One transformer or two for a remote outdoor power cabinet

Run one 15 kVA transformer and one distribution panel, not two transformers. Your two 120 V, 30 A tool circuits and your 240 V, 20 A battery-charger circuit come to 9.6 kVA of demand at an 80 % demand factor. The planned 5 kVA at 120 V takes the ultimate load to 14.6 kVA, still inside the rating. On the 120/240 V three-wire secondary you get 62.5 A per leg (15,000 VA ÷ 240 V). The ultimate draw is 60.8 A per leg once you split the future load across both legs. Two transformers would add a second 480 V feed, a second disconnect and a second gland set, and they buy nothing one panel can't do. The rest is arithmetic, a panel schedule and one cabinet to hold it all.

Count the load the way the transformer sees it

A receptacle panel never sees every breaker at full load at the same time. You size the transformer on demand, not on the sum of the breaker handles. An 80 % demand factor per receptacle branch is the planning allowance used in feeder sizing for tool panels like this one, and demand factor and load diversity guide sets out the method behind it. The arithmetic is four lines:

• 2 x 120 V x 30 A x 0.8 = 5,760 VA
• 240 V x 20 A x 0.8 = 3,840 VA
• Demand today: 9,600 VA = 9.6 kVA
• Future 5 kVA at 120 V: ultimate 14.6 kVA, still inside the rating

Then check the legs, because a 120/240 V secondary serves two 120 V halves. An overload is a per-leg event. Today's demand spread evenly is 9,600 ÷ 240 = 40 A per leg. Split the future 5 kVA into 2.5 kVA a side and each leg adds 2,500 ÷ 120 = 20.8 A, for 60.8 A against 62.5 A of capacity. Park all 5 kVA on one leg instead and that leg sees 40 + 41.7 = 81.7 A, which is 31 % over. That number sets the one panel rule that matters here: future 120 V branches land one per leg. Counted strictly at 100 % with no demand factor, the connected load is 12 kVA, and the same diversity argument still carries the 15 kVA choice.

One transformer or two

The two-transformer scheme fails on sizes before it fails on cost. Manufacturer catalogs step single-phase dry types through 10, 15 and 25 kVA; 12 kVA and 17 kVA don't exist as ratings. The split design therefore means 10 kVA plus 5 kVA. The 10 kVA unit runs at 9.6 ÷ 10 = 96 % of rating whenever the tools and the charger peak together. The single unit carries the same demand at 9.6 ÷ 15 = 64 %, which is where these transformers run cool for decades.

The rest of the comparison is counting parts:

Two secondaries in one cabinet would also give you two separately derived sources to keep apart. That is its own isolation exercise for two power sources in one cabinet. One transformer and one panel give you a single service point, a single door to lock and a single heat path. Split the design only when a written spec forces the charger onto its own source. For a tool panel beside a battery charger, one transformer wins every line of that table. For the pedestal and mounting side of a single 480-to-120 V transformer, the outdoor pedestal transformer enclosure walkthrough covers the rest.

The transformer you are buying

Specify a 15 kVA single-phase dry type, 480 V primary, 120/240 V three-wire secondary, ventilated general-purpose construction. Typical manufacturer data at that rating: 21.88 x 28 x 17.75 in [556 x 711 x 451 mm] and about 225 lb [102 kg]. The installation literature for these ventilated units calls for 2 in [50 mm] of clearance at the sides and back, and 6 in [150 mm] in front of the ventilation openings. Both numbers go straight into the cabinet sizing below.

The electrical one-liners, with the arithmetic:

• Primary current: 15,000 ÷ 480 = 31.3 A
• Primary fusing at 125 %: 31.3 x 1.25 = 39 A, so 40 A fuses in the disconnect
• Primary conductors: 8 AWG copper at 50 A (75 C copper ampacity, NEC); 6 AWG at 65 A if you want a 25 kVA swap later (25,000 ÷ 480 = 52.1 A)
• Secondary: 62.5 A per leg, landed on a 70 A two-pole main with 4 AWG copper at 85 A

The 70 A main stays under the 78 A ceiling that 125 % of 62.5 A allows. A 60 A main would cap the panel at 14.4 kVA, under your 14.6 kVA ultimate, so take the 70. Dry types lose about 1-3 % of rating as heat in manufacturer loss data. At 2 %, this one sheds 300 W at full load and about 200 W at the 9.6 kVA demand, because the winding share of the loss falls with the square of current. The transformer heat loss at partial load walkthrough takes that calculation to the next step.

One cabinet, two compartments

Start with what won't work, because it saves a round of quotes. Our deepest stock wall-mount box is 16 in [406 mm] deep against the transformer's 17.75 in [451 mm], and 225 lb [102 kg] belongs on a pad rather than on wall feet. The one-cabinet answer is a custom pedestal body built from one piece, with the two compartments sized from those installation clearances:

• Width: 21.88 in + 2 in + 2 in = 26 in [660 mm]
• Depth: 17.75 in + 2 in + 2 in = 22 in [559 mm]
• Transformer compartment: 28 in of transformer + 6 in above the vents = 34 in [864 mm]
• Panel compartment above it: 26 in [660 mm]
• Body: 60 in [1524 mm] plus a 6 in [152 mm] stub base = 66 in [1676 mm] overall

The transformer sits in the lower compartment, which keeps the heavy mass near the pad. That section gets louvered vent paths and a 3-6 mm [1/8-1/4 in] drain at the bottom — exactly the opening range NEC 314.15 allows in wet-location boxes — so rain and condensation leave by design. The upper panel section stays fully sealed: gasketed door, no vents, no field-drilled holes.

Both compartments carry manufacturer ratings for the specified design. The sealed section is rated NEMA 4X-type / IP66 by the manufacturer; not UL listed, and NEMA does not test or certify any enclosure. The vented section is built as Type 3R-type ventilation. The assembly's rating follows its weakest part: 3R where it vents, 4X where it gaskets. The 4X against IP66 primer sorts those labels if your spec mixes them. Keep the primary splices and the fused disconnect in the transformer compartment, below and apart from the 120/240 V panel; separating 480 V power from controls in one enclosure lays out that divider logic.

The heat check that sets the layout

Sealing the transformer in would cook the panel above it, and the numbers are short. A full-load 300 W loss through a 60 x 26 x 22 in [1524 x 660 x 559 mm] stainless body gives 300 ÷ (3.7 W/m²K x 4.45 m²) = 18 K of internal rise. That is 58 C inside on a 40 C [104 F] day. Breakers tolerate that; electronics don't, and the door gasket ages fast at those temperatures.

Vented, the transformer's 200-300 W convects out through the louvers and never crosses the divider. The sealed panel section carries only its own losses, about 20 W over 2.3 m², which is 20 ÷ (3.7 x 2.3) ≈ 2.5 K above ambient. Add a rule-of-thumb 10 K [18 F] of midday sun on a light stainless face and you sit near 53 C on the worst day. That is fine for breakers and over the 40 C design point for a controller or a mini PC. Shade the cabinet or face the door north before you hang electronics in it. The outdoor enclosure heat gain article and the 120 F PLC enclosure piece carry that arithmetic further.

The panel: circuits, balance and receptacles

The panel itself is a 12-space layout on DIN rail with a 70 A two-pole main:

Connected, each leg totals 100 A of breaker handle against the 62.5 A the transformer delivers on that leg. That is normal for a receptacle panel; the demand math above, 40 A today and 60.8 A ultimate, is what the transformer actually sees. The 240 V charger loads both legs evenly by construction. Every 120 V branch alternates legs, including the two spares, and that alternation is the whole balancing act.

Land the 30 A tool circuits on 30 A single receptacles and the 20 A branches on 20 A duplexes. Use GFCI breakers so the personnel protection lives inside the sealed door. Fit while-in-use covers that hold a wet-location rating with cords hanging out. Door-mounted receptacles put the tool cords at arm height and leave the bottom gland plate for the primary feed and any future conduit.

Mounting it at the site

Pour a pad, set the 6 in stub on it, and keep the space the code names. NEC 110.26(A) asks for at least 3 ft [900 mm] of clear depth in front of equipment at 0-150 V to ground in Condition 1. It also asks for at least 30 in [762 mm] of clear width or the equipment width, a door that opens a full 90 degrees, and 6.5 ft [2 m] of headroom. The primary terminations call for more depth where live parts face grounded metal, so keep that front zone generous.

Height works out on this body. NEC 240.24(A) keeps the grip of the highest breaker handle at or below 6 ft 7 in [2.0 m] above the standing surface, and the 66 in [1676 mm] cabinet puts your top row near 62 in [1575 mm]. The panel mounting height limits article collects the rest of those rules in one place.

Ground the cabinet on the equipment grounding conductor that arrives with the feed, sized from the fuse rating per NEC Table 250.122. Bond the door, the divider and the gland plate to that bus. The earth alone is never the fault path under NEC 250.4(A)(5), and a rod at a remote pedestal counts only as an auxiliary electrode under NEC 250.54. The grounding and bonding guide has the full method, and the cutouts and glands guide covers the gland plate layout — one row for the primary entry, spare space for future conduit.

What to put on the order

Send the cutout drawing with the gland size and receptacle centers, and the build starts from your approval of the drawing. Custom work is delivered in 10 business days, anywhere in the world, counted from that approval. A stock box counts from your order. If you would rather keep the transformer in its own vented outdoor housing — many ventilated dry types take a rain kit from their maker — put the panel alone in the 20 x 20 x 8 in 316L wall-mount enclosure. Take the 24 x 24 x 8 in 316L wall-mount enclosure if you want spare DIN rows.

Both are rated NEMA 4X-type / IP66 by the manufacturer and not UL listed, and the enclosure selector checks their inside dimensions against your panel. The decision holds either way: one 15 kVA transformer, one panel, future branches split across the legs, and one cabinet. Use custom enclosures if the transformer lives inside, a 316L wall-mount box if it doesn't.

Enclosures in this article

Questions