NEC Rules for Low Voltage Control Circuits
Question No. 1
I work for a control contractor and install typically 120 to 24vac Xformers for HVAC low voltage controls.
The Xformer VA is usually 250 or 500VA. Supplied from a 20 ampere breaker. What is the proper way
to protect this Xformer? What size conductors on the secondary?
Mike’s Response:
Primary Protection [Sections 4503(b), 72521(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 2403 (f) and 24021(c)(1) identifies that the secondary conductors
of a singlephase transformer having a 2wire (singlevoltage) secondary is considered protected by
the primary overcurrent protective device, provided the primary protection is in accordance with Section
4503.
Secondary Conductor Size. Class 1 conductors (secondary less than 30 volts) must be sized in
accordance with Section 72527(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 72524 Exception No. 2 and 2403(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 25020(a). Alternatingcurrent 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 72524 and 2403(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 4503(b), 72521(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 2403 (f) and 24021(c)(1) identifies that the secondary conductors
of a singlephase transformer having a 2wire (singlevoltage) secondary is considered protected by
the primary overcurrent protective device, provided the primary protection is in accordance with Section
4503.
Secondary Conductor Size. Secondary conductors must be sized in accordance with Table 31016:
No. 14 – 20 amperes
No. 12 – 25 amperes
No. 10 – 30 amperes
No. 8 – 40 amperes
Note: Alternatingcurrent circuits operating at 50 volts or more must be grounded in accordance
with Section 25030. This means a neutraltoground 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
