Size wire to breaker or nameplate?

[stickboy1375]

I've been doing a ton of generators lately, and i've been sizing the wire to the breaker size contained in the unit, but the nameplate is so much less... example, a 12kw generator will contain a 70 amp breaker, but has a nameplate of 50 amps output, so can I size my wiring to the nameplate or the breaker installed in the unit?

 

[iwire]

Breaker

 

[stickboy1375]

but why?

 

[david luchini]

but why?

240.4

 

[stickboy1375]

Let me rephrase the question, Why is the breaker sized so much higher than the actual output?

 

[Joethemechanic]

Let me rephrase the question, Why is the breaker sized so much higher than the actual output?

Due to the large rotating mass of the rotor, flywheel, crankshaft, harmonic balancer, a genset has a lot of stored mechanical energy. This stored energy can be used to power momentary loads in excess of the prime movers available output.


Have you ever noticed how much longer a generator takes to stop rotating than say, a car engine, after you turn it off?

 

[jumper]

Inrush current, like a HVAC unit or motor.

 

[iceworm]

Let me rephrase the question, Why is the breaker sized so much higher than the actual output?

disclaimer - I usually work on a lot bigger generators, but I think the principles are the same.

Generally speaking gen mfgs don't put any extra money in the equipment than they absolutely have to. The CBs, conductors, drivers, alternators are all as thin as they can be and still meet spec. Although in the case of a 12kw consumer grade they maybe even be a bit thinner than that.

As you noted, 12kw at 240V is 50A. But the gen is a continuous output kind of device, so the first disconnect is usually rated at 125% which would be 62.5A - next size up is 70A. Practically speaking, if the conductors past the first OCPD were sized at 62.5A you would be okay. Usually the gen can't put out any more than 50A continuous - the driver will slow down. And 62.5A rated conductors are okay for a 70A CB

ice

 

[Joethemechanic]

BTW, you can sense your voltage at the load end of your generator feeder. It makes for nicer voltage regulation at the load end. How you have to do it depends on the gen-set and the voltage regulator though.

As long as your voltage drop on the feeder is not too bad, the voltage regulator will adjust the field excitation enough to keep things pretty stable.

 

[iwire]

BTW, you can sense your voltage at the load end of your generator feeder. It makes for nicer voltage regulation at the load end. How you have to do it depends on the gen-set and the voltage regulator though.

As long as your voltage drop on the feeder is not too bad, the voltage regulator will adjust the field excitation enough to keep things pretty stable.

That sounds like something a DIY would do to compensate for a poor installation to start with.

Size the conductors properly to start with.

 

[Joethemechanic]

That sounds like something a DIY would do to compensate for a poor installation to start with.

Size the conductors properly to start with.

Like I'm the first guy to use the "in free air" rating of conductors on a construction site.


BTW, that is straight out of either a Delco or EMD manual from the 1950's

I can't inmagine what it's like to have an unlimited budget to set up a construction site that will only be in operation for a few months before everything is taken down and moved.

My customer has some portable dust collectors with 100HP 3 phase 1800 rpm motors. And at times they may be thousands of feet from the generator/generators. They need all the help they can get.


Just for size reference, here is one of the diesel powered units I was moving one day

I'm so DIY that I bought a 20,000 pound Hyster H200 forklift and a 6,000 pound Lull telehandler last year just to set the machinery up on jobsites.

 

[iwire]

Like I'm the first guy to use the "in free air" rating of conductors on a construction site.


BTW, that is straight out of either a Delco or EMD manual from the 1950's

I can't inmagine what it's like to have an unlimited budget to set up a construction site that will only be in operation for a few months before everything is taken down and moved.

My customer has some portable dust collectors with 100HP 3 phase 1800 rpm motors. And at times they may be thousands of feet from the generator/generators. They need all the help they can get.


Just for size reference, here is one of the diesel powered units I was moving one day

I'm so DIY that I bought a 20,000 pound Hyster H200 forklift and a 6,000 pound Lull telehandler last year just to set the machinery up on jobsites.

Joe ... the topic is a 12 KW genset at a home.

 

[Joethemechanic]

Oh, I thought he was just using that gen-set as an example. I figured he wanted to know why the breakers were sized larger than the cont rating on any sized gen-set

That large rotating mass with all it's inertia will carry you through a good sized motor start. Even if the windings in the gen are a little light the regulator will keep the voltage up while all the rotating mass carries it through. Most times the prime mover's output is the limiting factor of the continuous rating. it seems that the breaker on the genset end is really just there for short circuit protection.

I almost never have them trip, most times it exceeds the capacity of the prime mover and they go under frequency.