Q's on relationship between voltage drop and circuit protection

rlrobinhood

Member
Location
WY
Occupation
apprentice
Hi all,

I'm trying to understand the relationship between upgrading wire sizes to compensate for voltage drop and circuit protection. So, let me use an example:

A customer wants a 50 amp sub-panel 375' away from his main meter base. He will use this to power a 240 volt irrigation pump that requires 20 amp protection. Assuming an acceptable 5% voltage drop and 50 amp service at the subpanel, I believe the wire size should be #2 aluminum. The lugs on the sub-panel will accept this wire size. However, the 50-amp breaker (GE) at the main panel will not.

I believe there are two options to move forward on this:

1) Should the breaker on the main panel be up-sized to 100 amp to accept the upsized wire? I believe a 100 amp breaker is sized for #2 wire and the subpanel is rated at 100 amps.

2) Use a mechanical splice and splice the #2 aluminum wire to a short pigtail of #6 to allow for the connection to a 50-amp breaker.

I believe either of these is acceptable, what are your thoughts and preferences? Thanks in advance. I'm just struggling with the relationship between voltage drop and circuit protection. Many thanks!!
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I believe there are two options to move forward on this:

1) Should the breaker on the main panel be up-sized to 100 amp to accept the upsized wire? I believe a 100 amp breaker is sized for #2 wire and the subpanel is rated at 100 amps.

2) Use a mechanical splice and splice the #2 aluminum wire to a short pigtail of #6 to allow for the connection to a 50-amp breaker.
I would use option #1. The CB is only being used as a disconnecting means since the feeder is protected at its origin by the 50 amp OCPD.
 

gar

Senior Member
201121-0924 EST

I believe the wrong question is being ask.

The first question should be --- how much voltage drop can be tolerated at the motor under startup conditions, and what is the current at this motor voltage?

Suppose the motor is 15 A at 240 V with normal full load on the motor. As a guess assume starting current is 100 A at a motor voltage of 200 V, and that this is an acceptable starting condition. Then source impedance could be 40/100 = 0.4 ohms, not considering the actual vector math. Assume all impedance is resistive at 0.4 ohms.

The total loop length is 2*375 = 750 ft. Or 0.54 ohms at 1000 ft. Number 7 copper wire is about 0.5 ohms per 1000 ft at 25 deg C. So use #6.

The next problem to solve is breaker size. This will be determined by instantaneous trip level, and by duration of the high motor starting current. But breaker must be designed to at least protect the wire. However a smaller breaker could be used. The input breaker could be designed to protect the motor so long as it protects the wire as well.

If you undersize the wire and cause higher voltage drop to save money, then you may get unwanted breaker trips on motor startup.

A QO 35-70 A breaker will accept #8 to #2 wire sizes.

.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
#2 Al at 350 feet, 240 V supply, 50A load would have a voltage drop of 4.3%, better than your goal of 5%

#2 Al has a 75C rating of 90A so in general the largest breaker you could use is a 90A breaker

If you use a 50A breaker then your code required egc would be #6 Cu or #4Al (#10 or #8 upsized to match increased size of circuit conductors).

If you use a 90A breaker then a smaller egc is arguably code compliant, since the 90A rated circuit conductors are being protected by a 90A breaker and thus not increased in size.

(An alternative argument is that the wire is increased in size relative to the calculated load, and thus the egc must similarly be increased in size.)

IMHO the installation with the 50A breaker would be safer, but both would be code compliant making the safer install a design decision.

Jon
 

rlrobinhood

Member
Location
WY
Occupation
apprentice
Thanks all. So, disregarding the grounding situation. What I'm hearing is I you feel the #2 AL is appropriately sized to run the pump/provide 50 amp service with less than 5% voltage drop.

And that I still have two options and they both are acceptable:

1) a 90 Amp breaker at the main panel
2) a 50 amp breaker with a reducing pigtail or reducing pin to allow the wire to fit in the breaker.

Now with this being said and for my education, a 90 amp breaker still protects the wire. But, it provides no guarantees to the voltage drop at the end. But, a 50 amp breaker would both provide protection for the wire and trip when the voltage drop nears 5%. Are these accurate statements?

Lastly, what do you think of the 4.3% voltage drop for a 5 hp irrigation pump.

Thanks all. I'm really trying to learn the science and not just the "this is how we do it". Thanks!
 
Reducer pigtail, almost every time.

Thing is, you're not running a 90 amp feed, you're running a 50; the only reason to upsize the wire is for voltage drop. If you protect it at 90, the next person is going to assume 90 amps will be available and usable at the far end.

Add to that, the next person who comes along will see #2 on a 50 amp breaker and maybe stop and think about why somebody would do that (and hopefully get the right idea).
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Reducer pigtail, almost every time.

Thing is, you're not running a 90 amp feed, you're running a 50; the only reason to upsize the wire is for voltage drop. If you protect it at 90, the next person is going to assume 90 amps will be available and usable at the far end.

Add to that, the next person who comes along will see #2 on a 50 amp breaker and maybe stop and think about why somebody would do that (and hopefully get the right idea).
If they’re confused about the first scenario, the second will be no better.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
1 breakers do not trip on voltage drop. They trip on current. And not in a simple fashion; they allow a short duration overload, which can be several times the 'trip rating ' of the breaker.

Since a 5% voltage drop would mean more than 50A of current flow in the installation you described, the breaker will probably trip eventually at a 5% drop. But this might take hours.

The 50A breaker would likely not trip on a transient 200A current with corresponding voltage drop.

A 5 hp motor on this circuit is probably fine, but will probably experience 10% or so voltage drop during startup.

Jon
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'm sure there's a breaker size in between 50A and 100A that would also be an option. I know for sure that an 80A BR breaker takes #2. Not sure about GE, but breakers can usually take wire a couple sizes larger than the typical ampacity required, for this very reason.
 
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