Motor Calculation

[frank_n]

I need help sizing motor calculations. The job is to install the electric for a hood in a deli. There are 2 motors three-phase (intake and exhaust). I haven't done any work like this in a very long time.

First, I sized the feeder conductors. The large motor is 1-1/2 HP and the small motor is 1 HP. Using table 430.25 the large motor is 6.9 amps and the small is 4.8 amps. Using 125% of the largest motor plus the sum of the other motors, the total is 13.425 amps. Therefore I can use 14 THHN. (Will probably use #12 because I already have it)

Next, I needed to find the circuit breaker size (Square D QO). According to table 430.52, I multiply total amps by 800% (13.425 x 800% = 107.4 amps) 430.52(C)(1) Exception 1 allows the next larger standard size breaker 110 amps. Can this be right? Is a QO breaker considered "instantaneous?"

Next, I need to use figure out the feeder tap conductors. According to 430.28(3) I can use the same size (#14 THHN) as the feeder conductors.

Section 430.28 requires that the feeder taps be protected from an overcurrent device. 430.102 requires an individual disconnecting means for each controller. The disconnect shall be in sight of the controller. 430.52(B) & (C)(1) require the OCPD to carry the starting current for each motor. Using Time-Delay Fuses motor #1 OPCD would be (6.9 x 175% = 12.075a or 15 amp fuse) and motor #2 OCPD would be (4.8 x 175% = 8.4a or a 10 amp fuse).

Section 430.32 requires a Seperate Overload Device. This device depends on the service factor of the motor and is per the manufacturers specs.

The motors will be located on the roof and will require a disconnect. The disconnect does not have to be fused.

All of this looks good to me EXCEPT for the 110 amp circuit breaker.

Any experts out there care to critique me?

Frank

 

[frenchelectrican]

First, I sized the feeder conductors. The large motor is 1-1/2 HP and the small motor is 1 HP. Using table 430.25 the large motor is 6.9 amps and the small is 4.8 amps. Using 125% of the largest motor plus the sum of the other motors, the total is 13.425 amps. Therefore I can use 14 THHN. (Will probably use #12 because I already have it)

Frank ., #12 THHN/THWN will be fine that will handle it well.

Next, I needed to find the circuit breaker size (Square D QO). According to table 430.52, I multiply total amps by 800% (13.425 x 800% = 107.4 amps) 430.52(C)(1) Exception 1 allows the next larger standard size breaker 110 amps. Can this be right? Is a QO breaker considered "instantaneous?"

The QO series is not a instantaneous breaker a QO(B)320 breaker will handle both motor just fine unless stated otherwise.
[ side note I put in (B) that mean bolt in verison ]

also with standard breaker you can go high as 250% but normally most are sized at 175%

Next, I need to use figure out the feeder tap conductors. According to 430.28(3) I can use the same size (#14 THHN) as the feeder conductors.

you can but senice you mention #12 there so stick with #12.

Section 430.28 requires that the feeder taps be protected from an overcurrent device. 430.102 requires an individual disconnecting means for each controller. The disconnect shall be in sight of the controller. 430.52(B) & (C)(1) require the OCPD to carry the starting current for each motor. Using Time-Delay Fuses motor #1 OPCD would be (6.9 x 175% = 12.075a or 15 amp fuse) and motor #2 OCPD would be (4.8 x 175% = 8.4a or a 10 amp fuse).

Section 430.32 requires a Seperate Overload Device. This device depends on the service factor of the motor and is per the manufacturers specs.

The motors will be located on the roof and will require a disconnect. The disconnect does not have to be fused.

All of this looks good to me EXCEPT for the 110 amp circuit breaker.

Any experts out there care to critique me?

Frank

The remain question will come up here is the hood system will be tied into with the ANSAL system or not ? that will change the plan a little one of our members here have dangbat good diagram very simple to understand it clear.

That picture worth a thousand words.

However with the breaker just stay with common QO breaker as I mention size above but if it do trip you can upsize per nec art 430 or 440 one of the two place will mention there.

Merci,Marc

 

[Lcdrwalker]

Looks like both of you did a good job. The only thing that I would add is that I would use NEMA size 0 combination starters. The starters will give you the size of heater elements or electronic overloads. They can also easily connected to an Ansul and/or fire alarm system. Your rooftop 'toadstools' usually have three pole snap switches provided for disconnecting means.

 

[Jraef]

Edited note: I took longer typing my response...

<Snipped, OK to this point...>

Next, I needed to find the circuit breaker size (Square D QO). According to table 430.52, I multiply total amps by 800% (13.425 x 800% = 107.4 amps) 430.52(C)(1) Exception 1 allows the next larger standard size breaker 110 amps. Can this be right? Is a QO breaker considered "instantaneous?"

Wrong. The QO is a Thermal Magnetic "inverse time" breaker. I would use 125% of the load, rounded up to 20A, but the NEC allows up to but not exceeding 250%, rounded up to the next size. That doesn't mean you automatically use 250%, because if you do, you must size the conductors to the breaker rating. For example, your 13.425A, at 250% is 33+A, rounded up to the next breaker size makes it 40A. That means you would have to run wire rated for 40A, because you cannot protect 12ga wire with a 40A breaker.

The tricky part is, if the motors trip a 20A breaker on startup, you might have to up-size the circuit and then that means using larger wire. You could get around that by putting a restart timer on one of the motors, usually the supply fan, so that after a power failure they don't re-start at the same time. It's doubtful that the 6.9A fan motor would trip a 20A CB on starting current. The other alternative is to just run 10ga wire now and put in a 30A CB.

Next, I need to use figure out the feeder tap conductors. According to 430.28(3) I can use the same size (#14 THHN) as the feeder conductors.

Section 430.28 requires that the feeder taps be protected from an overcurrent device. 430.102 requires an individual disconnecting means for each controller. The disconnect shall be in sight of the controller. 430.52(B) & (C)(1) require the OCPD to carry the starting current for each motor. Using Time-Delay Fuses motor #1 OPCD would be (6.9 x 175% = 12.075a or 15 amp fuse) and motor #2 OCPD would be (4.8 x 175% = 8.4a or a 10 amp fuse).

Section 430.32 requires a Seperate Overload Device. This device depends on the service factor of the motor and is per the manufacturers specs.

The motors will be located on the roof and will require a disconnect. The disconnect does not have to be fused.

All of this looks good to me EXCEPT for the 110 amp circuit breaker.

Any experts out there care to critique me?

Frank

Not sure where you were going with all of this, you may be over reaching a bit. I'm assuming from the FLA's you quoted that you have a 3 phase 208V system. That means that you will need 3 phase motor starters for each of the motors. The simplest way to do this would be to buy "combination motor starters" which will have the proper size circuit breaker (or fused disconnect), contactor and overload device all in one box. Locate the combo starters within sight of the motors (using the right enclosure rating) and they will serve as the local lockable disconnect as well. Then all you need is a small gutter above them feeding them from your feeder breaker and a tap block (assuming they can be within 3 feet of each other). Just use the same size conductors for the feeder taps and you are done.

 

[tryinghard]

?I would use 125% of the load, rounded up to 20A, but the NEC allows up to but not exceeding 250%, rounded up to the next size. That doesn't mean you automatically use 250%, because if you do, you must size the conductors to the breaker rating.

This is not true; the conductors are protected by the overload protection not the breaker (OCP).

There is no disadvantage to size the OCP less than the 250%. The OCP in this application is only for short circuit & ground fault protection; the EGC might have to be larger than normal (but never larger than a phase conductor). Sizing it smaller or less does not give better protection. It needs to be sized from the appropriate NEC table, and not the motor nameplate, to accommodate LRA (start-up).

 

[Jraef]

This is not true; the conductors are protected by the overload protection not the breaker (OCP).

There is no disadvantage to size the OCP less than the 250%. The OCP in this application is only for short circuit & ground fault protection; the EGC might have to be larger than normal (but never larger than a phase conductor). Sizing it smaller or less does not give better protection. It needs to be sized from the appropriate NEC table, and not the motor nameplate, to accommodate LRA (start-up).

No, sorry. The OLs for each INDIVIDUAL motor circuit will not protect this FEEDER circuit in this case. Remember, he is branching it off to feed two individual motor starters. In the case of each individual motor circuit, that 250% would apply (assuming the OCP is an inverse time CB). But when he made this into a feeder circuit, article 430 ceases to apply.

 

[tryinghard]

...

First, I sized the feeder conductors. The large motor is 1-1/2 HP and the small motor is 1 HP. Using table 430.25 the large motor is 6.9 amps and the small is 4.8 amps. Using 125% of the largest motor plus the sum of the other motors, the total is 13.425 amps. Therefore I can use 14 THHN. (Will probably use #12 because I already have it)

Next, I needed to find the circuit breaker size (Square D QO). According to table 430.52, ?Can this be right? Is a QO breaker considered "instantaneous?"

Frank,

I only have my 2002 book with me right now but I think this stuffs the same. You really can use the Figure 430.1 at the beginning of Article 430 to locate this info and you may or may not find example D8 in Annex D helpful as well. If your service from the utility company is 208V 3ph I recommend using this for your calculations rather than 200, unless it really is 200V service.

Here?s another logical look at your application:

Feeder ? 430 Part III
6.6A = 1 1/2hp 208V 3ph (Table 430.250)
4.6A = 1 hp 208V 3ph (Table 430.250)

8.25A = 6.6 x 1.25 (1 1/2hp)
4.6A = (1hp)
12.85 = conductor ampacity
14 THHN = conductor size and type (Table 310.16 & 240.1(G))

Breaker (OCPD) for feeder ? 430 Part V
16.5A = 6.6 x 2.25 (1 1/2hp)
4.6A = (1hp)
21.1A = Breaker not to exceed (430.52(C))
20A = Breaker size, inverse time type

O.L?s (motor overload protection) ? 430 Part III
Both the 1 1/2hp & 1hp motors may already, factory, include this as ?TP? (Thermal Protector[tion]), see 430.32(A) & (B), this is common with these sizes. The motor nameplate will specify if these are included if not you must provide the OL protection as per 430.(A)(1) & (B)(1); usually 125% of the nameplate ampere not the table.

Tap Conductors
430.28 and usually (2) applies, ?at least 1/3 that of the feeder [ampacity]?.

Disconnect Means ? 430 Part IX
Location as per 430.102, usually next to the motor but in sight is compliant as well as a lockable supply.
Can be a snap switch for these sizes or cord and plug, see 430.109.
Ampere size must be at least 115% of the table amp as per 430.110.

 

[tryinghard]

Feeder ? 430 Part II not part III
6.6A = 1 1/2hp 208V 3ph (Table 430.250)
4.6A = 1 hp 208V 3ph (Table 430.250)

8.25A = 6.6 x 1.25 (1 1/2hp)
4.6A = (1hp)
12.85 = conductor ampacity
14 THHN = conductor size and type (Table 310.16 & 240.1(G))

 

[don_resqcapt19]

No, sorry. The OLs for each INDIVIDUAL motor circuit will not protect this FEEDER circuit in this case. Remember, he is branching it off to feed two individual motor starters. In the case of each individual motor circuit, that 250% would apply (assuming the OCP is an inverse time CB). But when he made this into a feeder circuit, article 430 ceases to apply.

Jeff,
Article 430 does apply. Take a look at 430.62. The maximum permitted OCPD for this circuit is 250% of 6.9A + 4.8A and rounded up to the next standard size....25 amps.

 

[frank_n]

Looks like both of you did a good job. The only thing that I would add is that I would use NEMA size 0 combination starters. The starters will give you the size of heater elements or electronic overloads. They can also easily connected to an Ansul and/or fire alarm system. Your rooftop 'toadstools' usually have three pole snap switches provided for disconnecting means.

As it turns out I do have to worry about a fire system. The building does not have a central system so I would have to install a horn/strobe in the kitchen. In the hood is a mechanical sensor that can be wired normally open or normally closed. I would use N/C to shut off the intake motor, but I have no idea what to do about the fire system.

What should I buy? Anyone have a diagram?

 

[augie47]

I may have missed something, but I see a problem. We are discussinga feeder breaker of 25 amps or greater, but I sw no mention of the individual motor GFSC protection. If this "feeder" breaker is the only GF-SC protection the 4.8 amp motor would be limted to a max SC protection of 15 amps so if only the one breaker is used for GF-SC protection, if it is sized fr both motors, it would eb oversized for the smaller motor.

Ground-fault/short circuit protection may have been addressed for each motor, but if so, I missed it.

 

[frank_n]

I may have missed something, but I see a problem. We are discussinga feeder breaker of 25 amps or greater, but I sw no mention of the individual motor GFSC protection. If this "feeder" breaker is the only GF-SC protection the 4.8 amp motor would be limted to a max SC protection of 15 amps so if only the one breaker is used for GF-SC protection, if it is sized fr both motors, it would eb oversized for the smaller motor.

Ground-fault/short circuit protection may have been addressed for each motor, but if so, I missed it.

A ground-fault short circuit would be protected by the breaker, right?

 

[augie47]

A ground-fault short circuit would be protected by the breaker, right?

each motor would be required to have some type G-F-S/C protection per 430.51. In reading, I only saw one means of providing that and that was what we were refering to as the "feeder" breaker. If that is the case the one breaker, sized for both motors, would not provide Code required protection for the smaller motor.

 

[frank_n]

each motor would be required to have some type G-F-S/C protection per 430.51. In reading, I only saw one means of providing that and that was what we were refering to as the "feeder" breaker. If that is the case the one breaker, sized for both motors, would not provide Code required protection for the smaller motor.

If there were a short circuit in "branch circuit" the fuse would blow.

 

[augie47]

my mistake..I missed the individual fuse in the 1st scenerio....

 

[misterbill1972]

sorry, I just cant help a yankee fan!

 

[roy g]

roy g

roy g

normally the intake (supply) motor contactor coil control is wired through the N/C switch of the ansul system. when the ansul sysatem is activated, the circuit opens, causing the intake(supply) fan to shut down. we recently completed resturant with simular system, and the AHJ indicated thet the exhaust fan should continue to operate with the ansul activated., to clear smoke and fumes.