Transfer Switch

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cmesaw

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Can this be done per NEC. I just talked to a customer that had another electricain tell him that if he installed a 200 amp transfer switch between his meter can and main panel It has to be fed from a generator suppling 200 amps. I have not found this in the book anywhere.
The generator the customer has is 25kw. it has a main disconnect sized at 100amps. I believe that as long as the generator suppling it has ocp rated for the wire and generator it would be fine.
Next question . Would the tranfers switch be the first means of disconnect?
Would I have to separate the neutrals and grounds?
I do thank you for this forum.
 
Usually a manual transferswitch says "service entrance rated" but, without fuses, everybody seems o think it cant be a service disconnect.
My question would be:
Could the service disconnect be installed after a non fused SE rated MTS?
 
cmesaw said:
. . . if he installed a 200 amp transfer switch between his meter can and main panel, it has to be fed from a generator supplying 200 amps. I have not found this in the book anywhere.
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You haven?t found it because it is utter nonsense. There is no such rule, and no need for such a rule. The ATS has to be rated at least as high as the current it could get from either source. But it can be rated higher than both sources. You can buy things rated higher than you need them to be, but not lower.
cmesaw said:
Next question . Would the transfer switch be the first means of disconnect? Would I have to separate the neutrals and grounds?
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I don?t know about this one. If you have a meter (with no disconnect) leading to an ATS, and a generator leading to the same ATS, and if the ATS then leads to a disconnecting means, that might be acceptable. Someone else will have to answer this one.
 
jrannis said:
Usually a manual transferswitch says "service entrance rated" but, without fuses, everybody seems o think it cant be a service disconnect.
My question would be:
Could the service disconnect be installed after a non fused SE rated MTS?
Click to expand...

The ATS in SE rated.
 
If the ATS is "service rated" then I see no problem.
In order to be service rated, I am almost positive it would overcuurent protetion (no true on manual) and charlie is 100% on target as to the switch ratings.
 
Would the tranfers switch be the first means of disconnect?
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If the ATS was service rated it would have breakers in it for the service feed and gen feed. These would serve as the disco. If the ATS is not service rated you would need to install service rated disconnecting means between the meter and ATS and gen and ATS.

Would I have to separate the neutrals and grounds?
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Any thing past the first over currant devise would be separated.

The loads that you want to carry how are they separated from the normal source?
 
charlie b said:
You haven?t found it because it is utter nonsense. There is no such rule, and no need for such a rule. The ATS has to be rated at least as high as the current it could get from either source. But it can be rated higher than both sources. You can buy things rated higher than you need them to be, but not lower.
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Um, what about 702.5(B)(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system comply with (a) or (b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
(b) Load Management -- deals with automatic load shedding.

So your generator doesn't have to be rated at 200A, but it must be rated to handle all the load on the panel (so what does your service calculation say your load is)?
 
Around here, if you have a manual transferswitch, it is assumed that you will also manually shed your loads.
With an automatic Transfer switch, loads have to shed automatically
 
charlie b said:

You haven?t found it because it is utter nonsense. There is no such rule, and no need for such a rule. The ATS has to be rated at least as high as the current it could get from either source. But it can be rated higher than both sources. You can buy things rated higher than you need them to be, but not lower.

I don?t know about this one. If you have a meter (with no disconnect) leading to an ATS, and a generator leading to the same ATS, and if the ATS then leads to a disconnecting means, that might be acceptable. Someone else will have to answer this one.
Click to expand...

If th TS has a manual means of disconnect, I would think that would be your main and the panel is now a sub...
 
I just did a Residential load cal. (Mike Holt's) With the customer. Comes to 113A demand load. (have gas heat and gas water heater, gas dryer) The customer has a generac 200 amp service rated ATS
And a 25kw generator rated at 104amps and 158 amp surge.
Would you say that it meets the NEC because the generator can carry the entire calculated load. Maybe not the 200 amp service but the calculated load. Or Does the surge count at all. What do all of you think??
 
cmesaw said:
I just did a Residential load cal. (Mike Holt's) With the customer. Comes to 113A demand load. (have gas heat and gas water heater, gas dryer) The customer has a generac 200 amp service rated ATS
And a 25kw generator rated at 104amps and 158 amp surge.
Would you say that it meets the NEC because the generator can carry the entire calculated load. Maybe not the 200 amp service but the calculated load. Or Does the surge count at all. What do all of you think??
Click to expand...

Thats fine. But how are you sheding loads?
 
Twoskinsoneman said:
I think he's saying with the genset able to handle the transfered load shedding isn't required.
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I just did a Residential load cal. (Mike Holt's) With the customer. Comes to 113A demand load.
Click to expand...
And a 25kw generator rated at 104amps and 158 amp surge.
Click to expand...
I do not see it that way from the OP.

Maybe not the 200 amp service but the calculated load.
Click to expand...

This is where you lost me. The only place this comes into play is the rateing of the transfer switch
 
What does the UL White Book have to say about a transfer switch that is service rated?

If memory serves me I think there is something in the White Book that says that there has to be a means externally to disconnect both the service and the generator or in other words both can be turned off without having to open the switch.
 
jwelectric said:
What does the UL White Book have to say about a transfer switch that is service rated?

If memory serves me I think there is something in the White Book that says that there has to be a means externally to disconnect both the service and the generator or in other words both can be turned off without having to open the switch.
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Your memory serves you well, that is one of the requirements.
 
The ATS has a 200 amp CB for the service side and another one for the generator side. But being in a 3R enclosure you still have to open.
 
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