I as a maintenance tech. have been asked to install more machinery. I looked at the panel that I am to get power from and see that it is listed as a 400 Amp Max. Already, I have a total of 23 20-Amp single poles and 5 30-amp 3 phase and 2 30-amp 2 phase circuit breakers. A total possible draw of 670 amps if everything was running at the same time. Is this OK? Since normally only half of what is connected is ever running at once. I in question about this.:-?
Panel calculations
- Thread starter IMNTRSD
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Bobhook149
Senior Member
Throw an amp probe on the panel in question and monitor at intervals during the day to see what the max draw is and go from there.
Bobhook149
Senior Member
what are you installing off the panel?
In some cases I may be installing off the panel. In other cases, I am asked to put additional motors on existing circuits. For instance, I have a 12 amp motor and a 5 amp motor on the same circuit. that is 17 amps on a 20 amp circuit. I cannot see putting any more on that circuit. If the amps drawn can possibly over draw the circuit rating, it seems illogical to me to add more to the circuit.
tryinghard
Senior Member
- Location
- California
The breakers total amps often do add up to more than the panel rating, and this can be fine.
Notice 408.30, you have to know the load at least as calculated per 220.1. Example D3 in Annex D helps to follow for a visual. Also notice a recent newsletter Commercial Loads - Part 1 from Mike Holt on this topic.
Notice 408.30, you have to know the load at least as calculated per 220.1. Example D3 in Annex D helps to follow for a visual. Also notice a recent newsletter Commercial Loads - Part 1 from Mike Holt on this topic.
No I have not. I'm not sure just how to accomplish that under the circumstances. Be patient with me - I have a lot being thrown at me at this moment. I need some direction.
tryinghard
Senior Member
- Location
- California
The circuit conductors are sized from the horsepower of the motor and tables in 430 x 1.25% - not the nameplate, the OCPD breaker is sized from 430 x 2.50% - not the nameplate. Really the only thing sized from the nameplate FLA are the OL's unless you have multiple motors. Multiple motors on a circuit are common but the feed must be sized per 430.
thank you - that was very helpful
tryinghard
Senior Member
- Location
- California
You betcha, these can be quite entailed but once your set up they just flow. I use a spreadsheet built from Article 220 and the examples.
Motors can be complicated as well; patients and persistence are required to determine this information.
Motors can be complicated as well; patients and persistence are required to determine this information.
I have 6 - 20amp circuits I can work with. These are already in use and we are concidering revamping our production layout. I have 3 belt sanders that pull 5.3 amps each that are 208 - 3ph.; 6 bench grinders that pull 4 amps each that are 110; and one torit that pulls 12 amps that is 208 - 3ph.
My thinking is: I can run the 3 belt sanders on 1 - 208 circuit (that would take 3 of the 6 20A circuits). Then I could run the torit on the remaining 3 20A circuits to make a 208/3ph circuit, (since it pulls 12 Amps). Then all I'd need to do is run 2 - 110 circuits for the remaining 6 bench grinders, (that would be 12 Amps per circuit, 3 bench grinders per circuit). Does that make any sense?
As tryinghard noted, you have to be careful here. At first glance this would appear to be a possible violation, depending on what shortcircuit devices you might have downstream. If these two motors don't have individual short-circuit/ground fault protection (with the 20 amp breaker being the only SCGF protection) then a 20 amp breaker would be a violation of 430.52. (5 x 2.5 = 12.5 so max breaker would be 15).
Without additional branch circuit protection downstream, it's often very difficult to have multiple motors on one SCGF breaker as the one breaker is often oversized for individual protection.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
You need to quit looking at nameplate amps for circuit sizing, look at the HP, then look up the ampacity in Table 430.248-430.250 and start sizing circuits from there.
Branch and Feeder overcurrent protection as well as circuit conductors are sized from the FLC in the NEC motor tables.
Overloads are the only thing you size from the motor nameplate amps, that's it.
There are a couple exceptions to this(like Home Depot air compressors that don't list HP, only amps) but for what your doing, the HP should be listed.
You need to get very familiar with Art. 430.
tryinghard
Senior Member
- Location
- California
A compliant application here will require a great amount of time and energy to qualify correctness. You cannot do this without NEC. A glance at minimum (these listed motors only) for your panel and feed size should include the following:
Motor------HP-----PH-----V-----Table A---Name Plate
Belt 1-----1.5 ?----3-----208-----6.6--------5.3
Belt 2-----1.5 ?----3-----208-----6.6--------5.3
Belt 3-----1.5 ?----3-----208-----6.6--------5.3
Bench 1---3/4 ?----1-----110-----6.4--------4
Bench 2---3/4 ?----1-----110-----6.4--------4
Bench 3---3/4 ?----1-----110-----6.4--------4
Bench 4---3/4 ?----1-----110-----6.4--------4
Bench 5---3/4 ?----1-----110-----6.4--------4
Bench 6---3/4 ?----1-----110-----6.4--------4
Torit-------5 ?-----3-----208-----16.7-------12
Panel & Feeder 430.62
Torit (Table Amp x 2.50)--250.0%--41.75
Belts (Table Amp)------------------19.8
Benchs (Table Amp)----------------38.4
Total Amp-------------------------99.95
Min OCPD-------------------------100
Min Conductor-------310.15
tryinghard
Senior Member
- Location
- California
See 430.53(A) you cannot combine these on one circuit because they are over 6A, unless you comply with (B) & (D). But then you need to see 430.109(F), 430.110(A) & (C).
Also each motor will need to be qualified with proper OL protection as well 430.32. While your code book's out review 430.6(A), again there's a lot to check.
tryinghard
Senior Member
- Location
- California
----------HP-----PH---------V-----Table A--Name Plate
Belt 1----1.5 ?----3--------208------6.6------5.3
Belt 2----1.5 ?----3--------208------6.6------5.3
Belt 3----1.5 ?----3--------208------6.6------5.3
Motor Circuit Conductor 430.24
Belt 1 (Table Amp x 1.25)----8.25
Belts (Table Amp)-----------13.2
Total Amp------------------21.45
Max OCPD-------25*
Min Conductor 310.15
* Violation of 430.52 in that the max OCPD allowed for one of these motors = 15A breaker
glene77is
Senior Member
- Location
- Memphis, TN
IMNTRSD,
I use this summary when working with motor circuits.
(1) Primary fuse protects for Short Circuit current.
--- 1 second at 400% maybe.
(2) Circuit Breakers protect for Start Surge current.
--- 10 seconds at 250% maybe.
(3) OverLoads protect for long term running conditions.
--- long term at 1.25% maybe.
I look for specs in that order,
and physically connect them in that order.
And,
just for reference,
here is a calc for Neutral Currents, 3 phase:
**
EL_IneutralCurrent_equation.txt
Ineutral =
SQRT (
I?A + I?B + I?C
- (IA x IB) - (IB x IC) - (IC x IA)
)
:smile:
I use this summary when working with motor circuits.
(1) Primary fuse protects for Short Circuit current.
--- 1 second at 400% maybe.
(2) Circuit Breakers protect for Start Surge current.
--- 10 seconds at 250% maybe.
(3) OverLoads protect for long term running conditions.
--- long term at 1.25% maybe.
I look for specs in that order,
and physically connect them in that order.
And,
just for reference,
here is a calc for Neutral Currents, 3 phase:
**
EL_IneutralCurrent_equation.txtIneutral =
SQRT (
I?A + I?B + I?C
- (IA x IB) - (IB x IC) - (IC x IA)
)
:smile:
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