Motor loads with other loads

Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Someone called me and said they used a 60 amp breaker on a hot tub with 6/3 nm. The inspector turned him down so he showed them 240.4(B) and passed. I told him that if the load of the tub was 48 amps then the calculated load would be 48 x 1.25 = 60 amps thus 6/3 nm @ 60C would not work.

Then I started to think.... If we had 2 motors we would calculate one motor at 125% and then use the others at 100%. So why can't we use the larger loads of the hot tub (motor or heater) and just use 125% of the larger and then add the smaller load.

I believe this section does not allow it. Do you all agree with me? If so does anyone know why you can't use the smaller load at 100%.

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 the sum of each of the following:
  • (1)
    125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A)
  • (2)
    Sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A)
  • (3)
    100 percent of the noncontinuous non-motor load
  • (4)
    125 percent of the continuous non-motor load.
Click to expand...
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Dennis Alwon said:
Most are listed as 48 amps but require a 60 amp cir. and overcurrent protective device
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Do you have a picture of a typical nameplate? If it lists an MOCP and MCA for the whole unit, that makes the question easy. Otherwise I guess we have to add up the loads ourselves and apply 430.24?

Which brings to mind a related question: when an AHU has heat strips, or a hot tub has an electric heater, do those loads count as continuous for applying 430.24 and the like?

The 125% factor in 430.24(4) is the usual factor seen for continuous loads. As such, 430.24(4) should have the usual exception for the case that the conductors are protected by 100% rated OCPD, although that omission is not relevant to your question.

Cheers, Wayne
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
wwhitney said:
Do you have a picture of a typical nameplate? If it lists an MOCP and MCA for the whole unit, that makes the question easy. Otherwise I guess we have to add up the loads ourselves and apply 430.24?

Which brings to mind a related question: when an AHU has heat strips, or a hot tub has an electric heater, do those loads count as continuous for applying 430.24 and the like?

The 125% factor in 430.24(4) is the usual factor seen for continuous loads. As such, 430.24(4) should have the usual exception for the case that the conductors are protected by 100% rated OCPD, although that omission is not relevant to your question.

Cheers, Wayne
Click to expand...

I believe all heater loads are factored at 125%
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
LarryFine said:
It seems that a packaged unit would not need us to consider the individual components.
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That is how I see it since they give us most of the info however they never tell you the wire size. If you did separate the load then 6/2 nm would probably work.

I am curious why you can use the higher of 2 or more motors and only use 125% on that motor.
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
wwhitney said:
Because the extra 25% is an allowance for motor starting surge, and the assumption is that multiple motors won't start exactly at the same time?

Cheers, Wayne
Click to expand...
That makes sense but there is no surge from the heater as there is on a motor. I am pretty sure that if the total load of a spa was 48 amps we would just multiply that load by 125%. That means both heater and motor load is set at 125%. It seems unnecessary.

Do you agree that 430.24 number 4 would not allow us to calculate the loads independently - motor at 125% and heater at 100%
 
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