UL 508A: 100% vs. 125% sizing for disconnect/OCPD

[EC Dan]

I have a resistive heating load of 200 A and a panel that will have a disconnect and/or OCPD, a contactor, and a control circuit (not important in this discussion). The plan was to have the disconnect and branch circuit protection be the same device, however it appears if I do that then it would have to be sized at 125% of the FLA, whereas if the disconnect and OCPD are separate (and the OCPD is a fuse) than both components can be sized based on 100% FLA. The relevant sections are:

• 31.6.3, which states the branch circuit protection shall be sized based on the ampacity of the field wiring, which leads to
• 28.3.8, which states the field wiring ampacity shall be 100% of the FLA
• 29.6.1 likewise states internal wiring shall be sized to the FLA of the load
• 30.2.2(a)(1) states a non-fused switch for a non-motor load shall be sized at 100% FLA

Ok, so no problem all components and wiring can be sized at 100% FLA, IF the disconnect and branch circuit are separate, however if I want to be efficient and combine the two into a fused disconnect, then 30.2.2(a)(2) applies instead which states a fused disconnect must be sized to 125% of FLA. Is this an accurate assessment of the situation?

A related question: UL 508A doesn't seem to distinguish between continuous and non-continuous load, except for the Exception to 30.2.1 which says the 125% sizing isn't required if the MCCB is rated at 100% load, however the NEC at least allows a maximum time at FLA to be considered continuous. Am I missing something there?

 

[don_resqcapt19]

What code are those citations from? What is the heating for? 424.4 requires the branch circuit for electric space heating to be sized at 125% of the load. If it is process heating, 425.4(B) says it is a continuous load.

 

[EC Dan]

This is for a UL508A panel.

 

[petersonra]

I have a resistive heating load of 200 A and a panel that will have a disconnect and/or OCPD, a contactor, and a control circuit (not important in this discussion). The plan was to have the disconnect and branch circuit protection be the same device, however it appears if I do that then it would have to be sized at 125% of the FLA, whereas if the disconnect and OCPD are separate (and the OCPD is a fuse) than both components can be sized based on 100% FLA. The relevant sections are:

• 31.6.3, which states the branch circuit protection shall be sized based on the ampacity of the field wiring, which leads to
• 28.3.8, which states the field wiring ampacity shall be 100% of the FLA

How would you know what the ampacity of the field wiring is going to be? What happens if the installer supplies wire that is larger than required? If you have a 200 FLA load, you would need to use 3/0 THHN copper, which is 200 A ampacity. But suppose the installer used 60 Deg C wire. Then it would be 250 MCM copper which has an ampacity of 215 Amps. Or suppose they wanted to use 75 deg C AL. That would be 250 MCM with an ampacity of 205 Amps.

• 29.6.1 likewise states internal wiring shall be sized to the FLA of the load
• 30.2.2(a)(1) states a non-fused switch for a non-motor load shall be sized at 100% FLA

Ok, so no problem all components and wiring can be sized at 100% FLA, IF the disconnect and branch circuit are separate, however if I want to be efficient and combine the two into a fused disconnect, then 30.2.2(a)(2) applies instead which states a fused disconnect must be sized to 125% of FLA. Is this an accurate assessment of the situation?

A related question: UL 508A doesn't seem to distinguish between continuous and non-continuous load, except for the Exception to 30.2.1 which says the 125% sizing isn't required if the MCCB is rated at 100% load, however the NEC at least allows a maximum time at FLA to be considered continuous. Am I missing something there?

 

[EC Dan]

How would you know what the ampacity of the field wiring is going to be? What happens if the installer supplies wire that is larger than required? If you have a 200 FLA load, you would need to use 3/0 THHN copper, which is 200 A ampacity. But suppose the installer used 60 Deg C wire. Then it would be 250 MCM copper which has an ampacity of 215 Amps. Or suppose they wanted to use 75 deg C AL. That would be 250 MCM with an ampacity of 205 Amps.

The field wiring terminals need to be sized according to the anticipated ampacity of the field wiring which is determined from 28.3.8 (for resistive process heating), and the field wiring terminals are marked to specify what type and temperature rating of conductor to use. If designed for 75 deg Cu, then the field terminal markings would prevent the use of 60 deg wire or Al wire.

 

[don_resqcapt19]

This is for a UL508A panel.

Once you leave the panel to go to the equipment, would not the NEC apply?

 

[Jraef]

Once you leave the panel to go to the equipment, would not the NEC apply?

Yes. So I don’t know what that UL 508A section would be referring to, I’ll have to look at the context. UL does not typically concern itself with field wiring, other than making room for it and determining terminal block ratings. My UL 508A documents are getting a little long in the tooth so I may not be up on all the changes, but I’m going to take a look later when I get back to the office. Seems odd.

 

[EC Dan]

Yes. So I don’t know what that UL 508A section would be referring to, I’ll have to look at the context. UL does not typically concern itself with field wiring, other than making room for it and determining terminal block ratings. My UL 508A documents are getting a little long in the tooth so I may not be up on all the changes, but I’m going to take a look later when I get back to the office. Seems odd.

Field wiring sizing in UL508A is discussed in terms of sizing both the field wiring terminals and the branch circuit protection:

28.3.8: For industrial process heating equipment employing electric resistance or electrode heating technology [...] or other fixed electric equipment used for industrial process heating the field wiring ampacity shall have an ampacity of 100 percent of the full-load current rating of the load involved.

31.6.3: Branch circuit protection for industrial process heating equipment employing electric resistance or electrode heating technology [...] or other fixed electric equipment used for industrial process heating the field wiring ampacity shall be sized based on the ampacity of the field wiring conductors supplying the equipment.

The guidance for sizing field wiring for this type of load in the NEC is more complicated:

425.22(E): Conductors for Subdivided Loads. Field-wired conductors between the heater and the supplementary overcurrent protective devices for fixed industrial process heating equipment shall be sized at not less than 125 percent of the load served. The supplementary overcurrent protective devices specified in 425.22(C) shall protect these conductors in accordance with 240.4. Where the heaters are rated 50 kW or more, the ampacity of field-wired conductors between the heater and the supplementary overcurrent protective devices shall be permitted to be not less than 100 percent of the load of their respective subdivided circuits, provided all of the following conditions are met: (1) The heater is marked with a minimum conductor size. (2) The conductors are not smaller than the marked minimum size. (3) A temperature-activated device controls the cyclic operation of the equipment.

UL508A makes no mention of sub-dividing the load of an industrial heater circuit, but if that part is ignored, then the three conditions can be met in this design, therefore the branch circuit could be sized at 100% of the load.