How do you size a circuit breaker?

[Randybee1]

I have a 7.5 kw head that I am wiring up. I do not plan to draw 7.5 kw but lets just say all my freezers, air compressor and AC motor kick on all at the same time and I see an instantaneous spiike of 8 KW. Do I size the circuit breaker for the max possible load? in this case 8KW? (35 amps).

Randy B

[mike90045]

NO

You consider the wire, and the wire size, determines what size breaker to protect it.  10ga wire can handle 30 amps generally, 12ga = 20a, and 14ga is good for 15a.   You could go smaller if you want to protect something else, but the WIRE is what the breaker is intended to protect.  If you go for larger wire, you can up-size the breaker to a larger breaker.

[Randybee1]

Makes sense.. thanks!

[Henry W]

mike90045 is correct.

You have a 7.5 kw head and I would not go over that rating by fusing it no more than 30 amps with proper gauge wiring installed. Another thing you need to know is LRA. (Locked Rotor Amps) This will help you determine how much amps it will take to start your compressors, electric motors and freezers. the manufactures should be able to give you this info.

I hope they never come on at the same time this will probably be to much and you can possibly damage the generator head and your equipment.

Another thing you need to keep in mind is when running any type of induction motor or freezer there is a unity power factor involved. this means you need to de-rate the generator head even more. Some generator heads can take motor starting better than others so you need to find this out also. In most cases it is always better to over size a generator head and limit the total output by a smaller breaker than the generator can handle. This will prevent generator overloading. For example if the need was to draw 8 kw I would go up to 12 kw generator head and fuse it at 35 amps.

Another thing is most generator heads are more efficent when they are loaded to aprox 50%.

Henry

[Thob]

In addition to wire size, you have to consider the maximum breaker that can be used for the load.  In the US, for example, the maximum breaker size for a standard 120v outlet is 20A (most outlets are only rated for 15A, but the code allows a 20A breaker).

What I do is take a small breaker box and use a 30A two pole breaker which is wired to the generator.  This serves as a "main" breaker and protects the generator.  I then put in 20A single pole breakers to 120V outlets, a 20A two pole breaker to a 240V outlet, etc.

[glort]

Surely you don't want to be just running everything from one circuit do You?

I'd be doing as throb suggests and putting in a pair of breakers to act as "Main Switches" for the genny then splitting up the individual circuits from there.
I'd just rate them the same as your normal household circuits and then space the loads out as need be.
The likelyhood of everything coming on at once is a Million to one ( or if you tried, it would take about a 1000 years to do it and then be unrepeatable anyway.) so I wouldn't worry about that.

The big advantage of having multiple circuits with the loads paced out is when you first fire the genny, you  can flick the breakers individually so you know the loads can only come on in increments. IF you have everything on one circuit, You'll have to run round and turn them all off then go back and turn them on one at a time.

I have my house wired with 4 Lighting circuits and 6 power circuits so I can isolate individual areas as I please.  I'm on grid power but I have well more than enough load to exceed the rating of the power pole and wires coming to the house so it's hand you have everything broken up.
As well as that, If I blow a fuse for the lights for instance, it's only 1 part of the house that goes out, not the whole thing being plunged into darkness.

Same as when my son had his 18th Birthday a few weeks back. They wouldn't keep the music down well after the 12 pm cut off so I just cut the power where they were. That quietened things down nice and effectively!  :0)

[Randybee1]

Surely you don't want to be just running everything from one circuit do You?

I backfeed 240 into my house from the generator in the garage.

[XYZER]

Surely you don't want to be just running everything from one circuit do You?

I backfeed 240 into my house from the generator in the garage.

Randy,
That is the way I do it. I have a breaker at the generator head rated for its output but the wire from the generator to the panel needs to be rated for that breaker or larger.