Parallel devices, Tap rules apply?

[NewHorizons]

Hi folks,

I have a question about feeding two devices each with 19.6A heaters (208V/1/60).

The heaters would be wired in parallel for the total load would be 19.6*2=39.2A.

The breaker would be 50A, (max breaker loading of 80%, 40A is greater than 39.2A so breaker ok).

Downstream of the breaker would be a 50A 2-pole contactor.

From the load side of the contractor, 2#6 (good for 75A from memory) wires to a j-box. From j-box, 2#10 wires (good for 30A) to each heater. All wire sizes are not corrected for temperature, conduit fill, etc. Just using straight values for example purposes.

Does this seem right? Do tap rules apply? Is overcurrent protection needed for each device (heater)?

If overcurrent protection is needed for each device, why would this situation be different than other circuits with multiple loads on it? Lights for example, often many lights are placed on a single circuit (one breaker).

Are devices wired in parallel considered taps?

Would things change (aside from load and possibly breaker and wire size changes) if the heaters were wired in 3-phase; one heater wired to L1-L2, second heater wired to L1-L3?

If this is not right, what would need to be done to wire the two 19.6A heaters to the 50A 2-pole contractor (in parallel)?

What specific sections of the NEC will help define the requirements for this situation?

Thanks!!
NH

 

[Smart $]

Welcome to the forum. :thumbsup:

Tap rules would apply for the #10 portion of the circuit. With no OCPD at load/device end, you'd have to comply with 210.19(A)(4) Exception No. 1(c)... essentially the #10 tap conductors can not be longer than 18 inches. Longer than 18 inches, tap rules of 240.21(B) would apply, which means you'd need load end OCPD and have to meet other conditions for the length and location of the tap conductors.

An alternative is to run #6 (or #8, should it be sufficient) all the way to both devices.

The advantage of running a 3Ø circuit is the conductor current is reduced by a ratio of (√3):2. In your case 19.6A*1.732=33.9A.

80% breaker loading (or rather 125% load factoring) is only required for continuous loads—those drawing full load continuously for not less than three hours. If this is a noncontinuous load, a 40A breaker and circuit is all that is required (even at 39.2A).

 

[iwire]

80% breaker loading (or rather 125% load factoring) is only required for continuous loads—those drawing full load continuously for not less than three hours. If this is a noncontinuous load, a 40A breaker and circuit is all that is required (even at 39.2A).

If it is fixed electric space heating we must treat (the branch circuit) as a continuous load.

424.3 Branch Circuits.


(B) Branch-Circuit Sizing. Fixed electric space-heating
equipment and motors shall be considered continuous load.

 

[Smart $]

If it is fixed electric space heating we must treat (the branch circuit) as a continuous load.

Good point if these are in fact fixed space heaters. OP is rather ambiguous on the matter.

 

[NewHorizons]

Thanks for the welcome and reply's!!

So, if I read the notes right, overcurrent protection would not be needed at each device (heater) if they are fed with the same wire size as the feed to the j-box? So, #6 from j-box to each device? Is this why parallel lights don't have overcurrent protection for each light, where all wires are the same size? Is this notion defined in a specific code reference? Is it implied because a tap rule would no longer apply because there was no reduction in wire size? Tap rules say tap feeders terminate into an overcurrent protection device, then to the load so any time tap rules apply, overcurrent protection is needed for the tap circuit.

What triggers tap rules, reduction in wire size?

If wiring in 3-phase would be the same idea? Just keep the wire size to each device the same as to the j-box no overcurrent protection for each device needed?

How would this scenario change if the devices were fractional HP motors when MCA and MOCP enter the picture?

Thanks!
NH

 

[Smart $]

Thanks for the welcome and reply's!!

So, if I read the notes right, overcurrent protection would not be needed at each device (heater) if they are fed with the same wire size as the feed to the j-box? So, #6 from j-box to each device?

Correct.

Is this why parallel lights don't have overcurrent protection for each light, where all wires are the same size?

Pretty much... but there are also different rules for feeder taps [240.21(B)] and branch-circuit taps [210.19(A)(3) and (4) Exceptions].

Is this notion defined in a specific code reference? Is it implied because a tap rule would no longer apply because there was no reduction in wire size?

Actually, it is not based on wire size directly. It is based on whether the OCPD where the circuit receives it supply is rated to protect the circuit conductor. Where it is not, you have a tap... and the tap rules then apply. See Tap Conductors definition in 240.2.

Tap rules say tap feeders terminate into an overcurrent protection device, then to the load so any time tap rules apply, overcurrent protection is needed for the tap circuit.

Feeder taps, yes.

What triggers tap rules, reduction in wire size?

As noted above, conductor ampacity vs. OCPD at point of circuit supply.

If wiring in 3-phase would be the same idea? Just keep the wire size to each device the same as to the j-box no overcurrent protection for each device needed?

Correct.

How would this scenario change if the devices were fractional HP motors when MCA and MOCP enter the picture?

Wiring motors is hard enough to understand even using specific case scenarios. Asking such a generic question opens a big can o' worms that I'm not in the right frame of mind to answer in a few short words. Perhaps someone else will step up to answer this one...

 

[kwired]

Are these heaters Fixed electric space heaters, or some other sort of heater?

The application will effect which code section applies.

210.23(C) does permit "infrared heating or other utilization equipment" to be connected to multioutlet circuits on 40 or 50 amp branch circuits.

Branch circuit tap conductors would be permitted to serve individual units in those instances.

There may or may not be other conditions mentioned in 424 or some of the sections for heating type equipment that follow 424.

 

[NewHorizons]

Thanks again. Yes, best to avoid the motor can of worms here. :thumbsup:

So it is basically a common sense approach. Use wire sizes that are rated for use on the overcurrent protection device throughout the circuit. If you downsize a wire where load reduces, you need overcurrent protection for that smaller wire size.

The heaters in this example are not space heaters but heaters that will operate 3 times a day for 45 minutes. They are defrost heaters in evaporators of a refrigeration system.

Thanks!
NH

 

[NewHorizons]

Thinking more about this, if we assume the wires internal to the evaporator that feed the resistive heaters are #10 (good for 30A uncorrected), think a case could be made that overcurrent protection would be needed at 30A at each evaporator regardless of wire size from j-box to the evaporator (#6, above the limits of the 50A breaker for the feed to the j-box)? There is no MOCP rating on the evaporator for the defrost heater feed.

I can't think of any reason that a resistive heater would draw more Amps if the right voltage is applied (no overloading issues), so there would be no issue to the internal #10 wires of the heater circuit (assumed above), but what would code require?

Thanks!
NH

 

[Smart $]

...
So it is basically a common sense approach. Use wire sizes that are rated for use on the overcurrent protection device throughout the circuit. If you downsize a wire where load reduces, you need overcurrent protection for that smaller wire size.
...

For feeders.

For branch circuits, no ocp is required, but branch-circuit taps are only permitted under a very limited set of conditions.

 

[Smart $]

Thinking more about this, if we assume the wires internal to the evaporator that feed the resistive heaters are #10 (good for 30A uncorrected), think a case could be made that overcurrent protection would be needed at 30A at each evaporator regardless of wire size from j-box to the evaporator (#6, above the limits of the 50A breaker for the feed to the j-box)? There is no MOCP rating on the evaporator for the defrost heater feed.

I can't think of any reason that a resistive heater would draw more Amps if the right voltage is applied (no overloading issues), so there would be no issue to the internal #10 wires of the heater circuit (assumed above), but what would code require?

Thanks!
NH

Does the supplied evaporator have separate supply terminals for the heaters, or are the heaters supplied through the evaporator power terminals? If the latter, and the evaporator is sold as a listed unit, you only have to concern yourself with wiring up to the power supply terminals. The wire sizing after the power terminals and integral with the manufactured unit are not under the purview of the NEC.

 

[NewHorizons]

For feeders.

For branch circuits, no ocp is required, but branch-circuit taps are only permitted under a very limited set of conditions.

Ok, but branch circuits don't have a change (reduction) in wire size. If there is a change (reduction), it is then a tap right? (tap since wire size smaller (less Amps) than the upstream ocp. Tap portion would need an ocpd to protect the smaller wires).

 

[NewHorizons]

Does the supplied evaporator have separate supply terminals for the heaters, or are the heaters supplied through the evaporator power terminals? If the latter, and the evaporator is sold as a listed unit, you only have to concern yourself with wiring up to the power supply terminals. The wire sizing after the power terminals and integral with the manufactured unit are not under the purview of the NEC.

The evaporators have several listings, UL, etc.. There are two separate circuits in the evaporator, one circuit for the fans and one circuit for the heaters. There are terminals where the power for each circuit is supplied (field wired). There is not one set of terminals that splits to serve each circuit within the evaporator.

 

[kwired]

Ok, but branch circuits don't have a change (reduction) in wire size. If there is a change (reduction), it is then a tap right? (tap since wire size smaller (less Amps) than the upstream ocp. Tap portion would need an ocpd to protect the smaller wires).

Depends on the application. As I mentioned before if these were infrared heaters - those are specifically mentioned, some luminaires are permitted to be directly tapped to a higher capacity circuit, but the "other utilization equipment is pretty vague wording, you may need to find other code sections that apply to the equipment you are using to make a determination.

If your defrost heaters are part of a listed unit - I think you must go by whatever is on the nameplate.

 

[Smart $]

Ok, but branch circuits don't have a change (reduction) in wire size. If there is a change (reduction), it is then a tap right? (tap since wire size smaller (less Amps) than the upstream ocp. Tap portion would need an ocpd to protect the smaller wires).

Yes there are branch circuit taps where the the tap conductor is of a lower ampacity than protected by the supply ocpd. No additional ocpd is required.

 

[Smart $]

The evaporators have several listings, UL, etc.. There are two separate circuits in the evaporator, one circuit for the fans and one circuit for the heaters. There are terminals where the power for each circuit is supplied (field wired). There is not one set of terminals that splits to serve each circuit within the evaporator.

Okay.

As I said earlier, OCPD is required if wired as a feeder tapped circuit.

OCPD is not required if wired as a tapped branch circuit... but the tap conductors are limited to 18". Refer to 210.19(A)(4) Exception no. 1 and item (c) thereunder.