Feeder neutral calculation for washer/dryers

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Greg1707

Senior Member
Location
Alexandria, VA
Occupation
Business owner Electrical contractor
I am running a feeder to a 100 amp panel that will supply 3 residential type electric dryers and 2 washing machines. The name plates on the washing machines state 10 amps. The dryer name plate states 30 amps. (is that really true? they draw 30 amps??).

However, my question is: does anyone have any idea what the maximum unbalanced load would be? The washing machines can contribute a maximum of 10 amps.
 

Smart $

Esteemed Member
Location
Ohio
I am running a feeder to a 100 amp panel that will supply 3 residential type electric dryers and 2 washing machines. The name plates on the washing machines state 10 amps. The dryer name plate states 30 amps. (is that really true? they draw 30 amps??).

However, my question is: does anyone have any idea what the maximum unbalanced load would be? The washing machines can contribute a maximum of 10 amps.
Is this 120/240 1?, 120/208 1?, or 208/120 3??

Doesn't matter what the dryers actually draw if the nameplate says 30A. But does it really say they draw 30A or does it say that is the minimum circuit ampacity (MCA)?

Anyway, 220.61(B)(1) implies the maximum [neutral] unbalanced load is determined by Table 220.54 for dryers. With only 3 dryers, you can reduce to 70% of 100%. No reduction for washers, and that assumes on different legs... so 10A. But if on same leg the max unbalanced is 20A.
 

kwired

Electron manager
Location
NE Nebraska
Sorta involved in a movie right now (paused, but want to get back to it)... but I believe there is no permitted reduction for the neutral of a 120/208 system... see 220.61(C)(1).

Since we are sizing the neutral for the feeder here, wouldn't it still depend on whether or not we have all three phases or just two phases and neutral making up the circuit, as well as load balancing?

Greg, since you have indicated this is a 208 volt system you may want to double check just what the nameplate of the dryers actually is for. Good chance it is rating at 240 volts and if so it will draw less connected to a 208 volt source. Most dryers I have seen are 5000 - 5500 watt @ 240 volts which is only about 23 amps max @ 240 volts, plus the motor which is usually 120 volt and maybe 5 amps max.
 

Smart $

Esteemed Member
Location
Ohio
Since we are sizing the neutral for the feeder here, wouldn't it still depend on whether or not we have all three phases or just two phases and neutral making up the circuit, as well as load balancing?
Does this 120/208 feeder meet the description?

Any portion of a 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system

IMO, it does...

There is as oddball chance the POCO would set up a 120/208 system compared to a common 208/120 system feeding several 120/208 services. However, I believe the gist of the subsection is that neutral currents of 120/208 loads do not cancel out like split-phase 120/240 loads. Even with balanced single phase loading, the feeder neutral will experience a magnitude of current about, if not equal that of the heavier-conducting ungrounded conductor.

Since the OP has no straight 208V loads, I don't see how the Code would permit any reduction in neutral sizing.
 

kwired

Electron manager
Location
NE Nebraska
Does this 120/208 feeder meet the description?

I don't know, the OP did not say. His question was about sizing the feeder neutral (I think). If he has a feeder consisting of all three phases and he balances the loads then no I don't think he has to comply with rules associated with that description. The description definitely applies to the branch circuits either way.
 

Smart $

Esteemed Member
Location
Ohio
I don't know, the OP did not say. His question was about sizing the feeder neutral (I think). If he has a feeder consisting of all three phases and he balances the loads then no I don't think he has to comply with rules associated with that description. The description definitely applies to the branch circuits either way.
From Posts 2 and 3...

Is this 120/240 1?, 120/208 1?, or 208/120 3??
I should have mentioned this is 120/208.
 

Smart $

Esteemed Member
Location
Ohio
He did not say whether it is single phase or three phase that I can see. It is obviously supplied by a three phase wye system at some point, but we do not know if it is a three wire or four wire feeder.
Being that I asked 120/208 1? or 208/120 3? and he replied 120/208, I have to assume 120/208 1? 3-wire.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Since we are addressing a feeder, 215.2 comes into play. The feeder grounded conductor can be reduced to the calculated load, but not less than the size required for the equipment grounding conductor per 250.122 (see 215.2).
So in this case the minimum size would be a #8.
 

Smart $

Esteemed Member
Location
Ohio
Since we are addressing a feeder, 215.2 comes into play. The feeder grounded conductor can be reduced to the calculated load, but not less than the size required for the equipment grounding conductor per 250.122 (see 215.2).
So in this case the minimum size would be a #8.
Well the calculated load on the feeder neutral is what is being questioned here. Refer to 220.61.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Understood, and if we actually have 120/208 1 phase from a 3 phase system as his terminology implies we have a different situation than a 208/120 3 phase as you have noted.
I simply wanted to remind those reading that there is a minimum regardless of the outcome of the calculated load.
I should have stated that.
 

kwired

Electron manager
Location
NE Nebraska
Being that I asked 120/208 1? or 208/120 3? and he replied 120/208, I have to assume 120/208 1? 3-wire.

OK. I don't pay attention to what is proper way to state 120/208 vs. 208/120 because I don't believe many actually pay that close attention. If I want to know specifically if it is two phase conductors or three, that is the way I will ask, to be certain they understand.
 

Greg1707

Senior Member
Location
Alexandria, VA
Occupation
Business owner Electrical contractor
OP responds

OP responds

Wow, thanks for all of the interest in my question. Let me try to clarify the situation. I have a three phase 120/208 two hundred amp panel. I am installing a one hundred amp two pole breaker in this panel and running two hots and one neutral to a 100 amp single phase sub panel. This panel will supply power for three household type dryers and two washing machines. The dryers are rated at 22 amps @ 208 volts. The washing machines are rated at 10 amps @ 120 volts.

I was just curious about the dryers and how one would calculate the unbalanced load, say for just one dryer? I am sure it is not much but was just curious if there is an actual number out there for these machines?
 

jumper

Senior Member
I was just curious about the dryers and how one would calculate the unbalanced load, say for just one dryer? I am sure it is not much but was just curious if there is an actual number out there for these machines?

Calculating the neutral/unbalanced load between two phases of a three phase system can be a tad tricky IIRC.

For linear loads it can anywhere from 0 to 100% of the phase load I think.

Non linear loads can create neutral loads of up to 1.732 times the phase current I believe.

Here is a link on the subject from SQ D.

http://static.schneider-electric.us...ormers/Harmonic Mitigating/0104ED9501R896.pdf

I would just run a full sized noodle for a 100A panel and forget the hassle of figuring out a possible size reduction for this app.
 
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