Loop Feed vs.
Radial Feed.
Loop and radial describe how the primary cable connects to a pad-mount transformer: one set of primary bushings at the end of a cable run, or two sets so the cable continues on to the next unit. Here is what each means on the unit, how loops are operated and how to spec it.
The short
version.
- Radial feed: one set of primary bushings. The cable ends at this transformer, usually at the end of a radial tap. If that cable or anything upstream fails, the transformer stays out until the fault is repaired.
- Loop feed: two sets of primary bushings, connected together inside the high-voltage compartment. The primary cable comes in on one set and continues to the next unit from the other.
- Loops run open. A loop is fed from both ends and normally has one open point. After a cable fault, crews isolate the bad section and move the open point to restore power from the other side.
- Loop-feed transformers don’t parallel or share load. At any moment each transformer is fed from one direction and serves only its own secondary load. Loop feed adds a second path for the primary cable, not extra capacity.
Radial vs.
Loop.
A simplified view of an underground primary circuit. Dots are primary bushings; boxes are switching points.
Simplified one-line view. Switching points are drawn as boxes for clarity. In the field, sections are opened and closed with the transformers’ loop switches (if fitted), loadbreak elbows or separate switchgear, following the utility’s procedures.
What Each
One Means.
Feed type describes the primary (high-voltage) side only. It doesn’t change the secondary voltage, the kVA rating or how the transformer carries load.
Radial feed
A radial transformer has one set of primary bushings. The primary cable comes from the source, usually as a tap off a feeder, and ends at this unit.
- Bushings: one on a single-phase unit; three (H1, H2, H3) on a three-phase unit.
- One path to the source. If the cable feeding it fails, the transformer is out until the cable is repaired or replaced.
- Simpler and usually less costly: fewer bushings and no loop switching in the high-voltage compartment.
- Common uses: a single transformer serving one site, the last unit on a run, and industrial sites where each transformer has its own feeder from upstream switchgear.
Loop feed
A loop-feed transformer has two sets of primary bushings, usually labeled A and B (H1A and H1B on single-phase; H1A–H3A and H1B–H3B on three-phase). Inside the high-voltage compartment the two sets are connected together. The cable comes in on one set and leaves on the other to feed the next transformer.
- Bushings: two on single-phase, six on three-phase.
- Fed from both ends (two sources, or two positions on the same source) and normally run with one open point, so every transformer is fed from one direction at a time.
- After a cable fault, crews isolate the faulted section and move the open point. Customers on both sides get power back by switching while the cable is repaired.
- Common uses: underground residential subdivisions, commercial developments and campuses, and utility underground systems built to a loop standard.
- A loop-feed unit can also sit at the end of a run, with the unused bushings capped. Check whether your utility’s standard allows that.
Common myth“Loop-feed transformers are connected in parallel and share the load.” They aren’t. The second bushing set lets the primary cable pass through. Each transformer still carries only its own secondary load, and the loop normally has an open point, so it isn’t a closed parallel path.
What Changes
on the Unit.
Feed type changes the high-voltage compartment: how many bushings there are, how the cables terminate and whether switching is built in.
Bushing count
The quickest way to tell a loop-feed pad-mount from a radial one is to count the primary bushings.
| Unit | Radial feed | Loop feed |
|---|---|---|
| Single-phase pad-mount | 1 (H1) | 2 (H1A, H1B) |
| Three-phase pad-mount | 3 (H1, H2, H3) | 6 (H1A–H3A, H1B–H3B) |
Secondary (low-voltage) bushings don’t change with feed type. A loop-feed unit needs room for the second bushing set and its cables, which can affect the high-voltage compartment size.
Dead-front vs. live-front
- Dead-front: no exposed energized parts in the high-voltage compartment. Cables terminate in insulated, shielded separable connectors (loadbreak elbows) that plug onto bushing wells or inserts. BPS pad-mount listings say: “Standard construction is dead-front with loadbreak elbow connections on the primary.”
- Loadbreak elbows can be operated with a hotstick to make or break load current within their rating. They are not designed to interrupt fault current.
- Live-front: exposed primary bushings with cable terminations. It’s an older design that needs more clearance in the compartment. BPS listings say: “Live-front construction is available on request and should be specified at the time of quotation.”
- Two separate choices. Feed type (radial or loop) and front type (dead or live) are specified independently.
Loop-feed switching
Many three-phase loop-feed pad-mounts are ordered with oil-immersed loadbreak switches, often called LBOR (loadbreak oil rotary) switches, operated with a hotstick from the high-voltage compartment. Common arrangements:
- No switch: the A and B bushing sets are bused together and the transformer is always connected to the loop. Sections are isolated with the elbows.
- One two-position (on/off) switch: disconnects the transformer winding while the loop stays connected through the unit.
- Two two-position switches: one for each side of the loop, so either cable can be opened at this unit. With both open, the transformer is de-energized.
- One four-position sectionalizing switch: a single handle chooses between feeding from A, from B, from both (loop closed through), or open. The exact positions depend on the switch design.
Switch positions and labels vary by maker and utility standard, so confirm the switch diagram with your utility or engineer. BPS listings don’t show switch options; tell us what your spec calls for.
Fusing
Fusing protects the transformer itself, on both radial and loop units. On a loop-feed unit the fuses sit between the loop and the transformer winding. A blown fuse takes that transformer off line while the loop keeps carrying power to the next unit.
- Bay-O-Net fuses: an oil-immersed expulsion fuse in a holder that can be drawn out with a hotstick from the high-voltage compartment for inspection or replacement. Some fuse links respond to both current and oil temperature.
- Current-limiting fuses: often a partial-range (backup) current-limiting fuse in series with the Bay-O-Net, to clear high-current internal faults.
- Other schemes exist, such as internal protective links or full-range current-limiting fuses. The utility or engineer usually sets the fuse type and rating.
BPS listings don’t list fusing, so include it in your quote request.
Which One
Do You Need?
Usually the utility or the engineer of record decides, based on how the primary system is built. These are the common reasons for each.
Choose radial when
- The transformer is a single unit at the end of a tap.
- The site has one primary feed and no plan to extend the cable.
- Each transformer gets its own feeder position in upstream switchgear.
- The utility standard calls for radial at that location.
- You want the simplest, lowest-cost high-voltage compartment.
Choose loop when
- The utility standard specifies loop feed, as is common in underground subdivisions and commercial developments.
- Several pad-mounts share one underground primary circuit.
- You need to restore service by switching after a cable failure.
- The cable may continue to a future transformer.
- The site has two feed points and you want to use both.
How Our
Listings Read.
What the feed-related lines on a BPS listing mean. The quoted text is taken from current listings.
| Listing text | Where it appears | What it means |
|---|---|---|
| “Radial feed, dead front” | Feed configuration line on single-phase and three-phase pad-mount listings | Catalog construction: one set of primary bushings, dead-front, with loadbreak elbow connections. |
| “Standard construction is dead-front with loadbreak elbow connections on the primary.” | Pad-mount listing FAQ | Cables connect with separable insulated elbows. No exposed live parts. |
| “Live-front construction is available on request and should be specified at the time of quotation.” | Pad-mount listing FAQ | Ask for live-front when you request the quote. |
| “Per application” | Feed configuration line on pole-mount, substation and dry-type listings | Connections are set by the project. Substation listings note throat or bus connections for a switchgear lineup. |
| “Plus/minus 2 x 2.5% full-capacity below and above nominal” | Taps line in every listing’s specifications | The no-load tap changer, used to fine-tune the voltage ratio. It has nothing to do with a radial tap or the feed type. |
Loop feed, loop switches and fuse arrangements aren’t shown as catalog options. If your utility or engineer specifies them, put them in the quote request and we’ll confirm what’s available and the lead time.
Loop vs. Radial
at a Glance.
The differences that matter when you spec a pad-mount.
| Feature | Radial feed | Loop feed |
|---|---|---|
| Primary bushing sets | One | Two (A and B) |
| Primary bushings, single-phase | 1 | 2 |
| Primary bushings, three-phase | 3 | 6 |
| Cable path | Ends at the transformer | Passes through to the next unit |
| Sources | One | Both ends of the loop, normally with one open point |
| After a primary cable fault | Out until the cable is repaired | Restored by switching; faulted section isolated |
| Load sharing between transformers | No | No |
| Built-in switching | Usually none; an on/off switch can be specified | None, on/off, two on/off or four-position (varies) |
| Typical use | Single units, end of a tap, industrial feeders | Underground subdivisions, commercial loops, campuses |
| On BPS listings | Standard: “Radial feed, dead front” | Ask when you request a quote |
If you have a utility spec or drawing, send it. It answers most of these.
- kVA, phase and type: single- or three-phase pad-mount.
- Primary and secondary voltage strings, such as 12470Y/7200 to 480Y/277.
- Feed: radial or loop, plus the utility standard or spec section if there is one.
- Front: dead-front (standard on BPS listings) or live-front.
- Switching: none, on/off, two on/off or four-position, per your spec.
- Fusing: for example Bay-O-Net with current-limiting backup, or the utility’s required scheme, with ratings if specified.
- Accessories: who supplies loadbreak elbows, inserts, caps and arresters.
- Quantity, site location and need date, plus any listing requirements (UL Listed options available).
Loop / Radial
Answers.
Send the spec sheet or drawing with your quote request to sales@bearpowersolutions.com, or call the team.
Call 601-912-3512Do loop-feed transformers share the load?
No. Loop feed means the transformer has two sets of primary bushings so the primary cable can pass through to the next unit. The loop normally runs with an open point, each transformer is fed from one direction at a time, and each one carries only its own secondary load.
Is loop feed more reliable than radial?
It restores service faster after a primary cable failure, because crews can isolate the faulted section and feed the transformers from the other end of the loop. It doesn’t prevent the outage itself, and it doesn’t help if the transformer itself fails.
How many bushings does a loop-feed pad-mount have?
Six primary bushings on a three-phase unit (two sets of three) and two on a single-phase unit. A radial unit has three and one.
Can a loop-feed transformer be used on a radial circuit?
Often, yes. The unused bushing set is fitted with insulated caps. Check your utility standard first, because some utilities want radial units wherever no loop is planned.
Are BPS pad-mounts loop feed or radial feed?
BPS pad-mount listings show radial-feed, dead-front construction as standard. If your project calls for loop feed, loop switches or a specific fuse scheme, include it in your quote request and we’ll confirm availability and lead time.
Spec’d loop
or radial?
Send kVA, phase, primary and secondary voltage, feed type, switching and fusing requirements, quantity and where it’s going. We keep hundreds of units in stock and will tell you what’s ready; typical lead time is 12–16 weeks. UL Listed options available.