MDP caculation
- Thread starter Dbronx
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Let me imagine what you got there!
You should know your loads first. Which loads are considered as 'continues' and which are not. You need to classify them in order to compute for the correct size of the wires (125% of continues load amps plus the rest of the load in the circuit). With the size of wires done, you decide which breaker size protects the wires used, keeping in mind that you will only load up the breakers up to 80% of its rating (if not indicated as 100%-rated breakers).
The purpose of the main circuit breaker is to protect the panelboard bus, the transformer, and the wiring between the transformer and the panelboard. Keep in mind that the main breaker will be the rated higher than the sub-panel breaker sizes, else, your main will trip ahead of the sub-panel breakers. The MDP breaker size is found by multiplying the largest single phase amps on any leg of the breaker by 125%. ( I am assuming you cannot balance the loads especially if there are single-phase loads).
Caculated MDP
Caculated MDP
The SOW specified 50 amp services for housing units. So a 3 phase 50 amp service at 80% would be 14,410.66 watts X 10 = 144,106.63 watts divided by 360 = 400.29619 X 1.25 = 500.37024
A 500 amp service with two cables in parallel 4C 250 with 2/0 ground.The 50 amp services will need #8 with #8 ground. Is that correct ?
Caculated MDP
The SOW specified 50 amp services for housing units. So a 3 phase 50 amp service at 80% would be 14,410.66 watts X 10 = 144,106.63 watts divided by 360 = 400.29619 X 1.25 = 500.37024
A 500 amp service with two cables in parallel 4C 250 with 2/0 ground.The 50 amp services will need #8 with #8 ground. Is that correct ?
jumper
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It we assume that these are SFDs and subject to the NEC, then you could apply demand factors and have a smaller service load.
Annex D has some good examples.
Annex D has some good examples.
Split unit Air conitioners caculation
Split unit Air conitioners caculation
Fifty 9K Air-conditioners will be installed at a large K-span building. THe 9K air-conditioner is rated at 6.9 amps 115V specified a 15A CB, 6.9 X 115 = 793.5 watts The largest load is heating a resistive load no 125% for largest motor caculated. So 793.5 X 50 = 39,677 watts divided by 360 for 3 phase load = 110.208 X 125% = 137.760.
A 150 AMP service is needed a 4C 1/0 with #6 ground is needed correct.
Split unit Air conitioners caculation
Fifty 9K Air-conditioners will be installed at a large K-span building. THe 9K air-conditioner is rated at 6.9 amps 115V specified a 15A CB, 6.9 X 115 = 793.5 watts The largest load is heating a resistive load no 125% for largest motor caculated. So 793.5 X 50 = 39,677 watts divided by 360 for 3 phase load = 110.208 X 125% = 137.760.
A 150 AMP service is needed a 4C 1/0 with #6 ground is needed correct.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Lights, fridges, stoves, WHs, etc???
What is the total speced load and sq.ft. per unit?
These are dwelling units,correct?
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
I am getting confused.
What exactly do you want to calculate?
Service, feeders or branch circuits?
For us to be able to chip-in, can you tabulate your loads per feeder. It would be a lot easier that way than wording it all out.:?:?
Fifty 9K Air-conditioners will be installed at a large K-span building. THe 9K air-conditioner is rated at 6.9 amps 115V specified a 15A CB, 6.9 X 115 = 793.5 watts The largest load is heating a resistive load no 125% for largest motor caculated. So 793.5 X 50 = 39,677 watts divided by 360 for 3 phase load = 110.208 X 125% = 137.760.
A 150 AMP service is needed a 4C 1/0 with #6 ground is needed correct.
This is a additional SDP for a large building, it is only for air conditioners loads. I want to caculate the service, then feeders .
Originally Posted by Dbronx
Fifty 9K Air-conditioners will be installed at a large K-span building. THe 9K air-conditioner is rated at 6.9 amps 115V specified a 15A CB, 6.9 X 115 = 793.5 watts The largest load is heating a resistive load no 125% for largest motor caculated. So 793.5 X 50 = 39,677 watts divided by 360 for 3 phase load = 110.208 X 125% = 137.760.
A 150 AMP service is needed a 4C 1/0 with #6 ground is needed correct.
This is a additional SDP for a large building, it is only for air conditioners loads. I want to caculate the service, then feeders .
Lemme try.
If you supply the load with 3-phase, it will be impossible for you to split your loads equally on the three phases. You will be forced to load 16 units on each of the two phases and the remaining 18 units loaded on the last phase.
Phase currents: 110.4 amps (16 x 6.9); 110.4 amps and 124.20 amps (18 x 6.9).
That would result into line currents of:
Ia =203.7 amps,
Ib = 191.22 amps, and
Ic = 203.7 amps.
So you take the largest current and design around that value. A 4/0 copper will do if the distance is just within 100 feet or a bigger wire if the distance is quite far.
203.7 amps x 1.25 = 254.625 amps. say 250 AT breaker.
Others, please check my math, been rusty for a while.
THe 9K air-conditioner is rated at 6.9 amps 115V specified a 15A CB, 6.9 X 115 = 793.5 watts The largest load is heating a resistive load no 125% for largest motor caculated. So 793.5 X 50 = 39,677 watts divided by 360 for 3 phase load = 110.208 X 125% = 137.760.
A 150 AMP service is needed a 4C 1/0 with #6 ground is needed correct.
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