20A breaker or 15A breaker?

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dahualin

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I am doing electrical system design for a commercial building project. Here comes a very simple question. There are a lot of heaters in this builidng, and most of them are small (3A or 5A FLA). Each heater will have their own dedicated circuit. The wire size is #12AWG minimum. How about the breaker rating in panelboard? 20A or 15A breaker? I always use minimum 20A breaker for commercial building unless some HVAC unit requires MOCP 15A.
If I have to go with 15A. Could you please let me know which article of NEC apply to it. Thanks.
 
In general, Circuit Breakers and/or Fuses are installed to protect the conductors. If you are installing #12 conductors then a 20-amp breaker should be fine. If the manufacturer requires a lower ampacity overcurrent device the installation instructions or labeling will need to indicate that it is required. If they need to be protected at less than 15-amps, then supplemental protection would need to be required.
 
I don't think heater has MOCP or MFA specified. I heard people talking about the closer the breaker rating to the load current the better protection for overload. Do we need worry about overload for heaters? I think the overload issue only apply for motor or compressor devices.

Any thought are really appreciated.
 
dahualin said:
If I have to go with 15A. . . .
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You don't have to, but why wouldn't you want to. Indeed, why would you not want to use #14 with a 15 amp breaker? Is there a cost advantage of using #12 and 20 amp breakers?
 
For commercial buildings, we always specify the minimum wire size #12AWG. Maybe that is company standard for this kind of project.
 
For commercial projects, we typically #12AWG minimum. Many customers also pass that requirement on to us in the bid specifications. Overkill, yes.
 
#12 Awg

#12 Awg

Our standard is to use #12 at a minimum. Probably ought to rethink that given the cost of copper. Are there any restrictions with #14 other than the appropriate OCPD?
 
We always spec #12 as a minimum also. Here are a few reasons (although I admit, some may not always apply for every project).

1. The small size of #14 makes it more likely to be damaged during pulling.

2. Most of the breakers are specified as 20A anyway.

3. There might be more load added in the future to a very lightly loaded branch circuit.

4. The few number of circuits that could be reduced from 20A to 15A don't usually make enough difference costwise to worry about.

5. The amount of wire that could usually be reduced to #14 isn't usually a lot.

6. Reducing the wire to #14 increases the voltage drop, and decreases the maximum length of runs.

7. Reducing the wire to #14 increases the losses in the wire.

BTW, I think our spec does allow #14 for control wiring.

Steve
 
steve66 said:
3. There might be more load added in the future to a very lightly loaded branch circuit.

4. The few number of circuits that could be reduced from 20A to 15A don't usually make enough difference costwise to worry about.

5. The amount of wire that could usually be reduced to #14 isn't usually a lot.

6. Reducing the wire to #14 increases the voltage drop, and decreases the maximum length of runs.

7. Reducing the wire to #14 increases the losses in the wire.
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Steve, these are among the reasons I like to use 20a circuits for many residential receptacle circuits.

Plus, for those who feel that the load calcs dictate actual receptacles-per-circuit count, I can place a few more receptacles on each circuit.

Well, I can anyway.
 
dahualin said:
I am doing electrical system design for a commercial building project. Here comes a very simple question. There are a lot of heaters in this builidng, and most of them are small (3A or 5A FLA). Each heater will have their own dedicated circuit.
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You may use either 15 or 20 amp overcurrent protection.

422.11(E) provides the rule. (2002)

422.11(E) Single Nonmotor-Operated Appliance. If the branch circuit supplies a single non?motor-operated appliance, the rating of overcurrent protection shall

(1)Not exceed that marked on the appliance;

(2)If the overcurrent protection rating is not marked and the appliance is rated 13.3 amperes or less, not exceed 20 amperes; or

(3)If the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes, not exceed 150 percent of the appliance rated current. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.
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That said IMO it is a poor design to provide dedicated circuits for each of these units. You are using up a lot of panel space for little if any benefit.

If it was up to me I would run at least a 20 amp circuit with three or four of these units on the circuit and specify a fused disconect at each unit with a fuse sized for the unit.

I might even consider running a larger circuit and adding more units per circuit depending on the layout and distances etc.
 
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