Temp Riser

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Probably overthinking this but trying to semi-design temp riser to 120/208v 100A panels on each floor(8 floors). I’m going to assume the calculated load is 80A.

Would you run a separate 80A feeder to each panel( keeping in mind VD as you go higher) or would you run say a feeder capable of say 300A and run that for every 3 panels and splice down on each floor? SB is splice box.


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I would use 750 kcmil aluminum with an ampacity of 385 amps and a 400 amp OCPD on the riser. Do you think that you'll really need more than 400 amps? If so then run one 750 riser for 4 floors.
 
infinity said:
I would use 750 kcmil aluminum with an ampacity of 385 amps and a 400 amp OCPD on the riser. Do you think that you'll really need more than 400 amps? If so then run one 750 riser for 4 floors.
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Other than the fact I would use 2 parallel 250 aluminum instead of a single 750, I am with infinity. You can either put 2-200A breakers, or 1-400A breaker. Either way, you are probably only 100 Feet max so VD should be a real problem.
 
Strathead said:
Other than the fact I would use 2 parallel 250 aluminum instead of a single 750, I am with infinity. You can either put 2-200A breakers, or 1-400A breaker. Either way, you are probably only 100 Feet max so VD should be a real problem.
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Thank you. You mean VD should NOT be a real problem??

2 sets #250 would give me about 500A.
 
infinity said:
I would use 750 kcmil aluminum with an ampacity of 385 amps and a 400 amp OCPD on the riser. Do you think that you'll really need more than 400 amps? If so then run one 750 riser for 4 floors.
Click to expand...
infinity said:
I would use 750 kcmil aluminum with an ampacity of 385 amps and a 400 amp OCPD on the riser. Do you think that you'll really need more than 400 amps? If so then run one 750 riser for 4 floors.
Click to expand...
Your using 60 degree column? Edit - Sorry aluminum your figuring. Sorry.
 
What type of load ? Is this a temp for construction purposes or are there any equipment loads being fed ?
How did you arrive at the 80 amp estimate ?
 
Alwayslearningelec said:
Thank you. You mean VD should NOT be a real problem??

2 sets #250 would give me about 500A.
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2 sets of 250 gives you 410 amps. Not sure what table you are looking at. Yes I meant shouldn't. Regarding the "calculation". The main load is lighting. With LED light now, if you use .5 watts per foot that is a lot. You didn't say how many square feet you are feeding, but most of the other power nowadays is battery chargers. So a floor of 8000 square feet should be easily safe at 50A single phase 240 volts.
 
Alwayslearningelec said:
Well they call for 100A panel so I just assume it might be loaded to 80%. It's for temp lighting and hand tools.
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My guess is that 80 amps is way too high for some LED temp lights and hand tools. Most hand tools are cordless anyway. Now if you had a bank of welders or electric heaters on each floor than that would be different. Having said that I would run a single 400 amp riser and tap off to each panel. If that proves to be too small than you can reconfigure by adding an additional riser later.
 
Strathead said:
2 sets of 250 gives you 410 amps. Not sure what table you are looking at. Yes I meant shouldn't. Regarding the "calculation". The main load is lighting. With LED light now, if you use .5 watts per foot that is a lot. You didn't say how many square feet you are feeding, but most of the other power nowadays is battery chargers. So a floor of 8000 square feet should be easily safe at 50A single phase 240 volts.
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infinity said:
My guess is that 80 amps is way too high for some LED temp lights and hand tools. Most hand tools are cordless anyway. Now if you had a bank of welders or electric heaters on each floor than that would be different. Having said that I would run a single 400 amp riser and tap off to each panel. If that proves to be too small than you can reconfigure by adding an additional riser later.
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Ok thanks. So once again I get confused with sizing conductors as it relates to the OCPD. So even if thought a panel could be rated for 100A if the calculated load is 50A the OCPD and conductors need to be sized to the calculated load? In short the rating/size of a panel MCB or bus could have no bearing on the conductor/OCPD size needed?
 
That is why I asked about the nature of the load. Most often "Load" is your key. You can't ignore breaker sizes but your conductor should be sized based on the load then the breaker sized to protect the wire
 
Using this as an example. There's an 800A main breaker being fed with 2 sets #500. The "branch" breakers in the panel and trip rating come out to about 875A. So is the calculated load the trip ratings?

Sorry calculations intrigue me but also confuse me sometimes.

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Nope. The actual load on each breaker was actually taken into consideration in determining the trip rating of that breaker. The total load was used to size the main breaker. Think of it as your home panel. Several individual breakers each sized for the load with a main sized on the total load not the total breaker size. On your graphic there should be an accompanying load sheet showing the load on each breaker and a total load. The wire sizes should be based on those loads (adjusted to breaker ratings)
 
Alwayslearningelec said:
The "branch" breakers in the panel and trip rating come out to about 875A. So is the calculated load the trip ratings?
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As Augie said that number is irrelevant. A load calculation of some sort is needed to know the load. Since this is for temp if you knew that each floor had X watts of lights and X temporary drops for hand tools you could fudge some type of load number.
 
Alwayslearningelec said:
Ok thanks. So once again I get confused with sizing conductors as it relates to the OCPD. So even if thought a panel could be rated for 100A if the calculated load is 50A the OCPD and conductors need to be sized to the calculated load? In short the rating/size of a panel MCB or bus could have no bearing on the conductor/OCPD size needed?
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Short answer is no. Sizing conductors isn't that difficult in general. First, like the English language, for the code every rule has an exception. So barring exceptions...
First a conductor can never have an ampacity that is less than the calculated load. Remember the conventional way of calculating the load starts with article 220, but using a 30 day study is another method. Regardless of the OCPD that rule can't be broken. Next, a circuit breaker or fuse must be rated to protect the conductor. So, if the conductor is rated 2100 amps the breaker must not be larger than 2100A. That is where the exceptions come in, in force. The main one is up to 801 amps you can size the OCPD to the next larger size. Another major exception is motor and HVAC loads, where the wire must be sized to the ampacity, but the breaker is sized to various other codes. But in my career, I have rarely seen an engineered job where they don't upsize the wire to the breaker size anyway.
 
Alwayslearningelec said:
So even if thought a panel could be rated for 100A if the calculated load is 50A the OCPD and conductors need to be sized to the calculated load? In short the rating/size of a panel MCB or bus could have no bearing on the conductor/OCPD size needed?
Click to expand...
In this case with your diagram it involves tap rules so you couldn't feed the 100 amp breaker with 50 amp conductors even if the calculated load were 50 amps. You could use a 50 amp breaker with 50 amp conductors.
 
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