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

[reno-speedster]

I am wiring up my 6/1-St5 setup and I am installing a master breaker for the panel but I want to know what size I should use.  According to the math the ST5 can turn out 45 amps (5kw divided by 110 volts = 45.45 amps) max but its more realistically something over 3.5 amps constant load with the 6/1.  I have it set to the 110 setting (rather than 220).  If I use the 40 amp breaker will that be good enough to keep from damaging the gen head or should I drop to a 30 amp breaker?

[sailawayrb]

Well, a 40A CB would protect the ST5, but a 30A CB is probably more than adequate given what the typical 6/1 can generate. 

Also make sure that the CB you select adequately protects the wire between your ST5 and your transfer switch/panel.  If you go with a 40A CB, you will want at least #8 gauge.  If you go with a 30A CB, you will want at least #10 gauge.  I say "at least" because if you have long wire run from your ST5 and your transfer switch/panel, you may need to go to even larger wire gauges.

[LowGear]

Hi Reno,

I know it's not much and it's a pet peeve but the voltages are closer to 120 and 240.  So 30 Amps at 120 Volts is 3.6 KW or 15 Amps at 240 Volts.  So a 240 20 Amp breaker would shut down at 4.8 KW.  From what I've read here and there you just don't get 4.8 KW out of a 6/1.

Gosh I hope I'm not wrong again.  I'm having one of those weeks.  Surely someone with hands on will check my math and reading comprehension skills.

Casey

[oliver90owner]

Not an electrician, but a CB will trip on a surge?  That may be needed to be taken into account.

You say typically you are drawing 3.5 amps at 120 - less than half a kilowatt?  Not much load for a diesel!!

Maybe you should be looking at protecting the engine rather than the gen head as it will be engine limited - if revs drop, voltage will drop and so will amps, so I²R losses may not be a problem with an over-rated fan (for the engine limited output) on the oversized head.  You could be 'black smoking' that engine without anything tripping.

Not really sure why you would want a 'master' breaker in that situation.  A switch would suffice as it would only trip on a serious surge. 

Further, not knowing what you are actually wiring for is not too helpful as there may be other over-riding considerations too take into account.

So you have a generator which will cope with a 7.5kW surge and 5kW continuous and an engine that can deliver little over 3kW continuous.  If all your loads are small, they will be taken care of by your 'slave' CBs?  how many outlets?  If they add up to less than the master you are simply protecting a very short length of wiring (assuming it is all in one panel) and they (one of them) should trip before the master anyway?

Conundrum.

Regards, RAB

[reno-speedster]

sorry, its 35 amps not 3.5 amps.  thats what you get for writing when you are tired.  I have 4 small buildings on the ranch and I am running wire to each of them plus one line to the shed where the tools are.  Each of the buildings will have a 15 amp breaker and the shed will have a 25 amp.  I will never run all of them to capacity at the same time but I want to make sure that I can not overload the generator so I am going to wire in a master breaker.  The generator is only about 4 feet away from the panel and I am using #8 wire between them.

[Grael]

If you are running only 120V, put a 40 amp single pole breaker in and be done with it. Usually a surge wont trip a breaker, that is if it is not too big and doesn't last too long.

[ronmar]

I run 15A on mine wired for 240 and havn't had any issues.  I am feeding my main panel with the ST-5 so If I inadvertently overload the generator by leaving something on that I shouldn't, the breaker will pop before the engine is pulled down to the point of browning out the rest of the house.

[Tom]

I use 20 amp breakers on my ST5. When charging batteries I pull 13 A continuous. My reasoning for the 20's rather than the 15's is that if I get an unbalanced load it can be carried by the 20 and a 15 may needlessly trip. Sounds like you are wiring for 120 VAC only so I'd go for the 40.

[Doug]

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 .

As a rule of thumb you want to rate all your parts at about 125% of what you expect as normal this aplies to wire size and trip settings. This gives you plenty of room for starting heavy loads like induction motors but leaves your system protected from shorts.

Under all conditions consult the CEC or NEC and local electrical codes for wiring methods, standards and tables.....   

[LowGear]

Call me old fashioned but the breaker needs to be sized to the wire and not the load.  The load determines the size of the wire. .  It’s kind of a circle dance. 

If you're running 12 gauge then a 20 Amp breaker and if you’re running 14 gauge then a 15 Amp one.  Runs that approach 100 feet should be gauged one click up or a 12 to a 10 and so forth.

Have we talked about power – Watts.  A 20 Amp circuit at 120 Volts is capable of around 2400 Watts and a 15 Amp at 120 Volts will deliver up to about 1800 Watts.  If you have an extra moment, check out the specs of your breaker.  They can be as poor as +or- 20%.  You’re asking your 6/1 to deliver 35 Amps at 120 Volts or 4200 Watts.  Again, from what I’ve read here and there it won’t give you 4.2 KW no matter of how big the Circuit Breaker is you put on it.

If I had a panel I’d use it.  Each building would be on its own circuit (breaker).  It really facilitates trouble shooting and allows modular development and maintenance of your program. 

And like Doug; the National Electrical Code has been around for decades because it works pretty darn good.  I live far enough away from the fire station that they have promised to bring fresh buns because the dogs should be just about right by the time they dig the refrigerator out of the rubble pile.

LowGear

[NoSpark]

A couple of months ago I had my 6/1 ST5 cranking out over 4200 watts in 120v mode for about 20 minutes, so it is possible. Don't ask me about breaker size though, because I didn't have one on it then  . My st5 puts out over 130v at 60hz in 120v mode so that probably helped. I plan on using 220v mode which gives me 119v. I will be using 20a CBs with 6 guage Alu wire.
 

[LowGear]

I wonder if 4.2 KW is a record?  It's certainly the best I read about.

I'm wondering if that #6 wire is going to be a pain in the fingers getting it into those holes that are designed more for 12 gauge.  Of course I'm curious why you're using aluminum wire.  Be sure and remember the nolox or some form of anticorrosive.

Casey

[sailawayrb]

I wonder if 4.2 KW is a record?  It's certainly the best I read about.

I have gotten 4.5 from my 6/1 when burning SVO/propane.  I suspect there's more to be had but I'm limited by my 40A CB and wouldn't want to press ST5 to it's limit either.  I don't think getting 4.5K is all that uncommon, but not many folks need 4.5K.  With gas appliances, I don't normally need more than 2K.

[Doug]

A #14 wire comes from a box with a 15 amp fuse or CB so what is its rating?

Generaly we do not want to load beyond 80 of the rating of the wire and breaker if you ask more of this than 12 amps it may trip.
It becomes increasingly more likely to trip as we aproach 15 . So don't go beyond 12 amps on a 15 amp breaker and you can basicaly sleep well knowing it will never trip unless there is a real problem.

Do you have any breakers in your home that never trip?
Odds are the ones that never trip stay below 12 amps, and the ones that do are in places like kitchens and bathrooms where we alway try and squeeze more power put that we should. They protection device trips and the system and tables on wich it was based time ana agina prove they are correct.

Specifics now READ the code book and understand what they mean when they talk about feeders and branches, and ratings.
Maybe I wasn't clear before so please buy a code book siplified and understand what is expect from residential wiring. Guessing can lead to fires.

A sinlge phase induction motor has a name plate rating of 14 amps can you run it on a 15 breaker?
No
Can you run it on a 20 amp breaker?
....
Lets see what the CEC says

Where is my code book? Damb wife cleaning up down here so I can't quote tables and specific section to look at to explain things. What does come off the top of my head is polyphase guys and 600 volts and above I am afraid.....

[Doug]

I still can't find my code book but let me try and explain a few things.

Generaly there is an inrush of current in the 3 to 5 times area of name plate. The code will allow you to use bigger CBs on a small to prevent nusance tripping. The code will tell you what you can use as min and max breaker sizes Jen.

That was my point.

Your 200 foot extension is a great point to bring up because we can also look at line losses and how to calculate them for branches and feeders ( 200 feet of 14 for a 15 amp circut is probably trouble

If I can find my book this evening I will actualy start and finnish a bunch of simple examples to show what you guys