Unit heater why 15A opposed to 20A

[mjmike]

This question is in regards to a 7.5Kw 480V unit heater. The branch circuit is required to be rated at a continuous load hece 11.2A. #12 conductor would be used. As for the breaker, 15A would work, but is there anything that would not allow a 20A breaker?

 

[kbsparky]

If the nameplate on the unit requires max overcurrent protection of 15, then using a 20 would be a violation of 110.3(b)

 

[mjmike]

Called mfgr, no max listed.

Aslo, this is similarly for an exhaust fan. Say an exhaust fan is very small 120V fractional HP, draws say 2A. Per 430.52 table, the largest thermal breaker is 2A X250% = 5A. 5A breaker is not standard so by exception, next standard is 15A. Typically though for exhaust fans in bathrooms, they are on 20A breakers. Is the 20A breaker a violation in this example?

 

[Dennis Alwon]

If this is a single non motor operated appliance then 422.11(E) would apply. If the unit is not marked and the load is 13.3 amps or less then 20 amps is the max OCPD

 

[Dennis Alwon]

Called mfgr, no max listed.

Aslo, this is similarly for an exhaust fan. Say an exhaust fan is very small 120V fractional HP, draws say 2A. Per 430.52 table, the largest thermal breaker is 2A X250% = 5A. 5A breaker is not standard so by exception, next standard is 15A. Typically though for exhaust fans in bathrooms, they are on 20A breakers. Is the 20A breaker a violation in this example?

Motors 1 HP or less is a horse of a different color. 430.42(A) or (c), also see 422.10(A)

 

[mjmike]

Motors 1 HP or less is a horse of a different color. 430.42(A) or (c), also see 422.10(A)

This is a confusing article.

1. In my example, the exhaust fans are hard wired and I believe this is for only cord and plug connected motors.

2. Upon reviewing this article, it is very confusing. It seems to go in circles. 430.42(C) says 15A branch circuit if overload is omitted per 430.42(A). This then refers you to 430.32(D)(2) for not permanently installed and manually started. Here it has an exception that says it can be on a 20A receptacle? Not following this whole article. Does not make sence, it seems to say you can only plug portable electric tools into a 15A receptacle. Can this bne clarified some? Thanks.

 

[Dennis Alwon]

This is a confusing article.

1. In my example, the exhaust fans are hard wired and I believe this is for only cord and plug connected motors.

2. Upon reviewing this article, it is very confusing. It seems to go in circles. 430.42(C) says 15A branch circuit if overload is omitted per 430.42(A). This then refers you to 430.32(D)(2) for not permanently installed and manually started. Here it has an exception that says it can be on a 20A receptacle? Not following this whole article. Does not make sense, it seems to say you can only plug portable electric tools into a 15A receptacle. Can this bne clarified some? Thanks.

The article does not state it must be cord and plug. 430.32(D) is for overload protection for permanent and non permanent installs. If it is permanent installs then you must use 430.32(B) but again this is for overload protection.

 

[mjmike]

The article does not state it must be cord and plug. 430.32(D) is for overload protection for permanent and non permanent installs. If it is permanent installs then you must use 430.32(B) but again this is for overload protection.

Still not seeing it. Lets say I have a chemical feed pump less than 1 HP that has a noraml 120V plug on the end. It appears that 430.32(D)2 exception is saying I can connect it to a branch circuit protected not over 20A. This looks like I can have a dedicated 20A duplex receptacle on 20A circuit and all is good, maybe even have a switched receptacle. However, looking at 430.42(C), it appears I can only be on a 15A circuit. I am just not seeing it.

How about a sump pump that gets plugged in? can this only be on a 15A branch circuit?

What If I have a thermal switch controlling the receptacle or the motor has integral overload protection. You seem to emphasize overload protection but not sure why.

Sorry for being so dense, but I just am not seeing it as the sections seem to be contradictive.

 

[Dennis Alwon]

Still not seeing it. Lets say I have a chemical feed pump less than 1 HP that has a noraml 120V plug on the end. It appears that 430.32(D)2 exception is saying I can connect it to a branch circuit protected not over 20A. This looks like I can have a dedicated 20A duplex receptacle on 20A circuit and all is good, maybe even have a switched receptacle. However, looking at 430.42(C), it appears I can only be on a 15A circuit. I am just not seeing it.

430.32(C) states that if the motor is cord and plug AND individual overcurrent protection is omitted then it must be on a 15 amp circuit. I don't see the contradiction. Most of these motors have overload protection built in.

How about a sump pump that gets plugged in? can this only be on a 15A branch circuit?

Same as above

What If I have a thermal switch controlling the receptacle or the motor has integral overload protection. You seem to emphasize overload protection but not sure why.

Because the sections stated are talking about overload protection not the branch circuit size. The size of the branch cir. may be determined by the presence of overload protection.

[/QUOTE]

 

[mjmike]

430.32(C) states that if the motor is cord and plug AND individual overcurrent protection is omitted then it must be on a 15 amp circuit. I don't see the contradiction. Most of these motors have overload protection built in.

Same as above

Because the sections stated are talking about overload protection not the branch circuit size. The size of the branch cir. may be determined by the presence of overload protection.

[/QUOTE]

I think my confusion is specifically where you stated "motor is cord and plug AND individual overcurrent protection is omitted then it must be on a 15 amp circuit". How did you get to the conclusion of omitted? I can't quite grasp this from 430.42C as it referrs back to 430.42A that referrs to 430.32 which requires the overload protection. Thank you for your help.

 

[Gregg Harris]

I think my confusion is specifically where you stated "motor is cord and plug AND individual overcurrent protection is omitted then it must be on a 15 amp circuit". How did you get to the conclusion of omitted? I can't quite grasp this from 430.42C as it referrs back to 430.42A that referrs to 430.32 which requires the overload protection. Thank you for your help.[/QUOTE]

Take a look at the first sentence under (C). Cord and Plug Connected it sends you to (A) Not over 1 Horsepower,

One or more motors without individual overload protection--------

430.42 Motors on General-Purpose Branch Circuits. Overload protection for motors used on general-purpose branch circuits as permitted in Article 210 shall be provided as specified in 430.42(A), (B), (C), or (D).
(A) Not over 1 Horsepower. One or more motors without individual overload protection shall be permitted to be connected to a general-purpose branch circuit only where the installation complies with the limiting conditions specified in 430.32(B) and 430.32(D) and 430.53(A)(1) and (A)(2).
70-321 (B) Over 1 Horsepower. Motors of ratings larger than specified in 430.53(A) shall be permitted to be connected to general-purpose branch circuits only where each motor is protected by overload protection selected to protect the motor as specified in 430.32. Both the controller and the motor overload device shall be approved for group installation with the short-circuit and ground-fault protective device selected in accordance with 430.53.
(C) Cord-and-Plug-Connected. Where a motor is connected to a branch circuit by means of an attachment plug and a receptacle or a cord connector, and individual overload protection is omitted as provided in 430.42(A), the rating of the attachment plug and receptacle or cord connector shall not exceed 15 amperes at 125 volts or 250 volts. Where individual overload protection is required as provided in 430.42(B) for a motor or motor-operated appliance that is attached to the branch circuit through an attachment plug and a receptacle or a cord connector, the overload device shall be an integral part of the motor or of the appliance. The rating of the attachment plug and receptacle or the cord connector shall determine the rating of the circuit to which the motor may be connected, as provided in 210.21(B).
(D) Time Delay. The branch-circuit short-circuit and ground-fault protective device protecting a circuit to which a motor or motor-operated appliance is connected shall have sufficient time delay to permit the motor to start and accelerate its load.

 

[mjmike]

Ok, I think some of this got clarified. Please let me know if I am correct.

Lets say you have 2 small pieces of equipment with a motor in it (like a chemical feed pump), both get plugged into a receptacle, manually started, are assumed non-permenantly installed, are less than 1HP, and both have thermal protection built in. Circuit plans would be a duplex receptacle for each piece of equipment with 20A receptacle and 20A breaker.

Starting with 430.32 per 430.32(D)2 we look to see if it has thermal protection. It does, but it's not required in the equipment as it can be the breaker sized per part IV. Next, we review 430.42 which is applicable because we are dealing with a General-Purpose Branch Circuit. The section is not applicable because we DO have thermal. However, if we didn't, 430.42(c) says we would have to plug into a 15A receptacle but can still have a 20A breaker because of 430.32(2)2 excpeption. Next we move to part IV. Per this section, specifically 430.52, we have to comply with A or D. However, because we have the thermal we do not have to have the breakers sized for 2.5% the motor load hence we can still have the 20A breaker (as opposed to 15 by 450.52(C)). Now comes a trick part, seeing how we have multiple loads on one branch circuit, we have to review 430.53 which is yet more confusion. Because are motors meet the requirements of 430.53(A)1,2, and 3, we can remain with a 20A receptacle and 20A breaker. However, if we have a say a 3/4HP motor that pluggs in, we would need a 15A receptacle and 15A breaker because we exceed the 6 amp load of the first requirement.

I'm not sure if I am right or not, but it is very confusing just to determine the receptacle and breaker size to plug something in. Did I miss the boat someplace?

 

[Gregg Harris]

Ok, I think some of this got clarified. Please let me know if I am correct.

Lets say you have 2 small pieces of equipment with a motor in it (like a chemical feed pump), both get plugged into a receptacle, manually started, are assumed non-permenantly installed, are less than 1HP, and both have thermal protection built in. Circuit plans would be a duplex receptacle for each piece of equipment with 20A receptacle and 20A breaker.

Starting with 430.32 per 430.32(D)2 we look to see if it has thermal protection. It does, but it's not required in the equipment as it can be the breaker sized per part IV. Next, we review 430.42 which is applicable because we are dealing with a General-Purpose Branch Circuit. The section is not applicable because we DO have thermal. However, if we didn't, 430.42(c) says we would have to plug into a 15A receptacle but can still have a 20A breaker because of 430.32(2)2 excpeption. Next we move to part IV. Per this section, specifically 430.52, we have to comply with A or D. However, because we have the thermal we do not have to have the breakers sized for 2.5% the motor load hence we can still have the 20A breaker (as opposed to 15 by 450.52(C)). Now comes a trick part, seeing how we have multiple loads on one branch circuit, we have to review 430.53 which is yet more confusion. Because are motors meet the requirements of 430.53(A)1,2, and 3, we can remain with a 20A receptacle and 20A breaker. However, if we have a say a 3/4HP motor that pluggs in, we would need a 15A receptacle and 15A breaker because we exceed the 6 amp load of the first requirement.

I'm not sure if I am right or not, but it is very confusing just to determine the receptacle and breaker size to plug something in. Did I miss the boat someplace?

"However, if we have a say a 3/4HP motor that pluggs in, we would need a 15A receptacle and 15A breaker because we exceed the 6 amp load of the first requirement."

A 3/4 hp motor would not exceed the 6 amp but a one hp would. 746 watts = 1 hp

1 hp motor, 746 divided by 120 = 6.2 amps

746 X 0.75 =559.5 (560) watts x 2 =1120 watts = 9.22 amps

 

[suemarkp]

Um, motors are not 100% efficient and they don't have unity power factor. For 120V motors, a good rule of thumb is 10A per horsepower + 4 amps. So a 1 HP motor would generally have a 12A nameplate. A 3/4HP motor would have about 7.5+2A = 9.5A nameplate. You could also look at the motor tables (Table 430.248) and look at what those require for amp draw of 115V single phase motors.

For the multi motor case of 430.53A, I think the point of the 6A exception is if you get above that you may be needing a larger breaker than 15A. If you had a motor with a 6A nameplate, you're looking at a 15A breaker just for it (6A * 250% for sizing the overcurrent device). So perhaps that begins the region where motors need to be more dedicated to their own circuits.