What Fuse and Wire size 100 H.P.?

True Test

New User
Location
Cary, Illinois
Hey Guys,

I am trying to figure out the Fuse and wire size for a 100 H.P. motor. I am an electrician and I dont do sizing on motors this big often. I think I figured it out from my code book with sizing the time delay fuse at 175% of the nameplate. But the charts that I am finding show the wire much larger than the 125% that the code says.

I have a nameplate rating of 112 A, for a 480 v, three phase motor. It looks like 196 amps, which I would use a 200 amp fuse for, and then I get 140 amps for my wire size that puts me at a #1 at 150 amps THHN. That does not seem right to me and the charts that I am finding tell me that I should be at a 2/0 or a 3/0. What am I missing? And what is correct? Also, I have a fused disconnect in my M.C.C. that is rated up to 200 amps that I was hoping I could use instead of ordering and replacing one for a 225A fuse.
 

kwired

Electron manager
Location
NE Nebraska
Hey Guys,

I am trying to figure out the Fuse and wire size for a 100 H.P. motor. I am an electrician and I dont do sizing on motors this big often. I think I figured it out from my code book with sizing the time delay fuse at 175% of the nameplate. But the charts that I am finding show the wire much larger than the 125% that the code says.

I have a nameplate rating of 112 A, for a 480 v, three phase motor. It looks like 196 amps, which I would use a 200 amp fuse for, and then I get 140 amps for my wire size that puts me at a #1 at 150 amps THHN. That does not seem right to me and the charts that I am finding tell me that I should be at a 2/0 or a 3/0. What am I missing? And what is correct? Also, I have a fused disconnect in my M.C.C. that is rated up to 200 amps that I was hoping I could use instead of ordering and replacing one for a 225A fuse.
Motor full load has to be taken from the tables at the end of 430, not the motor nameplate, part of the reasoning is if the motor is ever replaced (with same Hp anyway) the conductor should still be an acceptable size, those tables generally list the values for the worst efficiency/worst power factor motor you would ever find. Sizing overcurrent devices is not all that simple, but for most general duty motors you often can just go with some published data by the fuse or breaker manufacturer of the equipment you are using and usually be fine, problem is when you have some application where starting current is high, or high for longer than usual duration, but is still acceptable, then you may have a situation where you use higher setting devices as the code does permit when the normally used values will not allow for starting the motor.

The NEC FLC for a 100 HP 480 three phase motor is 124 amps. Your conductor must be at least 125% of 124 which gives you 155 amps, minimum conductor ampacity. Table310.15(B)(16) says a 75C 1/0 copper is only good for 150 amps - so you have to be larger than that as a minimum, you could have other adjustments that make it larger yet like ambient temp or number of conductors in a raceway.

General short circuit and ground fault protection by dual element time delay fuses is sized at 175% (see table 430.52) but that is a maximum value, that can be increased if doesn't allow the motor to start. Many other charts you find will recommend a 175 amp fuse, and I have seen a fair share of 100 hp well motors that only had 150 amp fuses that never trip on starting.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Hey Guys,

I am trying to figure out the Fuse and wire size for a 100 H.P. motor. I am an electrician and I dont do sizing on motors this big often. I think I figured it out from my code book with sizing the time delay fuse at 175% of the nameplate. But the charts that I am finding show the wire much larger than the 125% that the code says.

I have a nameplate rating of 112 A, for a 480 v, three phase motor. It looks like 196 amps, which I would use a 200 amp fuse for, and then I get 140 amps for my wire size that puts me at a #1 at 150 amps THHN. That does not seem right to me and the charts that I am finding tell me that I should be at a 2/0 or a 3/0. What am I missing? And what is correct? Also, I have a fused disconnect in my M.C.C. that is rated up to 200 amps that I was hoping I could use instead of ordering and replacing one for a 225A fuse.
Welcome to the forum.

You must use table values for the motor current, not nameplate, for selecting branch circuit conductor and OC protection.
100 HP @ 480 is 124 amps per Table 430.250
Conductor size is 124 times 125%=155 amp. This would be 2/0 @ 75 degree.
Max OC device per 430.52 = 124 times 175% for dual element fuses. This would be 217 amp. Next size up = 225 amp.

If this is just a garden variety motor and it is not a hard starting load the 200 amp fuses will probably be fine. Art 430.52 does not require you to use the max OC size.

Overload protection is another discussion.
 

kwired

Electron manager
Location
NE Nebraska
If this is just a garden variety motor and it is not a hard starting load the 200 amp fuses will probably be fine. Art 430.52 does not require you to use the max OC size.
Does the typical 60 - 100 hp crop irrigation well motor qualify as "garden" variety?;)
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
for motor circuits, you are allowed to separate the protective functions of a typical OCPD into two chunks.

The fuse or CB provides the short circuit protection. That is why you can exceed the "normal" values.

The overload protection is contained within the motor controller.
 

kwired

Electron manager
Location
NE Nebraska
for motor circuits, you are allowed to separate the protective functions of a typical OCPD into two chunks.

The fuse or CB provides the short circuit protection. That is why you can exceed the "normal" values.

The overload protection is contained within the motor controller.
The overload protection is often contained within the motor controller.

Sometimes it is within the motor, or maybe is a separate device that often is still at the controller location, and unlike short circuit and ground fault protection doesn't necessarily need to protect each conductor and in fact is not really protecting the conductor as much as it is protecting the motor. Three phase motors do require an overload device in all three lines, but a single phase motor only needs one device even if it runs on two ungrounded lines, what flows in on A has to flow out on B otherwise you have a fault instead of an overload.
 

kwired

Electron manager
Location
NE Nebraska
NEC is minimum sizes for safety. Has nothing to do with good engineering. You need to look at the motor curves in order to properly select the fuses that will protect it.
Yes, and no. The fuses generally only protect from short circuit and ground fault current, and protect the conductors more so than the motor. Overload protection is for protecting the motor, but incidentally does protect the conductor from overload as well.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
NEC is minimum sizes for safety. Has nothing to do with good engineering. You need to look at the motor curves in order to properly select the fuses that will protect it.
Yes, and no. The fuses generally only protect from short circuit and ground fault current, and protect the conductors more so than the motor. Overload protection is for protecting the motor, but incidentally does protect the conductor from overload as well.
Disclaimer - I rarely use fuses, most always use mag-only cbs
My inclination would be to lay out the motor starting curve on a fuse T-C curve and verify the motor would infact start. Especially if the LRC was high (energy efficient) or high inertial load (long time starting) That way one could just start out with 225% if needed - instead of waiting to see if it trips on starting at 175%.

And, if it is a high inertial load, also lay out the ovld curve. Be nice to know if you need class 30 overloads ahead of time.

Second disclaimer: I don't think I have ever used a combination fused starter - probably seen a few.
I would also want to verify that a starter that fits 175% (round-up) also fits 225% (round-down) fuse. Be a real bummer if the fuse size changes and the larger fuses didn't fit.

Probably severe overkill for an ordinary 100hp motor starter.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Suggestion -
Better yet, get a combination mag-only cb starter. They are maybe 20% more. This gets rid of a bunch of evils associated with fuses.

ice
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...

The NEC FLC for a 100 HP 480 three phase motor is 124 amps. Your conductor must be at least 125% of 124 which gives you 155 amps, minimum conductor ampacity. Table310.15(B)(16) says a 75C 1/0 copper is only good for 150 amps - so you have to be larger than that as a minimum, you could have other adjustments that make it larger yet like ambient temp or number of conductors in a raceway. ....
Don't forget voltage drop over distance.

Fuse sizing also depends on the TYPE of fuse you use.

I have a half dozen or so of those freebie "Slide Charts" that starter mfrs used to give out for free, but are getting rare. They all say that a 460V 100HP motor gets a 200A Dual Element Time Delay fuse. I have never had one of those be wrong. Yes it's "Engineering by catalog", but hey, this isn't rocket surgery...
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Don't forget voltage drop over distance.

Fuse sizing also depends on the TYPE of fuse you use.

I have a half dozen or so of those freebie "Slide Charts" that starter mfrs used to give out for free, but are getting rare. They all say that a 460V 100HP motor gets a 200A Dual Element Time Delay fuse. I have never had one of those be wrong. Yes it's "Engineering by catalog", but hey, this isn't rocket surgery...
mine is a square D. copyright 1987. says based on 1987 NEC. still works.

mine says 175A fuse, 200A TM breaker.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Engineer
I have never had one of those be wrong. Yes it's "Engineering by catalog", but hey, this isn't rocket surgery...
Those fuse/breakers recommendations are usually sized for the short circuit protection of the motor circuit and not the more critical running overload protection. To my knowledge they have not changed for more than 40 years. Another caution with the 'slide rules', is the thermal unit selection seems to be based on some type of secret formula, and is likely to err on the low side.
For example the Square D disclaimer reads roughly: Fuse sizes and circuit breakers trip amperes are approximate selections suitable for most installations. Thermal unit selections are not based on the NEC, but are selected from average full load currents.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
mine is a square D. copyright 1987. says based on 1987 NEC. still works.

mine says 175A fuse, 200A TM breaker.
Huh, I'm looking at one that says it was taken from the 2002 code, it says 200A fuses. I wonder if something changed? Not likely though, it's probably just a matter of interpretation of the ranges.
 

kwired

Electron manager
Location
NE Nebraska
Don't forget voltage drop over distance.
I think that was in reply to conductor sizing, but also don't forget conductor size, transformer impedance, etc. will effect how much inrush current is available when energizing the motor. Same fuse or breaker on same machine may work in one install and not another because of available current from the supply at startup.



Those fuse/breakers recommendations are usually sized for the short circuit protection of the motor circuit and not the more critical running overload protection. To my knowledge they have not changed for more than 40 years. Another caution with the 'slide rules', is the thermal unit selection seems to be based on some type of secret formula, and is likely to err on the low side.
For example the Square D disclaimer reads roughly: Fuse sizes and circuit breakers trip amperes are approximate selections suitable for most installations. Thermal unit selections are not based on the NEC, but are selected from average full load currents.
Correct about the overloads working for most applications. For Square D - the ones I am most familiar with, the slide chart also says it is for a NEMA 1 enclosure with three melting alloy units, probably intended to mean one of their own enclosures designed to house a single controller. If you size them using the digest you first need to know what starter you have, what it will be enclosed in (particularly the volume of the enclosure), how many thermal units are used, whether or not there is other heat producing equipment in the enclosure, and maybe some other details, then find the correct selection chart based on that information.
 
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