Calculating total amps to size panel using data

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Tainted

Tainted

Senior Member
Location
New York
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Engineer (PE)
I need to power (3) chiller banks

Each bank has the following electrical characteristics:

208V 3Phase
RLA = 677
MCA = 691
MOP = 700

I think these are continuous loads. Here's my calculation:

(3)(677)*1.25= 2539 amps

Does this mean I would need a 3000 amp distribution panel with 3000 amp fuses?

I don't even know if RLA is the same as FLA. I've always thought that FLA is 0.8*MCA. If we take (691)(0.8) it's 552.8 amps which is not even close to the RLA.
 
I am not entirely sure what you are asking but article 220 will tell you how to calculate the feeder load for these three devices. See 220.40.

I think you will find a 3000 Amp distribution panel is required.

You could potentially get by with using a 1000 A enclosed CB for each unit and just forgo the distribution panel entirely. But I don't know that it will save you all that much.
 
Strathead said:
Nope, MCA is 691*3=2,073A no need to include anything else. That is why they make the equipment provider label with MCA.
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In general that would be an overestimate of the required feeder size by twice 25% of the largest motor in one of the units. In that MCA includes that 25% extra, and for a feeder you only need to include it once, not three times.

But with such similar RLA and MCA numbers, I don't know what's going on in the MCA computation, so it's hard to say back out the extra 25%.

Cheers, Wayne
 
Tainted said:
I don't even know if RLA is the same as FLA. I've always thought that FLA is 0.8*MCA. If we take (691)(0.8) it's 552.8 amps which is not even close to the RLA.
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Manufacturers follow different rules than what is found in the NEC.
There is no need for you to know what goes into their MCA and MOP numbers. If you really want to know, you would need to look into the product standards.
 
230.90 Where Required. Each ungrounded service conductor shall have overload protection. (A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor.
(There are exceptions)
 
augie47 said:
230.90 Where Required. Each ungrounded service conductor shall have overload protection. (A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor.
(There are exceptions)
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Augie do we know that this is a service we are dealing with? 🤔
 
Basically the simple answer is you take MCA times 3= 2073. You might be able to squeeze it in to a 2000 instead of a 2500 if you can get the chiller data and add up all the loads individually which would allow you to take 125% of only the largest motor. (As Wayne said). Whether that will result in any big cost savings is questionable.
 
Nothing to do with any of the answers, but just curious, why is such a large load being powered by 208? I don't know if I've ever even seen a 208 circuit that big. Maybe something built in the 1920's we were tearing out.
 
augie47 said:
230.90 Where Required. Each ungrounded service conductor shall have overload protection. (A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor.
(There are exceptions)
Click to expand...
augie47 said:
Opps no. I don't know why I made that assumption... size, I guess...
If it's a feeder than, as Strathead notes, 240.;4
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Technically even if it is a service, 240.4 (A) and (B) apply. But, I was wrong before I was right. Not unusual.
 
Joethemechanic said:
Nothing to do with any of the answers, but just curious, why is such a large load being powered by 208? I don't know if I've ever even seen a 208 circuit that big. Maybe something built in the 1920's we were tearing out.
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Yeah good question. At 480V it would be a 1000A feeder. If they cant have 480V for safety or working clearance they could it at least bump it up to 240V and it would fit on a 2000A.
 
Joethemechanic said:
Nothing to do with any of the answers, but just curious, why is such a large load being powered by 208? I don't know if I've ever even seen a 208 circuit that big. Maybe something built in the 1920's we were tearing out.
Click to expand...
building service is only 208V 3-phase... My mechanical engineer gave me all these loads that he wants to power. He is trying to completely electrify cooling and heating with these chiller banks for the entire building.

I haven't been to the building yet but I'm almost 90% sure that the building might not have the available electrical infrastructure to handle that much power. It's a residential condo building.

Maybe I can ask utility to give me a new second service at 480V 3-phase?
 
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Tainted said:
building service is only 208V 3-phase... My mechanical engineer gave me all these loads that he wants to power. He is trying to completely electrify cooling and heating with these chiller banks for the entire building.

I haven't been to the building yet but I'm almost 90% sure that the building might not have the available electrical infrastructure to handle that much power. It's a residential condo building.

Maybe I can ask utility to give me a new second service at 480V 3-phase?
Click to expand...
How far is it from the transformer to the chillers? You might save a lot of money on wire
 
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