Generator Connection with disconnect switches

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mto12345

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Please see the one line diagram below, my question is a fundament one, if the two disconnect switches are sized correctly? How do you size a disconnect switch? 1712691672119.png
 
My questions:
1. What are the ratings of the two transfer switches?
2. What is the rating of the breaker within the MDP that supplies the first ATS?
3. Do I infer correctly that point 21 is fed from the MDP?
4. If that is a "yes," then what is the rating of the breaker that supplies point 21?
5. Why did you choose a 3 pole ATS?

My comments (taking note that we do not know how far along the design process is at this time, so some of these comments may be too early):
1. You have a generator rated at 750 KW (937 KVA at 0.8 pf). That gives you about 1130 amps. Yet you are feeding 2000 amps worth of ATS with it. This is a mismatch.
2. The right hand ATS is supplied without any overcurrent protection.
3. Even if the left hand ATS is fed by a fused disconnect (I don't have your list of symbols), the feeder to that disconnect has no overcurrent protection.
4. If the two disconnects are fused, then you should show the fuse ratings.
5. You are not showing the ratings of the two ATSs.
6. You are not showing any conductor sizes.

My answer to your question:
A disconnect should be sized at or above the calculated load it is intended to serve. If it is fused, the fuses should be sized to protect the conductors and the component immediately downstream. For example, the fuses in disconnect H8 must be able to protect transformer T8 and its primary conductors.
 
charlie b said:
My questions:
1. What are the ratings of the two transfer switches?
2. What is the rating of the breaker within the MDP that supplies the first ATS?
3. Do I infer correctly that point 21 is fed from the MDP?
4. If that is a "yes," then what is the rating of the breaker that supplies point 21?
5. Why did you choose a 3 pole ATS?

My comments (taking note that we do not know how far along the design process is at this time, so some of these comments may be too early):
1. You have a generator rated at 750 KW (937 KVA at 0.8 pf). That gives you about 1130 amps. Yet you are feeding 2000 amps worth of ATS with it. This is a mismatch.
2. The right hand ATS is supplied without any overcurrent protection.
3. Even if the left hand ATS is fed by a fused disconnect (I don't have your list of symbols), the feeder to that disconnect has no overcurrent protection.
4. If the two disconnects are fused, then you should show the fuse ratings.
5. You are not showing the ratings of the two ATSs.
6. You are not showing any conductor sizes.

My answer to your question:
A disconnect should be sized at or above the calculated load it is intended to serve. If it is fused, the fuses should be sized to protect the conductors and the component immediately downstream. For example, the fuses in disconnect H8 must be able to protect transformer T8 and its primary conductors.
Click to expand...
Please see responses in red to each question below:

1712751632054.png
 
Got it (I'm used to pads being utility fed and just realized HB is feeding the transformer) (y)
 
I agree with charlie that the feed to the 1600 amp ATS from the generator needs a disconnect/overcurrent device and you need to assure H10 is fusible.
also I assume the "420" amp breaker is actually a 400 amp based on the ATS rating.
 
augie47 said:
I agree with charlie that the feed to the 1600 amp ATS from the generator needs a disconnect/overcurrent device and you need to assure H10 is fusible.
also I assume the "420" amp breaker is actually a 400 amp based on the ATS rating.
Click to expand...

Thanks, So the feed from the Generator to the Transformer (T8) will have two disconnect switches. Or could i just have one disconnect switch (H8) but make it fusible?
 
Unless there is OCP at the generator you need to protect the conductors and the ATS on their supply side.

There still remains a problem in that H8 and H10 (1600 + 400) ratings excess the output capacity of the generator (1130). How is generator protection addressed.
 
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augie47 said:
Unless there is OCP at the generator you need to protect the conductors and the ATS on their supply side.

There still remains a problem in that H8 and H10 (1600 + 400) ratings excess the output capacity of the generator (1130). How is generator protection addressed.
Click to expand...
I will be upsizing the Generator to support the load (this will be in my new revision set). As far as the generator protection, you are right some sort of fusible or circuit beaker would need to support the OCPD. Outside of just adding a fusible disconnect switch is between ATS and generator is there anything else i can do? Also do i need to have both ATS to a 4 pole instead of a 3 pole?
 
A big flashing light suddenly appeared...we are looking at a 480/277 system... How do you meet the requirements of 230.95 & 215.10 ??

That will have a bearing on the 3 pole vs 4 pole question.
(Keep in mind I look at these thru the eyes of a dumbass inspector... Those answers need to be addressed by more knowledgeable Forum members).
 
shortcircuit2 said:
What Code section requires protection of the ATS on the supply side?
Click to expand...
Interesting question.. Is there any equipment, other than service, rated that doesn't need line side protection ?
110.10 or 110.3(B) possibly ?
In this case, 225.32 would require the protection for the feeder which would thus provide the ATS protection.
 
I would not call the conductors from the generator output terminals to the transfer switch feeder conductors.

So if the equipment (right hand transfer switch) is rated to comply with 110.9 & 110.10 and the conductors from the output terminals of the generator are rated 115% of the nameplate current rating of the generator per 445.13(B)...is an OCD needed?
 
IMO, the conductors do meet the Art 100 "feeder" definition.
I can't imagine a situation where conductors with no OCP would be allowed to enter a structure without immediate OCP.
 
Birken Vogt said:
That is a special case where a dead short can go on indefinitely with no ill effects.

It could be argued that the 115% generator rule is similar, but I never see it used in practice. I am not sure what it is supposed to accomplish.
shortcircuit2 said:
In solar...690.9(A)(1) conductors
Click to expand...
Thank you all for discussing my one-line connection. I have incorporated all your suggestions above. The only question I have is, is there a requirement that suggests adding an OCPD (fusible disconnect switch) between the ATS and generator?
Click to expand...
 
Thank you all for discussing my one-line connection. I have incorporated all your suggestions above. The only question I have is, is there a requirement that suggests adding an OCPD (fusible disconnect switch) between the ATS and generator?

Apparently we have two opinions on that. I feel it is required per 225.32 but others don't feel that is applicable.

(Please don;t overlook the GFP requirement in 230.95 & 215.10... that is going to be a problem if you are using fusible switches vs breakers)
 
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