Control Circuit Sizing (9-5-2K)
 

NEC Rules for Low Voltage Control Circuits

Question No. 1

I work for a control contractor and install typically 120 to 24vac X-formers for HVAC low voltage controls. The X-former VA is usually 250 or 500VA. Supplied from a 20 ampere breaker. What is the proper way to protect this X-former? What size conductors on the secondary?

Mike’s Response:

Primary Protection [Sections 450-3(b), 725-21(a)(1)]. Control circuit transformer having a primary current rating:

·       Less than 2 amperes. Where the transformer has a rating less than 2 amperes, the overcurrent protection on the primary must be set at not more than 300 percent of the rated primary current.

·       Between 2 and 8.99 amperes. Where the transformer has a rating between 2 and 8.99 amperes, the primary protection should not exceed 167% of the primary rating.

·       Greater than 9 amperes. Where the primary current is greater then 9 amperes, the primary protection should not exceed 125% of the primary rating.

Secondary Protection. Section 240-3 (f) and 240-21(c)(1) identifies that the secondary conductors of a single-phase transformer having a 2-wire (single-voltage) secondary is considered protected by the primary overcurrent protective device, provided the primary protection is in accordance with Section 450-3.

Secondary Conductor Size. Class 1 conductors (secondary less than 30 volts) must be sized in accordance with Section 725-27(a), provided they supply loads that do not exceed the following ampacities:

Secondary Conductor Size Maximum Primary 120V Protection* Maximum Secondary Load
No. 18 1.2 amperes (6 A x 24V/120V)    6 amperes
No. 16 1.6 amperes (8 A x 24V/120V) 8 amperes 
No. 14 4 amperes (20 A x 24V/120V) 20 amperes
No. 12 5 amperes (25 A x 24V/120V) 25 amperes
No. 10  6 amperes (30 A x 24V/120V) 30 amperes
No. 8 8 amperes (40 A x 24V/120V) 40 amperes

*Sections Section 725-24 Exception No. 2 and 240-3(f) require the primary overcurrent device rating shall not exceed the secondary conductor's ampacity multiplied by transformer ratio. These values are based on a secondary voltage of 24 volts.

Grounding. Section 250-20(a). Alternating-current circuits operating at less than 50 volts are not required to be grounded, except for the following conditions:

1. Where the transformer supply system exceeds 150 volts to ground

2. Where the transformer supply system is ungrounded

3. Where installed overhead outside of buildings

Example 250 VA at 120/24 volts:

Primary Current = 250 VA/120 volts = 2.08 amperes

Primary Protection = 2.08 X 1.67 = 3.47 amperes, Select 3 ampere fuse

Secondary Current = 250 VA/24 volts = 10.47 amperes

Secondary Conductors = No. 14 permitted to be protected by the 3 ampere fuse on the primary.

Note: Sections Section 725-24 and 240-3(f) require the primary overcurrent device rating
shall not exceed the secondary conductor's ampacity multiplied by transformer ratio.
Primary Protection = 3A X 120V/24V = 15 ampere, No. 14 conductor okay
 
Example 500 VA at 120/24 volts:

Primary Current = 500 VA/120 volts = 4.16 amperes

Primary Protection = 4.16 X 1.67 = 6.95 amperes, 7 ampere fuse

Secondary Conductors = No. 8 is required because the 7 ampere fuse on the primary. Primary Protection = 7A X 120V/24V = 35 ampere, No. 8 conductor.

Question No. 2

Mike, I recently started working at a control shop, and was asked how to protect a control circuit for a variable frequency drive controller(s). So after evaluating the load - two fans (88 watts at 115vac each) and a couple of lights (that had essentially no draw).

88w + (88w X 1.25) = 198 watts of draw, and 198w/115vac = 1.72amps of draw.

Then I did the motor protection route by fuses (dual element 175%) and came up with 3.01. Had a 2amp fuse available and tried it. The fuse blew, so I ordered 5 amp fuses for the circuit(s) and am waiting for them to arrive from manufacturer.

The control circuit is supplied from a 208vac to 115/131vac transformer rated 750va that is protected on the primary with a 5 amp fuse in each line. Does this meet the NEC requirements? Is this installation safe?

Our shop is a UL listing shop and we will have a UL inspector coming out to inspect the unit for certification. I would like to look good when the shop calls to have my work approved and sold to the school district without a glitch.

By the way is there another standard (NFPA, UL, NEMA, etc) for skid mounted equipment or motor control centers?

Mike’s Response: Since the secondary is above 30 volts, the secondary circuit conductors are not considered to be a Class 1 control circuit and all general requirements of the NEC applies.

Primary Protection [Sections 450-3(b), 725-21(a)(1)]. Control circuit transformer having a primary current rating:

·       Less than 2 amperes. Where the transformer has a rating less than 2 amperes, the overcurrent protection on the primary must be set at not more than 300 percent of the rated primary current.

·       Between 2 and 8.99 amperes. Where the transformer has a rating between 2 and 8.99 amperes, the primary protection should not exceed 167% of the primary rating.

·       Greater than 9 amperes. Where the primary current is greater then 9 amperes, the primary protection should not exceed 125% of the primary rating.

Secondary Protection. Section 240-3 (f) and 240-21(c)(1) identifies that the secondary conductors of a single-phase transformer having a 2-wire (single-voltage) secondary is considered protected by the primary overcurrent protective device, provided the primary protection is in accordance with Section 450-3.

Secondary Conductor Size. Secondary conductors must be sized in accordance with Table 310-16:

No. 14 – 20 amperes

No. 12 – 25 amperes

No. 10 – 30 amperes

No. 8 – 40 amperes

Note: Alternating-current circuits operating at 50 volts or more must be grounded in accordance with Section 250-30. This means a neutral-to-ground bond at the transformer with a grounding electrode must be installed.

Example 750 VA at 208/115 volts:

Primary Current = 750 VA/208 volts = 3.6 amperes

Primary Protection = 3.6 X 1.67 = 6.02 amperes, 6 amperes

Secondary Conductors = No. 8 is required because the 7 ampere fuse on the primary. Primary Protection = 6A X 208V/115V = 11 ampere, No. 14 conductor

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