Calculations for a Load Center

[stuartdmc]

I need your help. My electrical contractor is trying to size a load center to run several motors.
I'm helping him get through plan check and he doesn't think my calculations are correct.

If you would, and have a second, I have several motors all are 208v 3 phase,
We have 2 @ 70FLA, 5 @ 40FLA 2, @ 20FLA and 2 @ 25FLA.

My question is, I came up with a the total load of 430 amps correct? and therefore the load center shall be a minimum of 500amp panel.

Thanks for you comments.
Stu

 

[Stopmoving]

430.24 Several Motors or a Motor(s) and Other Load(s).
Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings
of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads.

Your total should be 465 amps prior to the addition of any other loads.

And your OCPD would be based on 430.62 based on the types used.

 

[Dennis Alwon]

You should not be using the FLA of the motors but rather Tables in art. 430
Table 430.250 for 3 phase.

 

[ray cyr]

I need your help. My electrical contractor is trying to size a load center to run several motors.
I'm helping him get through plan check and he doesn't think my calculations are correct.

If you would, and have a second, I have several motors all are 208v 3 phase,
We have 2 @ 70FLA, 5 @ 40FLA 2, @ 20FLA and 2 @ 25FLA.

My question is, I came up with a the total load of 430 amps correct? and therefore the load center shall be a minimum of 500amp panel.

Thanks for you comments.
Stu

Just curious, is this a panelboard with breakers feeding individual machines, or an MCC or an Art. 409 Industrial Control Panel?

 

[ray cyr]

430.24 Several Motors or a Motor(s) and Other Load(s).
Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings
of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads.

Your total should be 465 amps prior to the addition of any other loads.

And your OCPD would be based on 430.62 based on the types used.

How did you come up with 465 amps? I have tried a variety of calculations and haven't gotten that number yet.

 

[JWCELECTRIC]

Go with 600A panel or MCC in case there are future loads. Haven't seen a 500amp panel, do the panel Venders make one?

 

[suemarkp]

How did you come up with 465 amps? I have tried a variety of calculations and haven't gotten that number yet.

I'm thinking he added 150% of the largest motor, or added 125% of both the largest motors. Seems to me it should be 430 + 25%(70) = 447.5

 

[rodneee]

arrive @ 465

arrive @ 465

i have no clue what you guys are talking about as i only do new homes but i know how he arrived at his 465 number.

qty 2 times 70 equals 140 times 125% equals 175
qty 5 times 40 equals 200 200
qty 2 times 20 equals 40 40

qty 2 times 25 equals 50 50
total 465

 

[rodneee]

arrive @ 465 (easier to read format)

arrive @ 465 (easier to read format)

the page shifted my format . i will try again 2 units @ 70 = 140 X 125% =175. 5 units @40 = 200 2 units @20 = 40 2 units @25 = 50
........175 + 200 + 40 + 50 = 465......

 

[Dennis Alwon]

the page shifted my format . i will try again 2 units @ 70 = 140 X 125% =175. 5 units @40 = 200 2 units @20 = 40 2 units @25 = 50
........175 + 200 + 40 + 50 = 465......

Rod I don't believe you would take both 70 amp motors and multiply by 125%. Just the largest motor- if there are 2 or more largest motors then you use one of them not all.

1@70= 70 * 125%
1@70=70
etc

 

[charlie b]

I agree with Dennis, on both counts. Rare event, that. :grin: The "best available" answer, given the information in the first post, is 447.5 amps. You only count the one largest motor, not the pair of them, when you apply the 125% factor. But this is not the "right" answer, because we were given the wrong information. To properly size the MCC, you need to know the horsepower of each motor, not the nameplate FLA. Calculations are based on the amp values given in table 430.250, and the values given in post #1 do not correspond to any motors shown in that table.

 

[Dennis Alwon]

I agree with Dennis, on both counts. Rare event, that. :grin:

That's fine but don't make a habit of it. :grin:

 

[RETRAINDAILY]

go big

go big

at least 600 if you plan to stay small

 

[Pham Van Hung]

I need your help. My electrical contractor is trying to size a load center to run several motors.
I'm helping him get through plan check and he doesn't think my calculations are correct.

If you would, and have a second, I have several motors all are 208v 3 phase,
We have 2 @ 70FLA, 5 @ 40FLA 2, @ 20FLA and 2 @ 25FLA.

My question is, I came up with a the total load of 430 amps correct? and therefore the load center shall be a minimum of 500amp panel.

Thanks for you comments.
Stu

Rod I don't believe you would take both 70 amp motors and multiply by 125%. Just the largest motor- if there are 2 or more largest motors then you use one of them not all.

1@70= 70 * 125%
1@70=70
etc

Dear all,
I guess you are calculating amperage for ground fault and short circuit protection device for group of motor? aren't you?

If so you should not take FLA which is the rating marked on the motor nameplates, rather you should take the FLC (NEC 430. 250 table for FLC determination from motor HP)

The type of motors as well as voltage, HP ratings are not mentioned here in your post .
- You should refer to the NEC table 430.250 to get the FLC from motor HP, do not take FLA on the motor nameplate (NEC 430.6)
- and refer to NEC table 430.52 to get the multiplier (m) (corresponding to each type of motor and protection device you are going to choose).
- After that you apply the rule of largest FLC x m + FLC x number of smaller motor(s), and finally choose a protection device with the next lower value. Tell me what are your motor Hp ratings then I will try to size the protection device.

I agrre with Dennis to get only one highest ratings number.
HungPV

 

[Cold Fusion]

Stuart -
As dennis, charlie, and hung said, the ocpd is sized from table 430.250. Your EC would know that.

Motor nameplate FLAs are rarely the nice even numbers you listed.

Is this a homework question? It certainly sounds like one.

cf
PS: It's okay with me if it is - probably everybody else as well.