I have a table saw that is listed as 16 amp, 1 phase, 240 VAC. The manual specifies a 20 AMP slow-blow fuse. When I checked the NEC, I see that I can satisfy that requirement by installing a fused knife-switch (enclosed)ahead of the receptacle. My question concerns the wiring between the breaker panel and the knife switch. What size breaker do I need in the main panel, and what size wiring do I need between that breaker and the fused disconnect?
Motor Circuit Protection Question
- Thread starter bmenozzi
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Motor Circuit Protection Question
This thread was temporarily moved to the Relocation Forum for review by the Moderators. We have since learned that the question is intended to assist in negotiations with an electrical contractor. This is not, as I had briefly suspected, a DIY situation.
I am returning this thread to the NEC topic area, with apologies to the original poster for the delay.
[ October 28, 2004, 03:44 PM: Message edited by: charlie b ]
This thread was temporarily moved to the Relocation Forum for review by the Moderators. We have since learned that the question is intended to assist in negotiations with an electrical contractor. This is not, as I had briefly suspected, a DIY situation.
I am returning this thread to the NEC topic area, with apologies to the original poster for the delay.
[ October 28, 2004, 03:44 PM: Message edited by: charlie b ]
Re: Motor Circuit Protection Question
I assume this is a 3HP saw with NEC table FLA of 17A. This is pretty healthy for 1-phase operation, but anyway.
Based on info given, I'd probably use 30A/2-pole bkr. in panel & #10 cu wire. Normally, would probably use 25A fuses in the local fused sw., but if the manual says to use 20A, then use 20A.
Now, Code minimum could be 20A bkr. in panel & #12 wire.
I assume this is a 3HP saw with NEC table FLA of 17A. This is pretty healthy for 1-phase operation, but anyway.
Based on info given, I'd probably use 30A/2-pole bkr. in panel & #10 cu wire. Normally, would probably use 25A fuses in the local fused sw., but if the manual says to use 20A, then use 20A.
Now, Code minimum could be 20A bkr. in panel & #12 wire.
- Location
- Massachusetts
Re: Motor Circuit Protection Question
There is no minimum overcurrent device size only a maximum. Also the wire size is based on the load not the overcurrent device.
The opening post asked;
For the wiring between the panel and the disconnect if it is a 17 amp motor, the NEC would allow 14 AWG with a 40 amp standard breaker.
[ October 29, 2004, 05:18 AM: Message edited by: iwire ]
I disagree, that is not the code minimum.
There is no minimum overcurrent device size only a maximum. Also the wire size is based on the load not the overcurrent device.
The opening post asked;
I believe they will have the recommended fuse at the fused disconnect.
For the wiring between the panel and the disconnect if it is a 17 amp motor, the NEC would allow 14 AWG with a 40 amp standard breaker.
[ October 29, 2004, 05:18 AM: Message edited by: iwire ]
Re: Motor Circuit Protection Question
The motor plate is marked as 3.0 HP (2.2 KW), Code J. It is a single-phase, 240 VAC motor. The instruction manual for the saw states that the feed should be "not less than # 12 wire and protected by a 20 Amp timelag fuse." My NEC book is a 1984 edition, so I wouldn't be surprised if some of this had changed. According to table 430-148, the Full-Load Current in Amperes for a 230V single-phase motor is 17 Amperes. According to the instruction manual, the motor Branch-Circuit Short-Circuit and Ground-Fault Protection should be provided by a 20 Amp timelag fuse. When I go to Table 430-152, I find that the "Nontime Delay Fuse" value for a Code Letter J (full-voltage, resistor or reactor starting) is 300% of full load. In this case, it would be 17 Amps x 3 = 51 Amps. However, the "Instantaneous Trip Breaker" value is 700%, which would be 119 Amps. The intent is to have my electrical contractor install a sub-panel to feed a new woodworking shop that I am having built in an unfinished basement. So the motor feed will be supplied from that breaker panel. So, which do I specify, 50 Amps or 110 Amps? If he has to use a 110 Amp instantaneous trip breaker in the distribution sub-panel, then the motor branch-circuit and the fused disconnect switch have to be rated for 110 Amps. The fuses he will install in that disconnect should be 20 Amp timelag fuses, and the motor circuit conductor and the plug will be sized at 20 Amp. If I only need to have a 50 Amp breaker supplying the motor branch-circuit, then the rest of the motor branch-circuit will be sized accordingly. I'd hate to pay for a 110 Amp circuit if I only really need a 50 Amp circuit.
The motor plate is marked as 3.0 HP (2.2 KW), Code J. It is a single-phase, 240 VAC motor. The instruction manual for the saw states that the feed should be "not less than # 12 wire and protected by a 20 Amp timelag fuse." My NEC book is a 1984 edition, so I wouldn't be surprised if some of this had changed. According to table 430-148, the Full-Load Current in Amperes for a 230V single-phase motor is 17 Amperes. According to the instruction manual, the motor Branch-Circuit Short-Circuit and Ground-Fault Protection should be provided by a 20 Amp timelag fuse. When I go to Table 430-152, I find that the "Nontime Delay Fuse" value for a Code Letter J (full-voltage, resistor or reactor starting) is 300% of full load. In this case, it would be 17 Amps x 3 = 51 Amps. However, the "Instantaneous Trip Breaker" value is 700%, which would be 119 Amps. The intent is to have my electrical contractor install a sub-panel to feed a new woodworking shop that I am having built in an unfinished basement. So the motor feed will be supplied from that breaker panel. So, which do I specify, 50 Amps or 110 Amps? If he has to use a 110 Amp instantaneous trip breaker in the distribution sub-panel, then the motor branch-circuit and the fused disconnect switch have to be rated for 110 Amps. The fuses he will install in that disconnect should be 20 Amp timelag fuses, and the motor circuit conductor and the plug will be sized at 20 Amp. If I only need to have a 50 Amp breaker supplying the motor branch-circuit, then the rest of the motor branch-circuit will be sized accordingly. I'd hate to pay for a 110 Amp circuit if I only really need a 50 Amp circuit.
Re: Motor Circuit Protection Question
Using # 12 conductors
? Table 430.148 gives a 3HP motor an full load current of 17 amps
? 430.22(A) requires conductors be sized at 125% of full load current
? 17 x 125% = 21.25 amps
? Table 310.16, #12 THHN 75 deg C column = 25 amps
? Table 430.52 allows up to 250% for the inverse time breaker
? 17 x 250% = 42.5 amps
? 240.6 states we can go UP to the next STANDARD size breaker
? you can use a 45 amp breaker and # 12 conductors
Roger
Using # 12 conductors
? Table 430.148 gives a 3HP motor an full load current of 17 amps
? 430.22(A) requires conductors be sized at 125% of full load current
? 17 x 125% = 21.25 amps
? Table 310.16, #12 THHN 75 deg C column = 25 amps
? Table 430.52 allows up to 250% for the inverse time breaker
? 17 x 250% = 42.5 amps
? 240.6 states we can go UP to the next STANDARD size breaker
? you can use a 45 amp breaker and # 12 conductors
Roger
- Location
- Massachusetts
Re: Motor Circuit Protection Question
However I am sticking with my answer of a 40 amp breaker. I do not think we can apply 240.6 for this application due to the wording in 430.52(C)
Roger I forgot about the 125% so I was mistaken about using 14 AWG.
However I am sticking with my answer of a 40 amp breaker. I do not think we can apply 240.6 for this application due to the wording in 430.52(C)
Bob
ron
Senior Member
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- New York, 40.7514,-73.9925
Re: Motor Circuit Protection Question
How about #10AWG since this circuit is less than 100A and we have no idea what the terminations are rated for?
How about #10AWG since this circuit is less than 100A and we have no idea what the terminations are rated for?
- Location
- Massachusetts
Re: Motor Circuit Protection Question
OK Roger you got me twice in the same thread.
Now I'm mad :roll:
Ron 60 C or 75 C 12 AWG is rated 25 amps.
Bob
OK Roger you got me twice in the same thread.
Now I'm mad :roll:
Ron 60 C or 75 C 12 AWG is rated 25 amps.
Bob
Re: Motor Circuit Protection Question
It is agreed that minimum standards would be #12 wire with as large as a 45A bkr. It can be argued that these conductors are feeders & that minimum standards would limit the bkr. to 40A. Yes, the smallest bkr. that could be used needs to be only large enough to carry the starting current of the motor.
For a common, practical, design (without all the facts) - a 30A bkr. & #10 wire should be given consideration. NEC is not a design manual.
Notice charlie b's reasoning behind this question.
It is agreed that minimum standards would be #12 wire with as large as a 45A bkr. It can be argued that these conductors are feeders & that minimum standards would limit the bkr. to 40A. Yes, the smallest bkr. that could be used needs to be only large enough to carry the starting current of the motor.
For a common, practical, design (without all the facts) - a 30A bkr. & #10 wire should be given consideration. NEC is not a design manual.
Notice charlie b's reasoning behind this question.
- Location
- Massachusetts
Re: Motor Circuit Protection Question
A 17 amp load on 12 AWG is well within the rating of the conductor.
bmenozzi has stated:
Also IMO bmenozzi should be given the minimums allowed so that when he talks with his electrical contractor he will have the NEC facts.
He can always ask for more than minimum.
What will that do for us that 12 AWG on a 45 amp breaker would not?
A 17 amp load on 12 AWG is well within the rating of the conductor.
bmenozzi has stated:
This being the case the run length will not be long enough to be a problem.
Also IMO bmenozzi should be given the minimums allowed so that when he talks with his electrical contractor he will have the NEC facts.
He can always ask for more than minimum.
Re: Motor Circuit Protection Question
Kiloamp7,
The NEC articles and sections provide the guide to arrive at a safe installation.
As far as bare minimums, so are some very safe cars in the 20k range, so why would I need to buy one in the 50k range if I didn't want overkill.
Roger
[ October 30, 2004, 08:04 AM: Message edited by: roger ]
Kiloamp7,
and your point is?
The NEC articles and sections provide the guide to arrive at a safe installation.
As far as bare minimums, so are some very safe cars in the 20k range, so why would I need to buy one in the 50k range if I didn't want overkill.
Roger
[ October 30, 2004, 08:04 AM: Message edited by: roger ]
Re: Motor Circuit Protection Question
Sorry. When I made my 1st post, I didn't know about the sub panel. The 2nd post by bmenozzi explained that the bkr. in question would be in a residential subpanel. Guess I did not read his 2nd post close enough. So volt drop would probably not be an issue.
Not always accustomed to designing to minimum standards, so maybe sometimes I forget that there are sometimes slightly less expensive ways to accomplish a given task.
Sorry. When I made my 1st post, I didn't know about the sub panel. The 2nd post by bmenozzi explained that the bkr. in question would be in a residential subpanel. Guess I did not read his 2nd post close enough. So volt drop would probably not be an issue.
Not always accustomed to designing to minimum standards, so maybe sometimes I forget that there are sometimes slightly less expensive ways to accomplish a given task.
Re: Motor Circuit Protection Question
he said he was going to put a fused knife switch ahead of the receptacle,in this case the rating of the receptacle cannot be exceeded by the overcurrent protection device.If a receptacle is installed for a disconnect what is the purpose of the fused switch?
[ November 01, 2004, 02:08 PM: Message edited by: jap2525 ]
he said he was going to put a fused knife switch ahead of the receptacle,in this case the rating of the receptacle cannot be exceeded by the overcurrent protection device.If a receptacle is installed for a disconnect what is the purpose of the fused switch?
[ November 01, 2004, 02:08 PM: Message edited by: jap2525 ]
Re: Motor Circuit Protection Question
If instruction book says to protect with 20A time-lag fuses, then these fuses need to be somewhere in series with the ckt. for this saw.
Since panelboard is a ckt.-bkr. type, & not a fusible-type, the purpose of the fused switch is to hold the fuses, right?
If instruction book says to protect with 20A time-lag fuses, then these fuses need to be somewhere in series with the ckt. for this saw.
Since panelboard is a ckt.-bkr. type, & not a fusible-type, the purpose of the fused switch is to hold the fuses, right?
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