Help with article 230 Part III Service calcs.

[donf]

Greetings, I am 3 class nights away from finals for Part 1 of the NEC.

We are doing service feeder calcs for a single family dwelling and for the life of me, I cannot find a demand factor listed in the code.

What I am trying to locate and verify is the 25% demand factor used in calculating the "Largest appliance motor".

For example, the diswasher, we were given a 1kW D/W and told to get the largest motor calc for that D/W simply multiply the 1kW by 25% (1000va*0.25=250va).

The 250VA would then be added to the loads.

Where is this calculation supported in the NEC 2008 code?

 

[augie47]

I think you may be talking about the 75% demand factor in 220.53

 

[Dennis Alwon]

Look at 220.18(A). That I believe is what you want.

220.18 Maximum Loads.
The total load shall not exceed the rating of the branch circuit, and it shall not exceed the maximum loads specified in 220.18(A) through (C) under the conditions specified therein.
(A) Motor-Operated and Combination Loads. Where a circuit supplies only motor-operated loads, Article 430 shall apply. Where a circuit supplies only air-conditioning equipment, refrigerating equipment, or both, Article 440 shall apply. For circuits supplying loads consisting of motor-operated utilization equipment that is fastened in place and has a motor larger than hp in combination with other loads, the total calculated load shall be based on 125 percent of the largest motor load plus the sum of the other loads.

 

[jumper]

How about this?

220.50 Motors. Motor loads shall be calculated in accordance
with 430.24, 430.25, and 430.26 and with 440.6 for
hermetic refrigerant motor compressors

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.

 

[hurk27]

I would not consider the motor of a dishwasher as the main item to base the load from, Maybe its just me, but most dishwashers motors are very small, and are only a fraction of the dishwashers load, to me its the heating element that grabs my attention, some of these can be many times the current of the motor?

Am I wrong of thinking this way?

 

[jumper]

I would not consider the motor of a dishwasher as the main item to base the load from, Maybe its just me, but most dishwashers motors are very small, and are only a fraction of the dishwashers load, to me its the heating element that grabs my attention, some of these can be many times the current of the motor?

Am I wrong of thinking this way?

I agree. I missed the DW part.

 

[hurk27]

I agree. I missed the DW part.

I could be wrong, as code doesn't really define what kind of load a dishwasher is, but basing it on the motor to me is wrong?

 

[jumper]

I could be wrong, as code doesn't really define what kind of load a dishwasher is, but basing it on the motor to me is wrong?

A dishwasher is a mixed load, as you stated, IIRC it has a pump motor and a heating element. I doubt that it is ever going to be the largest motor load.

MCA would be based on both.

The teacher probably made a mistake or the OP read the question wrong, IMHO.

 

[e57]

IMO - a (Household) DW is an "Appliance" first then it is a mixed load - Rememebering this off the top of the head since I do calc's on spreadsheets that I just plug the load into on... But more than 4 appliances "fastened in place" is 75%....

OK - I'll look it up....

220.14 A) Specific Appliances or Loads.​

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An outlet for a specific
appliance or other load not covered in 220.l4(B) through
(L) shall be calculated based on the ampere rating of the​

appliance or load served.

220.53 Appliance Load - Dwelling Unites).​

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It shall be
permissible to apply a demand factor of 75 percent to the
nameplate rating load of four or more appliances fastened
in place, other than electric ranges, clothes dryers, spaceheating
equipment, or air-conditioning equipment, that are
served by the same feeder or service in a one-family, twofamily,​

or multifamily dwelling.

Uh - from the 05 code - so go check whatever you are being tested on.....