DECIDE THE GENERATOR SIZE

[binwork91]

How to size the generator?
For example, I have one generator to feed one panelboard, my load is 1700A, and my main breaker at the panelboard is 2000A.
are we using 2000A to size the generator or 1700A to size the generator?
I think we using 1700A to size the generator, because as long as generator is able to provide the 1700A load.
However, if for some reason, we add 100A load to the system, the 2000A will not trip, but generator is overload.

Please help. Thank you.

 

[ptonsparky]

Can you automatically shed loads while on generator?

 

[binwork91]

Can you automatically shed loads while on generator?

1. if yes, does it mean generator never overload?

2. if no, what will be the answer?

 

[ptonsparky]

Start with reading Article 700.
Optional Standby System? 702.4(B) (1)&(2)

 

[zbang]

1- no. (Well, it'll never overload if the breaker trips .)
2- it depends.

Load shedding lets you remove non-essential loads when the generator is running, or when the gen is getting close to it's max output. Whether you implement that or not depends on the loads supplied and how you want them to operate; we can't answer that.

Generators are seldom sized to the panel/switchboard they're feeding, but to the loads they're expected to carry and for how long. You also have to account for the transfer switch size (usually matches the generator output). First, look at whether the protected loads are legally-required, optional, "process-critical", etc (see Articles 700-702). Once you have those, you can start to work on sizing.

You also need to consider start-up timing and loading- if there are a lot of high-inrush loads, the generator needs to be bigger to accommodate those surges.

If you can give us a lot more info about the application, we can be more specific.

 

[petersonra]

1700 A is a suspiciously round number. How did you calculate that number.

 

[binwork91]

1- no. (Well, it'll never overload if the breaker trips .)
2- it depends.

Load shedding lets you remove non-essential loads when the generator is running, or when the gen is getting close to it's max output. Whether you implement that or not depends on the loads supplied and how you want them to operate; we can't answer that.

Generators are seldom sized to the panel/switchboard they're feeding, but to the loads they're expected to carry and for how long. You also have to account for the transfer switch size (usually matches the generator output). First, look at whether the protected loads are legally-required, optional, "process-critical", etc (see Articles 700-702). Once you have those, you can start to work on sizing.

You also need to consider start-up timing and loading- if there are a lot of high-inrush loads, the generator needs to be bigger to accommodate those surges.

If you can give us a lot more info about the application, we can be more specific.

I understand that the generator need to supplying the full load, and if there is inrush load, I need to consider inrush current.
I also notice generator usually does not size base on OCPD.

let's back to my example, if 1700A load is the max current I need for my system, and all equipment are running.
in this case i only need 1700A load, but I need a main breaker for it, so 2000A is my main breaker.
if generator I size is base on 1700A, then my system should work fine, since i only need 1700A, main breaker will not trip due to overload.

what if in the future, someone want to add 100A load to the panel, because my main breaker is 2000A, so they think 1800A load should be fine.
However, my generator only able to provide 1700A load. in this case, my generator is overload, and it may cause fire. That is my question come from.
if I size my generator base on my main breaker 2000A, then generator never overload, because the breaker will trip.

My opinion is I should size base on main breaker, but I can not find anything to support my opinion. That is why I want to know how to size the generator.

 

[zbang]

And we're telling you that. You're also confusing things by saying "1700 amp" without telling us if that's the total load evenly distributed on a 3-phase feed (566 amps/phase) or the per-phase load; or if that's the con.

What you seem to be neglecting is the generator's own breaker and protection. Pretty much any modern generator won't let itself be overloaded to the point of causing an unsafe or destructive condition. There's also the point that the prime mover of any generator can only deliver a given amount of power before it stalls/shuts down (an extra 100 amps on a 1700 amp feed is not going to risk a fire, but it could shut down the generator).

 

[binwork91]

And we're telling you that. You're also confusing things by saying "1700 amp" without telling us if that's the total load evenly distributed on a 3-phase feed (566 amps/phase) or the per-phase load; or if that's the con.

What you seem to be neglecting is the generator's own breaker and protection. Pretty much any modern generator won't let itself be overloaded to the point of causing an unsafe or destructive condition. There's also the point that the prime mover of any generator can only deliver a given amount of power before it stalls/shuts down (an extra 100 amps on a 1700 amp feed is not going to risk a fire, but it could shut down the generator).

This answer help a lot.
In my example, I want to say, 1700A per phase on 3 phase.
If generator has it own breaker and protection, then 2000A main breaker it work as a switch, and does not provide any protection. The only reason to have 2000A main for the panelboard is because panelboard need a main as per 408.36. right?

 

[retirede]

1700 A is a suspiciously round number. How did you calculate that number.

Test question?

 

[petersonra]

Test question?

I was thinking another guy who determined his load by adding up the ratings on all the circuit breakers.

 

[petersonra]

If generator has it own breaker and protection, then 2000A main breaker it work as a switch, and does not provide any protection. The only reason to have 2000A main for the panelboard is because panelboard need a main as per 408.36. right?

The PB does not necessarily need a main. It needs to be protected. If the PB is only being fed by the generator it is likely the generator protection could also serve to protect the PB.

 

[zbang]

...or if the panel is protected by upstream OCPD...
...of if there's an ATS involved which might have protection involved....

Most of the generator vendors have papers on their web sites about sizing and installation.

 

[WA_Sparky]

Personally I always give a little wiggle room for future additional loads. Especially on smaller gens. This is quite a significant size gen. Might be worth a conversation with the client to see what their future intentions are. Also If this is new construction and not an industrial application (industrial tends to have their equipment running at all times), you could just size gen for estimated load. If I was given the project down the road following your design to add the 100A future load, first thing id do is meter it. Most likely 1700A estimated isnt going to be actual peak usage, then id add the load once its confirmed the system can handle it.