largest motor neutral

[treyp3]

When sizing a neutral for residential service and the largest motor is 120v, should I ad the 25% of largest motor to the neutral columb? If its multi family served by one feeder, should the largest motor be counted for every unit. Also I assume 240v largest motor means that there would be no addition to the neutral for largest motor (or should I use the largest 120v motor here). Stallcup's master study guide is contradicting itself.

 

[jbelectric777]

How many units? Do the calculation and if they are 100 or 200 amp services the SEU cable already has a downsized neutral (70%) max unbalanced load. If its an apartment building with a big service use 70% if its 240V single phase if its 208 three phase there is no derating allowed, the neutral has to be the same size as the ungrounded conductors (hots)

 

[kwired]

Neutral needs sized minimum same size as GEC for a service or EGC for a feeder. After that it needs sized to max unbalanced load of the ungrounded conductors that may be imposed on it.

When sizing conductors the 125% factor for continuous loads is for the temperature rating of terminals on an overcurrent device. Neutral conductors unless for the very few exceptions that allow an overcurrent device are allowed to be sized at 100% load at the applicable temperature rating. They also actually have a better chance of landing on a terminal that is rated 90 degrees C than an ungrounded conductor has of having a 90 degree termination.

Add: for three phase wye systems with non linear loads you still need to watch for additional heating from harmonic currents though.

 

[treyp3]

Im using a study book. The question in the book is, "what size neutral conductor is required for a multifamily dwelling 120/240 single phase". I have no problem with the rest of the neutral calc., (fixed appliances, lighting, range, dryer ). There are 15 units, and the largest motor is a 1200va garbage disp. (given by book). The answer they have given for largest motor nuetral is 300va total (1200 x 25%). This does not take the 15 units into account. Shouldnt you multiply the 300va times the 15 units? Furthermore, in some questions they leave out the highest 120v motor in the neutral calc. and refer to 210.19(A)(1)Ex2. What gives? Should I even be using the largest motor in a neutral calc.?