Generator Question.

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hillbilly

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Got a customer who does wood working, and has purchased some 480V/3 equipment.
20HP Planer, 15HP Edger, etc....I figure he needs less than 100A to run what he wants.

The service into his building is 240V/1/60, and 3 phase is not available anywhere near his site (5 miles away).

He's also got a 350KVA 480V diesel generator that he wants to use to power this equipment.
This (used) generator has a 100A 3 pole fusible disconnect mounted on it's exterior that's apparently been used in the past as the disconnect for this generator. The conductors from the generator terminals to this disconnect are #2cu.
This seems to be a violation of 445.13.

I understand that the conductors from the generator output to the first overcurrent device must be sized for 115% of the generator capacity.
In this case 500A...correct?
Does this mean that the generator disconnect has to be rated for 500A?
That may be a stupid question:)
My mind says yes, but I'm having a problem understanding the reason.

All I need is 100A from this generator.
Can I not just fuse my feeder from this generator at 100A to power a 100A MLO panel inside the building?

If I have to set a 500A disconnect at the generator and feed it with 500A cable, can I then use the 10' tap rule to power a 100A Main breaker panel from this 500A disco?

I tried to sell this guy on using inverters to get 3 phase power for this equipment, but he's not interested.
This guy is a multi-millionaire that made his money building, owning and operating large equipment, and this wood working is just a hobby..albeit a large one.:rolleyes: He's quite a character.
He wants to use this generator, and this 3 phase equipment.

What would you do?
Any ideas are appreciated.

steve
 
Steve -

--- The conductors from the generator terminals to this disconnect are #2cu.
This seems to be a violation of 445.13.
Yes, I would agree

--- I understand that the conductors from the generator output to the first overcurrent device must be sized for 115% of the generator capacity.
I would consider 100%. See 445.13 exception

---Does this mean that the generator disconnect has to be rated for 500A?
Probably. The NEC doesn't prohibit you from under protecting the conductors. However, you are going to need a pretty healthy disconnect just to terminate the conductors from the generator.

All I need is 100A from this generator.
Can I not just fuse my feeder from this generator at 100A to power a 100A MLO panel inside the building?

If I have to set a 500A disconnect at the generator and feed it with 500A cable, can I then use the 10' tap rule to power a 100A Main breaker panel from this 500A disco?
Sounds like you are suggesting a 500A fused disconnect at the gen, fused at 100A, feeding an MLO or MCB panel. That's a good solution. You may have to get fuse adapters.

I tried to sell this guy on using inverters to get 3 phase power for this equipment, but he's not interested.
How are you going to get a 20hp and a 15hp, plus other stuff to run off of a 240V, 60A service???

What would you do?
Work with the customer to get him exactly what he wants. Design and install an easy to use, convenient, cost effective system with enough extra for future growth.

Don't ask him how he wants it installed. Rather tell him what you are going to install to meet his specs.


If the price is too high, work with the customer to shave the specs to get the price down.


cf
 
hillbilly said:
Got a customer who does wood working, and has purchased some 480V/3 equipment.
20HP Planer, 15HP Edger, etc....I figure he needs less than 100A to run what he wants.

The service into his building is 240V/1/60, and 3 phase is not available anywhere near his site (5 miles away).

He's also got a 350KVA 480V diesel generator that he wants to use to power this equipment.
This (used) generator has a 100A 3 pole fusible disconnect mounted on it's exterior that's apparently been used in the past as the disconnect for this generator. The conductors from the generator terminals to this disconnect are #2cu.
This seems to be a violation of 445.13.

I understand that the conductors from the generator output to the first overcurrent device must be sized for 115% of the generator capacity.
In this case 500A...correct?
Does this mean that the generator disconnect has to be rated for 500A?
That may be a stupid question:)
My mind says yes, but I'm having a problem understanding the reason.

All I need is 100A from this generator.
Can I not just fuse my feeder from this generator at 100A to power a 100A MLO panel inside the building?

If I have to set a 500A disconnect at the generator and feed it with 500A cable, can I then use the 10' tap rule to power a 100A Main breaker panel from this 500A disco?

I tried to sell this guy on using inverters to get 3 phase power for this equipment, but he's not interested.
This guy is a multi-millionaire that made his money building, owning and operating large equipment, and this wood working is just a hobby..albeit a large one.:rolleyes: He's quite a character.
He wants to use this generator, and this 3 phase equipment.

What would you do?
Any ideas are appreciated.

steve
Click to expand...

It seems to me like if the conductors left the generator you would increase the size of the conductors 115%.

We often see generators with one large breaker and one smaller breaker installed on the generator. Leading me to believe that the line side of the breaker would not know if you were the POCO or a generator.

I do not see how you could have a problem using the 100amp fused disconnect on the generator, most likely 100kaic. Check the hp rating of the disconnect. Sometimes it is very,very low.

Might be a whole different ballgame if you were installing a panelboard.

BTW,
I would ask the nutty professor if he thought that size unit would have a stable output with such a small load.
 
jrannis said:
I do not see how you could have a problem using the 100amp fused disconnect on the generator, most likely 100kaic. ---.
Click to expand...
I don't think I have ever seen a 100A disconnect that will take parallel 250kcmil. Did you have a different idea of wire size between the gen terminals and the first OCPD?

cf
 
Being a Brit, I'm not greatly familiar with what may or may not be code compliant there.
From a technical perspective I don't see a problem with fusing the output at a lower level than the rating of the generator provided the fuses can interrupt generator fault current.

I suspect that, with the genset running at about 10% of rated output, it might be rather inefficient. That might not be an issue for your well-heeled customer and if he already has the plant why not use it?
 
Besoeker said:
I suspect that, with the genset running at about 10% of rated output, it might be rather inefficient. That might not be an issue for your well-heeled customer and if he already has the plant why not use it?
Click to expand...

That's the way that he looks at it.

Thanks for the replies.

As far as the #2 cu that's already installed, that cable was installed by whomever installed the 100A disconnect on the generator chassis.
It was not factory installed.

My biggest question is.....is that allowed by 445.13?
The way that I read it is I have to run 500A cable from the generator output connection to a 500A (first means) (fused) disconnect.

I just don't get having to go through a 500A disconnect to utilize 100A from this generator.
If it was a POCO service, it wouldn't be required.
By applying the 10' Tap Rule (240.21(B)(1), I can use the 100A disco as installed.
It's not clear to me.....well 445.13 is pretty clear.:smile:

This is not a new generator, and has seen some use, although the owner says that it works well, so the 100A disco is a later addition.

Thanks again
steve
 
jrannis said:
This is what he wrote.
Click to expand...

Originally Posted by hillbilly The conductors from the generator terminals to this disconnect are #2cu.

steve
Click to expand...

Yes, he did say that is the existing installation.

However, as steve commented, for the conductors between the gen term box and the first ocpd. Art 445 requires the conductors be good for at least 420A and likely should be 484A. The #2s are not code compliant.

Even so, I still agree with what I think you are saying: The #2s are big enough for the intended load.

It's a legal issue, not a physics issue

cf
 
What is the name plate rating on the gen set? It should be in the 421 amp range. You could parallel 4/0cu to a 500amp rated disconnect and then drop the fuse rating to 100 and go from there.
Talk about an elephant pulling and little red wagon!
 
hillbilly said:
---As far as the #2 cu that's already installed, that cable was installed by whomever installed the 100A disconnect on the generator chassis.
It was not factory installed.
Yes - got that

My biggest question is.....is that allowed by 445.13?
No
The way that I read it is I have to run 500A cable from the generator output connection to a (ocpd)
Yes

500A (first means) (fused) disconnect.
No, you can under protect the conductors all you want.

---By applying the 10' Tap Rule (240.21(B)(1), I can use the 100A disco as installed.
No. This would be the first time I ever saw a tap rule applied to a gen output, before the first ocpd.

It's not clear to me.....well 445.13 is pretty clear.:smile:
Yes
Click to expand...

steve -
Recomendation: Treat this as a light industrial - not as residential. It certainly will ease the decision making. From your description, your customer will agree.

Tap Rule Issue:
Are you saying the tap rule applies to the conductors between the gen term box and the first ocpd? I've never heard of a tap rule being applied where there is no upstream OCPD. I don't think 240.21.B.1 applies.

The existing #2s:
Art. 445 requires conductors capable of 484A between the gen term box and the first ocpd. No way out of that.

The first ocpd:
The existing 100A disconnect, except (as already mentioned) maybe the horsepower rating, is big enough for the load and meets code. No law against under protecting conductors. The problem is the required conductors won't fit the lugs.

The choices appear to be (SQD anyway):
200A with a 2 lug kit
400A - 2 lug
600A (comes with 2 lugs per)

What ever disconnect you use, fuse with 100A and take off with your 100A wire and feed 100A panels. Can one even get a 100A, 3ph, 480/277 panel? Probably can - not sure I've ever seen one though.

cf
 
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Cold Fusion said:
steve -

Tap Rule Issue:
Are you saying the tap rule applies to the conductors between the gen term box and the first ocpd? I've never heard of a tap rule being applied where there is no upstream OCPD. I don't think 240.21.B.1 applies.


I agree, I was just thinking out loud.
The tap tules are for Feeder Taps, and for it to be a feeder, it has to have upstream OCP.

I'm pretty sure that I can get a 100A 3 phase 600V MLO panel .

I'm going to look at the generator again just to make sure that it doesn't have a Main breaker located on it or in it.
Sure would make it more simple if it did.

I started out in industrial many years ago working on 480V 3 phase.
I having to refresh my memory, which at my age takes longer and longer.:)

Thanks again for the reply.

steve
Click to expand...
 
I am confused by all the references in this thread about 445.13 when talking about the feeders coming from the generator.

I though that Article 445 was addressing the generator and 225 would address the feeders.

Has the code panel members started mixing these articles together?
 
jwelectric said:
I am confused by all the references in this thread about 445.13 when talking about the feeders coming from the generator.
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Click to expand...

I may have caused that confusion.

What I have is a 350KVA 480V generator, and the only OCP that is on the generator is a 3 pole 100A fusible disconnect that is connected directly to the generator output terminals by #2cu conductors.

I believe that these #2 conductors are the ones referenced by 445.13.

100A is all the power that I need from this generator, but I don't believe that I can legally (code wise) just connect to this 100A disconnect and power a 100A MLO panel.

That about the extent of it.

steve
 
Remember that generators do not have an unlimited ability to supply current that is the size of the generator is selected based upon the load to be served.
It is important to take the generators ability to provide enough overcurrent for enough time to trip a breaker on thermal (overload).
The maximum trip time in seconds for breakers rated up to 250v with 300% of the breakers rating is 50 sec for breakers with up to a 30at, 80 sec for breaker to 31-50at, 140sec for a breaker to 51-100at etc.
The question is if that generator can product enough over current to trip the breaker. Currents less than 300% would take longer to trip the breaker of course ref NEMA AB4.
Short circuit (faults) are another concern as the magnetic pick up of breakers is 6-7x the rating on small breakers such as those in the 15-30 a range and 10x for the larger breakers. Can a generator produce enough current with a bolted fault condition to trip the breaker magnetically.
There are manufactures how do make breakers with trip characteristics that are more in line with those required by generators.
 
I'm completely confused, if all you need is 100A why can't you just add a 100 amp disconnect immediately off the generators terminals?

I don't see the difference in adding a 100 amp panel to utility feeders with thousands of amps available to adding this disconnect to the generator.
 
jmd445 said:
I'm completely confused, if all you need is 100A why can't you just add a 100 amp disconnect immediately off the generators terminals? ---
Click to expand...
You can. However, NEC 445 requires the conductors between the gen terminals and the first OCPD be sized at 115% of the gen FLA. For this gen, that requires conductors suitable 484A - parallel 250kcmil.

I've never seen a 100A disconnect that will accept parallel 250s. So you end up with a higher amp rated disconnect. My recomendation was to size the fuses down to 100A. Adapters are available for 400A panels to take the smaller size fuse.

This is not a physics or safety issue. it is an NEC legal issue, an equipment availability issue.

cf
 
Even though there is a requirement to protect the cables does the generator have the capablitiy to generate a great enough over current for a long enough period of time to even trip the breaker?
This question is often ignored because because one wants to assume that the cable is protected be the breaker.
 
You are supplying two different systems/supplies to the building wiring, I do not see this as a permitted installation. There are conditions that permit more than one source, I do not see the residential dwelling you are working on as complying with those conditions.
 
Pierre C Belarge said:
You are supplying two different systems/supplies to the building wiring, I do not see this as a permitted installation. There are conditions that permit more than one source, I do not see the residential dwelling you are working on as complying with those conditions.
Click to expand...



First off, this is not a residence, it's a wood working factory.

The building has a existing 240V single phase electrical system.

The generator will supply specific pieces of equipment that require 480V 3 phase power.

The installation is allowed by 230.2(D).

There will be no interconnection between the two systems, other than the earth connection.

steve
 
hillbilly said:
First off, this is not a residence, it's a wood working factory.

The building has a existing 240V single phase electrical system.

The generator will supply specific pieces of equipment that require 480V 3 phase power.

The installation is allowed by 230.2(D).

There will be no interconnection between the two systems, other than the earth connection.

steve
Click to expand...


Steve
Sorry, I thought this was a dwelling.

230.2(D) is for services, a generator is not a utility provided service.

I would still find out if this is permitted by your local AHJ.
 
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