3P Check my Math

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Cletis

Senior Member
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I punched something in real quick and used all appropriate articles without a calculator for the moment til tonight. Can you check my math

Running new feeder off a 3Phase panel 120V/208V off a 90 or 100 amp 3 phase OCPD.

Will be feeding a 125 amp load center with 2 - Machines. 1- 60 amp single phase (MCA 56amps), 1-70 amp (MCA 66 amps)

Does running a #2 or #1 4 wire off a 90 amp 3 pole or 100 amp 3 pole sound correct?

Machine power coming mostly from heaters

Total Load (32,400VA)

15,600 amps phase A
7,200 amps phase B
7,200 amps phase C

Question is #2Al 3 pole 90 or #1 3 pole 100 amp breaker for feeder ?

These numbers are crude and may be off a bit. I"m having trouble with the unbalanced load determining my main ocpd right now for some reason.
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I may have missed my math so let's see what others say, but using MCA, I get 105 amps on one phase.
 

Cletis

Senior Member
Location
OH
I may have missed my math so let's see what others say, but using MCA, I get 105 amps on one phase.

Crap! I was afraid of that. I can only come out of this panel with a max 100 amp 3 pole ocpd. I hope someone else comes up with 99.9 amps on that unbalanced phase :D
 

Cletis

Senior Member
Location
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Making sure. These 2 machines are on a 2 pole 60 (208V) and 2 pole 70 (208V). May have missed that in first post
 

Smart $

Esteemed Member
Location
Ohio
Making sure. These 2 machines are on a 2 pole 60 (208V) and 2 pole 70 (208V). May have missed that in first post
What are the actual loads per breaker? Noncontinuous or continuous? (Thermostatically-controlled heaters suggest noncontinuous).

ETA: Nevermind... I see the MCA's in the OP... be back shortly...
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
Crap! I was afraid of that. I can only come out of this panel with a max 100 amp 3 pole ocpd. I hope someone else comes up with 99.9 amps on that unbalanced phase :D

You might check with your supplier, some panels with a 100 amp max OCP device will still accept a sub-feed lug assembly greater than a 100 which would allow you to feed a M/B panel (with consideration to relevant tap rules)
 

Cletis

Senior Member
Location
OH
You might check with your supplier, some panels with a 100 amp max OCP device will still accept a sub-feed lug assembly greater than a 100 which would allow you to feed a M/B panel (with consideration to relevant tap rules)

Ok. I found some more field information which was slightly off from what I was told. This is gonna be close
On 2 machines I got 39 amps and 46amps on full blast. Supposendly, the machine fires up for 10-15 min full blast then it cycles down to 60% rest of day. 10 hr work days. The SO cords on machines are #8. The circuits they are on now are number 8 and have been running fine for a year or so he said. The internal fuse blocks are 50 amp ones.
 

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Cletis

Senior Member
Location
OH
More pics

Also to note. We have 214Volts phase to phase at the panel we are leaving and running #1 Aluminum MC 4 wire on a 90 amp 3 pole or 100 amp. I'm guessing this is a contiuous load maybe ??? So, with this new info i'm calculating the following on the proposed 3 phase 90 or 100 amp load center

Phase A 13,200
B 4,800Va
C 4,800Va

In Reality
 

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augie47

Moderator
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Location
Tennessee
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State Electrical Inspector (Retired)
math comes back at 91 amps based on your nameplate.
IMO compliant, no; get by, probably
 

Cletis

Senior Member
Location
OH
math comes back at 91 amps based on your nameplate.
IMO compliant, no; get by, probably

Augie, would you mind posting your math on that? I'm trying to learn this all tonight and get it straight for once and all. It is the unbalanced leg that's throwing me off. I need to see how you got the 91

Thanks
 

Smart $

Esteemed Member
Location
Ohio
Augie, would you mind posting your math on that? I'm trying to learn this all tonight and get it straight for once and all. It is the unbalanced leg that's throwing me off. I need to see how you got the 91

Thanks
There's various ways to do it. Conventional and code compliant method is to add 1/2 of each load in VA then divide by 120V.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
There's various ways to do it. Conventional and code compliant method is to add 1/2 of each load in VA then divide by 120V.
If you know the current contributions for each line to line load and the load is known to be high PF, then you can do simple vector math to add the two currents that combine on each phase conductor. That is not how the NEC does it though.

For loads with low PF, the vector math gets more complicated and the results can be unexpected.
 

Cletis

Senior Member
Location
OH
If you know the current contributions for each line to line load and the load is known to be high PF, then you can do simple vector math to add the two currents that combine on each phase conductor. That is not how the NEC does it though.

For loads with low PF, the vector math gets more complicated and the results can be unexpected.

These machines are around 20yrs old. I"m guessing that means low PF ?
 
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