Automatic Transfer Switch.

[nkalghatgi]

I have a MCC with feeder protection device at 125 A. The disconnect swicth for MCC is at 200 A. The ATS serves the generator (generator CB at 100A), mini power center (CB at 70A) and MCC via its disconnect.
What size should my ATS be?

 

[templdl]

I have a MCC with feeder protection device at 125 A. The disconnect swicth for MCC is at 200 A. The ATS serves the generator (generator CB at 100A), mini power center (CB at 70A) and MCC via its disconnect.
What size should my ATS be?

Is this a test question or a course exercise?

 

[nkalghatgi]

This is my actual design.

 

[augie47]

If I follow correctly, the ATS is supplied by a feeder protected at 125 amps and by a generator feeder protected at 100 amps.
In that case the ATS would need to be rated at 125 amps as that is the most load it would ever be called upon to carry.
From that standpoint, the size of any switches, OCP devices etc on the downstream side of the ATS is irrelevant.

Keep in mind Art 701 and Art 702 would require the generator and ATS be sized to carry the connected load.

 

[kingpb]

I have a MCC with feeder protection device at 125 A. The disconnect swicth for MCC is at 200 A. The ATS serves the generator (generator CB at 100A), mini power center (CB at 70A) and MCC via its disconnect.
What size should my ATS be?

As a helpful tip, it would be very useful to sketch out a simple one-line diagram. Many of us are visual learners :dunce:

 

[nkalghatgi]

From utility service meter, the power goes to ATS. From ATS it goes to mini power center and MCC. The mini power center is protected by 70A breaker and MCC is protected by 125A breaker. The MCC has a disconnect switch at 200 A.
One more connection is from generator to ATS. The generator CB is 100A.

Hope this gives a better picture.

 

[kingpb]

Based on my understanding of your configuration, I would say you need a service entrance rated ATS with a 175A or 200A rating depending on standard size.

You won't need it to be SE rated if you are going to put a main breaker ahead of the ATS, which you did not mention.

It would appear that the intent of the DEG is that it will not power all the loads, in that case some load shedding will need to be performed so that you don't force the DEG into load rejection. However, under normal operating you will be powering the miniPC and the MCC through the ATS hence the required rating.

 

[hbendillo]

Well, your are basically feeding almost 200 amperes worth of load if you add up the MCC rating and mini-power center. Do you want to be able to use that full load capacity? Course the generator won't be able to handle it. I say make it simple and just get a 200-amp transfer switch and you are covered. You never said what the service size was coming into the transfer switch from the utility.

 

[ahmedats]

Automatic Transfer Switch

Automatic Transfer Switch

I have a MCC with feeder protection device at 125 A. The disconnect swicth for MCC is at 200 A. The ATS serves the generator (generator CB at 100A), mini power center (CB at 70A) and MCC via its disconnect.
What size should my ATS be?

According to my understanding, i think you have to use at least 200 Amp. ATS. this site may help you.:lol:

 

[Jraef]

As a helpful tip, it would be very useful to sketch out a simple one-line diagram. Many of us are visual learners :dunce:

LOL, I just was in a similar circumstance. User was having me design an MCC and at the last moment as we go over the load list, drops a bomb that that one of the 30A feeders (FDS) was "going to a 5kW generator". I asked him why we were powering a generator (rhetorical question, but I wanted to see how bad the situation was). He seemed to think this 30A FDS was going to become the backfeed FROM the generator if the power failed. He wanted to feed the MCC bus, but only power a couple of 1HP motors that HAD to run if the power failed. No ATS, no MTS, no way to isolate ONLY the two little 1HP motor starters from the rest of the 800A bus MCC. Basically, no forethought whatsoever. I drew out a one line for him and showed him how it needed to work (added an ATS and separated the loads), that eventually helped him get it. Took a while for it to sink in though. I had to keep going over the power flow again and again, I even found a red felt pen to make it more clear.

"When the utility is working, the power goes like this. When the utility power fails, the ATS switches the generator on, and the power from the generator ONLY goes like this to just these two motor starters."

"Why do we have to separate the loads?"

"Because, that generator can't power all 800A can it?"

"No, but why would it have to?

"Because if you connect it to the MCC bus, how is it NOT going to?"

"We turn off everything else."

"How are you going to do that?" (no automation involved by the way).

"We have a guy turn them off."

"What if he is in the bathroom when the power fails?"

"Why is that a problem?"

Around and around we went for 20 minutes...

 

[Phil Corso]

Jraef...

I dont know if your'e old enough to rember the show, "Can You Top This?" but you opened the door, so here goes:

An assignment took me to a mid-east refinery. I met a young, aggressive, site electrical engineer who was anxious to talk about his newly acquired expertise. It was the first time that underground conduit had ever been installed in this country. Direct-buried cable was the country?s usual installation technique. So, he proudly announced that they had just completed "pulling" a relatively long length of 20 kV, 3-phase, lead-sheathed cable through the conduit, in one length. Of course I congratulated him because the feat, while not impossible, was certainly very, nay extremely difficult.

We walked the route of the underground conduit, and about half way, I noticed a splice vault, with a cable splice in it. I said "If you pulled the cable in one length then why is there a splice in it?". He replied "Well the construction plan called for one, so..."

Regards, Phil Corso

 

[Jraef]

Jraef...

I dont know if your'e old enough to rember the show, "Can You Top This?" but you opened the door, so here goes:

An assignment took me to a mid-east refinery. I met a young, aggressive, site electrical engineer who was anxious to talk about his newly acquired expertise. It was the first time that underground conduit had ever been installed in this country. Direct-buried cable was the country?s usual installation technique. So, he proudly announced that they had just completed "pulling" a relatively long length of 20 kV, 3-phase, lead-sheathed cable through the conduit, in one length. Of course I congratulated him because the feat, while not impossible, was certainly very, nay extremely difficult.

We walked the route of the underground conduit, and about half way, I noticed a splice vault, with a cable splice in it. I said "If you pulled the cable in one length then why is there a splice in it?". He replied "Well the construction plan called for one, so..."

Regards, Phil Corso

Gotta love the quick thinking though!