What sized breaker to use on my 6/1 - ST5 combo

[trigzy]

Hi,
     I'm with Doug on this one, read the code book, follow it.  People spent years on it, it works well, and covers almost every situation you can name.   The one point I will add, and it's been partially covered before:

Your fuse/breaker is USELESS unless the engine & drive system attached to your alternator have the power to actually open the fuse/breaker.

Lets say you have a 5HP Diesel driving a 50kW head.  Say you ingnore me, and size the breaker for the 50kW head.  You overload the generator, and depending on the exitation type, the engine either stalls (breaker has done nothing), or slows down to a point where the field partially collapses, Volts & Hz sink, A increases, but maybe (probably) not high enough to trip the breaker (breaker has (probably) done nothing once again).  In the latter case, if the breaker does not trip your engine lugs at wide open throttle, low RPM, until you or something (over temp sensor??) stops it.  Not a good scenario.   

Use the code book to get the breaker size, but size it to the GEN HEAD and ENGINE!  The code book assumes that generators are commerically built units where one will always find the engine is rated for more kW than the gen head.  Remember that most listeroid installations are the reverse, ie. Genhead has a higher kW rating than the engine powering it, so the code book can be misleading in this case.

Steve

[Jim Mc]

Once you get above about 80% the rating of a biscut breaker your into the range where they start to get a little warm and possibly trip ....

Yeah,  They might trip.  But the problem is that according to the trip curves, shown here:

http://ecatalog.squared.com/pubs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/0730CT9801R108.pdf

a 40A Square D QO breaker isn't guaranteed to trip until you hit 125% of its rating.  And it looks like it could pass 125% indefinitely.  Can the ST head do that???

That's why motors are usually protected with a thermal overload relay.  Generators look a lot like motors from this standpoint.

I'd suggest something like this:

http://www.factorymation.com/s.nl/sc.2/category.64/.f

If I use the 40 amp breaker will that be good enough to keep from damaging the gen head

Remember the OP asked about protecting the ST head, not his house wiring...

[LowGear]

Is there some reason that no one until now has mentioned "permit".  Might I suggest a couple of guidelines.

     1).  If the project is more than $500 then build it to code.
     2).  If your building to code then get a permit so your insurance will cover it, the damage it causes and the property owner won't be criminally liable for IT.

Casey

[Stan]

Now you're being a real killjoy Casey!     I propose we all just give up and join the cardboard condo bunch and put all our faith in the "system" and then who needs generators or stuff like that.

Stan

http://jameshowardkunstler.typepad.com/

[Doug]

Boy this has degenerated into something confusing, things we should try to avoid.

I still can't fidn my code book and I don;t think I am going to bother looking anymore.

The important things to get from all this has however been said.

How and when a breaker trips is dependent on the rating and as Jim eluded too there is an inverse time delay curve for all protection devices. The more above the setting the faster it will trip, and the lower down the chart we get into the gray area where it may or may not trip.....

Code book is full of correct answeres.

Permits where required.

Correct sizing of protection devices for cowardination  protectionand control (  Well put trigzy ).

Last there is the isue of line loss and wire size for long runs and there are tables that will tell you exactly how to deal with that.

Doug
 

[reno-speedster]

OK back to reality


Its out on my ranch, its powering outbuildings. 

Its a stand alone system, not hooked to the grid.

Each of the buildings will have a separate / individual breaker sized for the load/ wire size / and run length.


I am asking about the size of a master breaker to protect the motor.  30 or 40 amp?

[Tom]

If you want to prevent "brown outs" go with the 30. With the 40 you will be able to bog down the engine and will have low voltage issues under a heavy load.

[LowGear]

I'm hurt and dismayed.  Ole Sour Dough Stan calls me a wimp and Doug has a code book but can't be bothered to give us The Truth.  But I've burned out on a couple of threads myself so I understand.  

My sugestions here have been basic.  When working with electromotive forces I think it gets a lot more complicated.  I went to a tax seminar with a neighbor once that was all about how taxes in the US were a voluntary program.  He just got sentenced to 5 years in a Federal prison 3000 miles from home.  He's 65.  It could be a life sentence.  So "Code Kicks Ass" here at Camp Aloha.

The Answer to your question is:

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When the 30 Amp pisses you off because it "pops" too often then put the 40 in.

Casey

[Jim Mc]

When the 30 Amp pisses you off because it "pops" too often then put the 40 in.

well said.

Or, start with the 40.  If smokes starts to roll out of the ST head, replace it with a 30   

[sailawayrb]

If you want to prevent "brown outs" go with the 30. With the 40 you will be able to bog down the engine and will have low voltage issues under a heavy load.

I dunno if I would depend on a CB to protect against "brown outs".  I suspect your sensitive electronics would start smoking before the CB would trip.  If you want to protect against brown outs, I'd recommend building a low/high voltage monitor/relay that you put in between your generator and your transfer switch.  That way, if voltage goes low for any reason, the relay will trip and disconnect the generator from transfer switch.  I tried to post photo of the low/high voltage monitor/relay I built but our new photo gallery must be broken because it would not upload my 1.4MB file.

I use a 30A CB in the low/high voltage monitor/relay and a 40A CB at the ST5 (wired 120 only).  My setup allows by-passing the low/high voltage monitor/relay so to get 40A to transfer switch from ST5 when necessary.  My transfer swirch is rated for 50A as I also have portable gas genset that can provide 7KW continuous and 12K surge (wired 120/240).  All my connection cables are rated for 50A.

[LowGear]

Ah Yes my brother Mc.  Sometimes fun escapes in the face of logic.  So there you have the two best answers Mr. Reno.  30 or 40.

LowGear

[sailawayrb]

Any chance of getting that relay schematic emailed if you can't get it posted ?
Jens

Sure thing Jen.  The component hookup that actually does the mission is very simple although you will need to work with some heavy gauge wire that can make fitting everything into the panel box challenging.  I also added voltage and frequency meters on mine just for the heck of it (you really don't need these meters if you have Kilo-Watt meter). 

Mine also only has to handle one leg of 120 as I wired my ST5 for 120.  If you wired your generator for 240, you will need twice the components that are in mine.  If you are 240, let me know and I'll work something up for you.

Anyhow, I will sketch this out by hand, scan it to PDF, and hopefully be able to post it and pictures of what my panel looks like on inside/outside in next few days...assuming the upload problem has been resolved.  Failing that, I will just email all this to you. 

Bob B.

[sailawayrb]

I am using 240V. I have a contactor ready to go for the job but have not got the sensing circuitry planned yet. I am primarily concerned with under voltage and my initial plan was to just use a voltage sensing relay. The initial plans did not call for over voltage sensing.
Looking forward to see how you have implemented the sensing .....

Jens

Until I get the schematic/photos posted...

I use a Magnercraft 831VS-120A for voltage sensor relay.  You can set the upper and lower voltage thresholds, as well as adjustable time delay.  It can purchased here for about $44:

http://www.bb-elec.com/product_family.asp?FamilyId=225

I use a Magnercraft W6240ASX-1 solid state relay (in lieu of mechanical contactor) to do the actual 120VAC drop in and pull out.  Mine is rated at 40A and adequate for my 6/1 ST5 120 wired setup.  It can be purchased here for about $25:

http://www.bb-elec.com/product_family.asp?FamilyId=224

The 831VS-120A voltage sensor relay I am using has two connectors that are used to sense the voltage, and also has two other connectors that can be used to open or close a circuit (triggered by exceeding the upper or lower voltage thresholds) that can be used to control your high current relay or contactor.  Jens, for your 240 project you would probably want to get the 831VS-240A voltage sensor relay.  I say probably because I could not find the spec sheet on bb-elec website to actualy see how it works.  However, I would think you would either want to sense 240 (i.e., using two Hot wires) or both 120 legs (using each Hot and Neutral wires).

[mobile_bob]

this code business caused me some concern as well, because i am planning an offgrid home
and would like to have insurance,, realizing of course insurance companies look for anything they can
to skirt paying off in case of a loss especially if they can prove the system is not up to code, installed by a licensed
installer, UL or CSA listed (depends which side of the boarder you are on) components, and all that.

what i found was the code is much more lax when it comes to buildings where humans do not dwell, so

i figure to put all the power generation/storage/conversion equipment in a steel container or concrete bunker
well away from the house, and bury the power feed line to the house, and bring it into a standard house metered entrance
and breaker panel,, then wire the house to code, with permits and signed off inspections on the house.

i figure that way if the house burns down the insurance company has a bigger challenge proving an issue
and if the bunker goes up in a flaming mushroom cloud i will just take that loss myself.

at least my house will be covered, also

because the utility power is so close to my place i may well go ahead and have the power brought in to a pole out by
the bunker and mount a yard light on it, i would have a small basic bill each month and have access to switching over to
utility power if i had to, call it my backup system i guess.

my original intention was to put everything in the attached garage in a dedicated room, but fear of possible hazards
and then having the insurance investigator poking around after a fire made me rethink having it all under the same roof.

last thing i would want is a fireplace fire burn down the place and then have some A$$hole inspector from the insurance company
try and blame the fire on my non approved, non permitted, non inspected power system.

btw, thanks Bob for the links to the over/under voltage relays
looks like cheap insurance in protection of the load side of the system.

bob g

[sailawayrb]

Bob G,

You probably know this already, but if you are starting from a clean sheet (i.e., don't already have CB panel or power at your place), you may be better off using a transfer panel in lieu of conventional CB panel and transfer switch.

http://nooutage.com/TRAN-PNL.HTM

The advantages are you only have one switch to throw to go from utility to generator, you can have many more circuits, and much cheaper to boot.  Having your generator in seperate building from your house makes a lot of sense for the reasons you mentioned.  Also gives you a place to spend time away from the household.