS not the rules for the generator.
With a manual transfer switch it is possible to directly turn off enough load circuits before operating the transfer switch.

 

[bcm]

Yes, I thought MTS would solve the problem, but the client wants to have ATS, so it looks like I may be able to use Article 240(G) in conjunction with 445.12 & 445.13 to convince them the design I'm must show a way to keep the load small enough so as not to overload the generator they want to use. Is there anything else in the Code I may have overlooked?

 

[GoldDigger]

Yes, I thought MTS would solve the problem, but the client wants to have ATS, so it looks like I may be able to use Article 240(G) in conjunction with 445.12 & 445.13 to convince them the design I'm must show a way to keep the load small enough so as not to overload the generator they want to use. Is there anything else in the Code I may have overlooked?

Not necessarily entirely in the code, but there has been a lot of confusion around the question of whether the generator is operated as a separately derived system or not. This affects how the EGC and EGC to neutral bonding will be handled and whether the ATS would have to switch the neutral as well as the ungrounded leads. In either case, make sure that you run an EGC wire between the ATS and the generator.

 

[gadfly56]

First Things First

First Things First

Before you get all fancy trying to shed this and that, sit down and do a load calculation for the two services. A real one, not the rule-of-thumb sort of thing most folks do. If it turns out your A/C loads are 230 amps during the dog days of August, stop there and tell your client the bad news. Let him decide if he wants to risk frying one of the sites. And don't forget the surge factor when the A/C kicks on. Even if the generator can handle the full steady-state load for both sites, can it handle both A/C's kicking on at the same time? Murphy's Law says that is exactly what will happen.

 

[eHunter]

Yes, I thought MTS would solve the problem, but the client wants to have ATS, so it looks like I may be able to use Article 240(G) in conjunction with 445.12 & 445.13 to convince them the design I'm must show a way to keep the load small enough so as not to overload the generator they want to use. Is there anything else in the Code I may have overlooked?

Most cell companies have detailed engineering data on each of their tower huts and also have detailed utility bills that show what the monthly and demand usage is.
I have performed extensive services for cell companies, they are not to be trusted. With that said, don't get yourself in a legal bind by engineering a solution that possibly can cause tower downtime trying to save them money at their urging instead of installing a second or proper sized genset.
If there is even a remote question about the genset capacity, mitigate your liability by either upsizing the single genset or add a second unit to fully support the load.
If possible can you divuldge what state and county this installation is in?
That may answer why they are trying to avoid the upsizing or additional genset.

 

[GoldDigger]

Before you get all fancy trying to shed this and that, sit down and do a load calculation for the two services. A real one, not the rule-of-thumb sort of thing most folks do. If it turns out your A/C loads are 230 amps during the dog days of August, stop there and tell your client the bad news. Let him decide if he wants to risk frying one of the sites. And don't forget the surge factor when the A/C kicks on. Even if the generator can handle the full steady-state load for both sites, can it handle both A/C's kicking on at the same time? Murphy's Law says that is exactly what will happen.

Actually, Mr. M's law further provides that since the power was interrupted briefly, all of the A/Cs will be failing to start against head pressure and will just try to draw LRA until their overloads cut out. (But the generator would not last that long anyway.)
If the A/C loads turn out to be significant, the customer may want to invest in timers to delay the A/C startup and to stagger it among units.

 

[qcroanoke]

Actually, Mr. M's law further provides that since the power was interrupted briefly, all of the A/Cs will be failing to start against head pressure and will just try to draw LRA until their overloads cut out. (But the generator would not last that long anyway.)
If the A/C loads turn out to be significant, the customer may want to invest in timers to delay the A/C startup and to stagger it among units.

If they are newer wall mount units I would bet they have time delays for the compressors already installed. Should also be adjustable so they won't both try to start at the same time.
I would look at locking out one of the units on generator run too.

 

[bcm]

Load Shedding: 1 Gen, 2 Loads

I gave the client the bad news last week about needing load information or even better,using a larger gen. I didn't hear back from them until yesterday, so I wondered if they were reseaching, having 2nd thoughts, etc. They still want to continue down this path. I'm trying to get more load information from them. They so far have been completely unable to provide me with anything - not even an existing panel schedule for each of the 2 services! I'll keep you apprised of how this progresses. These people always seem to want me to be God - creating from nothing!

Thanks for all your input! It's given me a lot to work with and think about!

bcm

 

[bcm]

Most cell companies have detailed engineering data on each of their tower huts and also have detailed utility bills that show what the monthly and demand usage is.
I have performed extensive services for cell companies, they are not to be trusted. With that said, don't get yourself in a legal bind by engineering a solution that possibly can cause tower downtime trying to save them money at their urging instead of installing a second or proper sized genset.
If there is even a remote question about the genset capacity, mitigate your liability by either upsizing the single genset or add a second unit to fully support the load.
If possible can you divuldge what state and county this installation is in?
That may answer why they are trying to avoid the upsizing or additional genset.

The site is in Milton, Florida. In my experience, cell companies have almost NEVER been able give me even remotely detailed electrical data on their equipment. Then they're unhappy with my solutions based on 200A. I agree they're not to be trusted. I'm guessing that since sites are always have a 200A service when built, they have 50KW gens in a warehouse somewhere and don't want to have to buy a bigger one for this special situation. Either that or they don't want to pay for more lease area as I suspect a gen double in rating would be larger in size too.

 

[ptonsparky]

I gave the client the bad news last week about needing load information or even better,using a larger gen. I didn't hear back from them until yesterday, so I wondered if they were reseaching, having 2nd thoughts, etc. They still want to continue down this path. I'm trying to get more load information from them. They so far have been completely unable to provide me with anything - not even an existing panel schedule for each of the 2 services! I'll keep you apprised of how this progresses. These people always seem to want me to be God - creating from nothing!

Thanks for all your input! It's given me a lot to work with and think about!

bcm

and they usually start with burnt offerings

 

[gadfly56]

and they usually start with burnt offerings

So that's what I smell when I open up the RFQ! :lol:

 

[ActionDave]

The site is in Milton, Florida. In my experience, cell companies have almost NEVER been able give me even remotely detailed electrical data on their equipment. Then they're unhappy with my solutions based on 200A. I agree they're not to be trusted. I'm guessing that since sites are always have a 200A service when built, they have 50KW gens in a warehouse somewhere and don't want to have to buy a bigger one for this special situation. Either that or they don't want to pay for more lease area as I suspect a gen double in rating would be larger in size too.

Call me crazy but I'm not so sure that one 50kw generator would not be enough to handle both sites. One is outside so only the one in the shelter needs A/C. Depending on the size of the rectifiers they may not even draw 50amps. The rest of loads all run off the batteries.

The cell company may not have detailed load data, but they do have power bills.