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#210406 06/20/13 02:55 PM
Joined: Oct 2002
Posts: 830
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Could someone refresh me again on finding the load on 3 phase feeder? I have a combination of 3 phase motors and single phase motors. After finding the total amperage of several motors, and taking the largest at 125%, do you just take the total amperage to size the wire? I know I should know this, but it's been a while since I figured 3 phase load, and about 30 years since I took it in a code class and I just want to make sure. Thanks

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sparkync #210407 06/20/13 03:46 PM
Joined: Oct 2002
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Those loads are:

Motors:
20.3 amps 3 phase@ 125% = 25.38 amps (208/230 volts)
10.0 amps 3 phase@ 100% = 10.0 amps """""""""""""""
7.0 amps 3 phase@ 100% = 7.0 amps """""""""""""""
7.8 amps 1 phase@ 100% = 7.8 amps """""""""""""""
6.88 amps 1 phase 100%= 6.88 amps """""""""""""""
2.33 amps 1 phase 100% = 2.33 amps """""""""""""""
4.73 amps 1 phase 100% = 4.73 amps ( 115 volts)

Defrost heaters:
26.09 amps 1 phase 100% = 26.09 amps ( 208/230)
7.80 amps 1 phase 100% = 7.80 amps ( 208/230)

Lighting load:
15 amps 1 phase @ 100% = 15 amps ( 115 volts)

Total amps???? 113.01 ????

Is this right?? Thanks

sparkync #210408 06/20/13 06:00 PM
Joined: Jul 2004
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That is what it looks like to me. \

You still have a decision to make. Do you use #2cu and round up to a 125a breaker (minimum legal) or go with something bigger to give you a better chance of starting those motors if a couple come on at the same time.

I would certainly want some kind of load control to keep them all from trying to start at one time (like after a power hit)


Greg Fretwell
sparkync #210411 06/20/13 08:01 PM
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Greg, I was actually figuring on running 2/0 aluminum, good for 135 amps in the 75 degree column (can't seem to get #1 or 1/0 here, I'll have to double check on that). I am sort of wondering what size main breaker to put on it to compensate for start up though. Can I go up on the feeder breaker or is that just for branch circuits??

sparkync #210412 06/20/13 08:26 PM
Joined: Oct 2002
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I'm getting a little worried over the price I give for this job. I didn't think they would take it, but they took it right away:( Makes me think I'm missing something here. Consist of running approx. 45ft. feeder from inside panel in 2" conduit through block wall through (1) 2" LB, 2 90's, 1 45 up into a weatherproof 125 amp 3 phase panel to be mounted on roof of a new "freezer, cooler unit" with the motors I listed for condensing units and evaporators. Then from the panel to the units on roof. I gave a price of $ 5500.00, but am thinking to up it more. One thing I didn't figure was disconnects at the units, though most already have disconnects on them ( used units ). No final contract as of yet. Any input before I make my call to the customer to verify my price??? Sort of scary for me now. Thought I had it together, but am having doubts now frown

sparkync #210420 06/20/13 11:19 PM
Joined: Jul 2004
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Nobody ever went broke charging too much money.
You certainly want to put in the price of disconnects as an additional cost, if you are not sure the units have them. You can always make them happy by saying they were not necessary and they pay the lower number.
There is also something we always built into our contracts ... RISK.
Add some extra money for that stuff you are not thinking about or find after you start.


Greg Fretwell
sparkync #210424 06/21/13 07:58 AM
Joined: Oct 2002
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I have figured a 125 amp 3 phase panel on this job. If I have to up the main breaker to compensate for the "start up load" of the motors, then I will have to go with a larger rated panel, correct???

sparkync #210426 06/21/13 10:22 AM
Joined: Jul 2004
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Yes you are limited by the rating of the panel.
The compressors should have start up delays on them so that will help you out with that power failure thing and the heaters are pure resistive loads.
You are being somewhat conservative with the 1P loads counting all 3 phases when you are using 1 or 2 but be sure you balance them as well as you can.
I think you will be OK but that is really a Scott question when you get to this "design" stuff.


Greg Fretwell
sparkync #210427 06/21/13 01:15 PM
Joined: Oct 2002
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Just for further information input, when I listed the loads above, I put one down twice, so the total load will be 7.8 amps less = 105.21 amps instead of 113.01

sparkync #210428 06/21/13 02:32 PM
Joined: Oct 2002
Posts: 830
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Just found out Sq. D don't make over a 100 amp plug in breaker. That's the kind of panel I was going to have to come out of to feed the subpanel. A whole new ball game here. Now I have no option but to come out of a 6ft. traugh outside with a 200 amp wp disconnect, then feed the subpanel from there:( It's going to be fun tapping into the traugh connections:( Hope they left a pigtail and I don't have to bust a split bolt thing. There's already around 4 disconnects there now with meters. I think they should have have an architech do this planning for them. I seem to be the designer here. A lot of work... Well, on with the program:) Got to make a living somehow.... Thanks for all the input..

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